Measurements of the Distribution And

QUEENSLAND UNIVERSITY OF TECHNOLOGY

SCHOOL OF PHYSICAL AND CHEMICAL SCIENCES

MEASUREMENTS OF THE DISTRIBUTION AND

BEHAVIOUR OF BERYLLIUM-7 IN THE NATURAL

ENVIRONMENT

Che Doering

B. App. Sc., M. App. Sc.

A thesis submitted in partial fulfilment of the requirements of the degree of

Doctor of Philosophy

2007

KEYWORDS

Beryllium-7, environment, environmental radioactivity, radionuclide, cosmogenic, cosmic- rays, atmosphere, deposition, soil, surface air, atmospheric transport, erosion, depositional flux, areal activity density, Brisbane, Southeast Queensland, Australia

ABSTRACT

Beryllium-7 is a cosmogenic radionuclide produced in the atmosphere through the spallation of nitrogen and oxygen nuclei by cosmic-ray-produced neutrons and protons. It is carried in the atmosphere attached to aerosols and is deposited on land and ocean surfaces by wet and dry deposition processes. Beryllium-7 decays by electron capture to lithium-7 and has a half-life of approximately 53 days. It is a potentially useful radionuclide for studying different natural processes.

This thesis presents a collection of scientific papers on the occurrence of beryllium-7 in the natural environment, particularly in the Southeast Queensland region of

Australia. It shows the results of experimental measurements and discusses their implications. Overall, this thesis contributes to advancing our understanding of the distribution and behaviour of beryllium-7 in the natural environment and provides a foundation for the development of nuclear techniques for the evaluation of environmental problems.

iii

LIST OF PUBLICATIONS

Doering, C., Akber, R., Heijnis, H., 2006. Vertical distributions of 210Pb excess, 7Be and

137Cs in selected grass covered soils in Southeast Queensland, Australia. Journal of

Environmental Radioactivity 87, 135–147.

Doering, C., Akber, R., submitted. Beryllium-7 in near-surface air and deposition at

Brisbane, Australia. Journal of Environmental Radioactivity.

Doering, C., Akber, R., submitted. Describing the annual cyclic behaviour of 7Be concentrations in surface air in Oceania. Journal of Environmental Radioactivity.

TABLE OF CONTENTS

CHAPTER ONE 1

INTRODUCTION

1.1 Environmental radioactivity and beryllium-7 1

1.2 Research objectives 2

1.3 Link between the scientific papers 2

CHAPTER TWO 4

LITERATURE REVIEW

2.1 Introduction 4

2.2 Nuclear properties 4

2.3 Activity measurement 5

2.4 Cosmogenic production 5

2.5 Production through nuclear detonations 6

2.6 Activity size distribution 7

2.7 Atmospheric residence time and concentration 8

2.8 Surface air concentrations 9

2.9 Deposition 15

2.10 Distribution in ocean waters 20

2.11 Distribution in soils and grasses 21

2.12 Environmental applications 24

2.13 Annual effective dose 28

2.14 Summary and conclusions 28

References 29

CHAPTER THREE 43

VERTICAL DISTRIBUTIONS OF 210Pb EXCESS, 7Be AND 137Cs IN SELECTED

GRASS COVERED SOILS IN SOUTHEAST QUEENSLAND, AUSTRALIA

3.1 Introduction 46

3.2 Materials and methods 47

3.2.1 Soil sampling sites 47

3.2.2 Soils 47

3.2.3 Soil sampling method 50

3.2.4 Analytical technique 50

3.2.5 Radon-222 exhalation measurements 51

3.3 Results 52

210 7 3.3.1 Pbex and Be areal activity densities 52

210 222 3.3.2 Accumulation of Pbex on the land surface and Rn activity

fluxes 56

3.3.3 Fallout of 137Cs on the Southeast Queensland landscape 60

210 7 3.3.4 Natural radionuclide signatures ( Pbex and Be) for use in

erosion studies 62

3.4 Conclusions 64

References 65

CHAPTER FOUR 68

BERYLLIUM-7 IN NEAR-SURFACE AIR AND DEPOSITION AT BRISBANE,

AUSTRALIA

4.1 Introduction 71

4.2 Sampling and analytical techniques 72

4.3 Results and discussion 73

4.3.1 7Be concentrations in near-surface air 73

4.3.2 7Be deposition 75

4.3.3 Net cumulative 7Be areal activity density 78

4.4 Conclusion 80

References 81

CHAPTER FIVE 83

DESCRIBING THE ANNUAL CYCLIC BEHAVIOUR OF 7Be CONCENTRATIONS IN

SURFACE AIR IN OCEANIA

5.1 Introduction 86

5.2 Data and methods of analysis 86

5.2.1 Data sources 86

5.2.2 Measurement technique 90

5.2.3 Data treatment 90

5.3 Results and discussion 92

5.3.1 Results of sinusoidal model 92

5.3.2 Atmospheric processes controlling the 7Be annual cycle 94

5.4 Conclusions 100

References 101

CHAPTER SIX 104

CONCLUDING REMARKS

6.1 Summary and conclusions 104

6.2 Future directions 106

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LIST OF TABLES

Table 2.1. Average surface air concentrations of 7Be at different locations. Data is

sorted in order of descending latitude. (pp. 10–12)

Table 2.2. Annual 7Be deposition and rainfall at different locations. Data is sorted in

order of ascending annual rainfall. (pp. 16–18)

Table 2.3. Total 7Be areal activity densities at undisturbed sites. Data is sorted in

order of ascending sampling date. (pp. 22–23)

Table 3.1. Site location, annual rainfall rate, and physical and chemical properties of

the soils (moisture content, composition and particle size distribution are

all expressed as percentage weight of the sample). (p. 49)

210 7 137 Table 3.2. Accumulated Pbex, Be and Cs areal activity density in the surface 10

cm and portion present in grass. (p. 53)

Table 4.1. Summary of near-surface air 7Be concentration measurements at Brisbane

during the years 2002–2005. (p. 74)

Table 4.2. Summary of 7Be deposition measurements at Brisbane during the years

2004–2006. (p. 76)

Table 4.3. Comparison of measured and calculated net cumulative 7Be areal activity

densities for May 2003. (p. 79)

Table 5.1. Location of sampling stations, measurement periods, mean 7Be

concentration in surface air, and long-term annual average rainfall. (pp.

88–89)

Table 5.2. Summary of results of sinusoidal model fitting. (p. 93)

Table 5.3. Values of linear correlation coefficient (R) and Spearman correlation

coefficient (ρ) between mean of monthly 7Be deviations and long-term

mean of monthly rainfall. (p. 99)

viii

LIST OF FIGURES

Figure 3.1. Location of the sampling sites and map of Australia (inset). (p. 48)

210 7 Figure 3.2. Pbex (top) and Be (bottom) cumulative areal activity density profiles

normalised to unity at 10 cm depth. The solid line indicates the best

fitting exponential given by Equation 3.1 to the data. Error in individual

data points is typically 10 %. (p. 54)

Figure 3.3. 226Ra and 210Pb areal activity density plotted against 238U. The solid

indicates the line of equality. Errors in 226Ra, 210Pb and 238U are typically

5, 10 and 10 %, respectively. (p. 59)

Figure 3.4. 137Cs cumulative areal activity density profiles normalised to unity at 10

cm depth. The site key is the same as that shown in Figure 3.2. The solid

line represents the best fitting linear function to the data. The dotted lines

indicate the 95 % confidence region. Error in individual points is

typically 10 %. (p. 61)

Figure 4.1. Percent deviation of monthly average 7Be concentrations from the annual

mean at Brisbane during the years 2002–2005. Uncertainty in 7Be

concentrations is generally less than 10 %. (p. 74)

Figure 4.2. Monthly 7Be deposition (columns) and rainfall (solid circles) at Brisbane

during the years 2004–2006. Uncertainty in monthly 7Be deposition

measurements is generally less than 10 %. The dashed line shows the

trend in rainfall. (p. 76)

Figure 4.3. Monthly 7Be deposition (Y) plotted against monthly rainfall (X). The

solid line represents the linear least squares regression line. (p. 77)

Figure 4.4. Calculated net cumulative 7Be areal activity density (columns) and

measured rainfall (solid circles) at Brisbane during the years 2000–2006.

The solid line is the decay curve for 7Be activity deposited before January

2000. The dashed line shows the trend in rainfall. (p. 79)

Figure 5.1. Map of Oceania showing location of 7Be measurement stations. (p. 87)

Figure 5.2. Frequency representation of normalised (top panel) and un-normalised

(bottom panel) 7Be concentration data for the monitoring station at

American Samoa. (p. 91)

Figure 5.3. Latitude-time isopleth diagram of the zonal averaged ratio of the mean of

monthly 90Sr concentrations in surface air. From Rehfeld and Heimann

(1995). (p. 96)

Figure 5.4. Mean of monthly 7Be deviations (filled circles) at American Samoa (top

panel), Brisbane (middle panel) and Cape Grim (bottom panel), and mean

of monthly 90Sr deviations (open circles) at the equivalent latitude. (p. 98)

STATEMENT OF ORIGINAL AUTHORSHIP

The work contained in this thesis has not been previously submitted for a degree or diploma at any other tertiary education institution. To the best of my knowledge and belief, the thesis contains no material previously published or written by another person except where due reference is made.

Signed: ……………………………..

Date: ……………………………..

CHAPTER ONE

INTRODUCTION

1.1 Environmental radioactivity and beryllium-7

Radionuclides occurring in our natural environment can be classified in three

general categories: (1) primordial radionuclides, i.e., nuclides which have survived since

the time when the elements formed, and their progeny nuclides; (2) cosmogenic

radionuclides, formed continuously by the interactions of cosmic-ray particles with matter;

and (3) artificial radionuclides, introduced by human activities, e.g., by detonations of

nuclear weapons. Radioecology is the study of the occurrence of these radionuclides in the

natural environment. This includes the atmosphere, hydrosphere and lithosphere. Studies of

environmental radionuclides can provide us with useful geophysical information, such as

the timescales over which natural systems are changing or evolving and the underlying

mechanistic causes. They can also be used to assess the radiological or radioecological

effects of ambient radioactivity.

Beryllium-7 (7Be) is a cosmogenic radionuclide formed in the atmosphere when

cosmic-ray-produced neutrons and protons disintegrate the atomic nucleus of nitrogen and

oxygen into lighter fragments. It has a relatively short half-life of approximately 53 days.

7Be is found naturally in air, rainwater, soils and sediments, vegetation, as well as lake, estuarine and ocean waters. The use of 7Be as a tracer of natural processes has recently been realised, and a number of nuclear techniques have been developed to support ongoing research in this area. Nevertheless, most applications of 7Be require a detailed

understanding of its distribution within different natural systems. Such an understanding is

still incomplete. Therefore, studies of 7Be, such as those presented in this thesis, will greatly add to our understanding of the behaviour of this radionuclide in the environment and its future use for the evaluation of environmental problems.

1.2 Research objectives

The overall objectives of this research were to (1) investigate the distribution of

7Be in soils within Southeast Queensland in order to evaluate the potential use of this radionuclide as a tracer of soil erosion; and (2) investigate the atmospheric cycle of 7Be within the troposphere, both in Southeast Queensland and the Oceania region. These research objectives were met through a number of subsidiary aims, which were to (1) measure 7Be activity in soils and grasses and identify factors controlling the areal activity

density of this radionuclide; (2) measure 7Be activity in wet and dry deposition and identify

factors controlling the depositional flux of this radionuclide; (3) measure 7Be activity in surface air; and (4) describe the annual cycle of 7Be concentrations in surface air and identify factors controlling this behaviour.

1.3 Link between the scientific papers

This thesis is presented as a collection of scientific papers which have been published or submitted for publication in peer reviewed and international journals.

A comprehensive review of the 7Be literature is presented in Chapter Two. The

purpose of this review is to examine the current state of knowledge surrounding 7Be. It covers aspects of 7Be nuclear properties, activity measurement, production, distribution and

behaviour in natural systems, as well as applications for tracing natural processes. This

chapter provides a foundation of understanding for the results and discussions presented in

each of the research papers that follow.

Measurements of the distribution of 7Be in soils and grasses at sites in Southeast

Queensland are presented in Chapter Three as a published paper. These are the first such measurements to be reported for the region. It is recognised that soil erosion is a major factor contributing to land degradation in Southeast Queensland. This is due in part to high rates of population growth and urban development. Nuclear techniques can be used to assess rates of short- and medium-term soil erosion. However, of critical importance to

such assessments is knowledge of the distribution of radionuclides within the soil profile

and factors that affect their areal activity density. In addition to 7Be, distributions of

210 naturally occurring lead-210 excess ( Pbex) and the artificial radionuclide caesium-137

(137Cs) are also reported in this chapter.

Results of 7Be deposition measurements are presented in Chapter Four as a

submitted paper. Such measurements are fundamentally important for understanding factors

that deliver 7Be to terrestrial systems and affect its areal activity density in soils and overlying vegetation. The measurements presented in this chapter were performed over three complete years so that any seasonal or annual changes in the depositional flux of this radionuclide could be observed. The study compares values of 7Be areal activity density calculated from measured depositional fluxes with those measured directly in soils and grasses. It identifies seasonality in rainfall and the 7Be half-life itself to be important factors

controlling the areal activity density of 7Be in Southeast Queensland. This chapter also presents measurements of 7Be in surface air and investigates the relationship between 7Be deposition and surface air concentrations of this radionuclide.

A study of the annual cycle of 7Be concentrations in surface air in the Oceania region is presented in Chapter Five as a submitted paper. The annual cycle is a defining feature of this radionuclide in surface air and is thought to be the product of a number of meteorological factors. This study analyses data collected throughout the Oceania region over the past years and shows that the observed annual cycle can be described by a sinusoidal model. In addition, by comparison with available artificial radionuclide data for the Southern Hemisphere, it identifies stratosphere-to-troposphere exchange and vertical transport of air within the troposphere to be active atmospheric processes controlling the observed 7Be annual cycle at different measurement stations in the region.

CHAPTER TWO

LITERATURE REVIEW*

*The literature survey for this review was finalised on January 31, 2007.

2.1 Introduction

Beryllium-7 (7Be) is a cosmogenic radionuclide produced in the atmosphere as a result of neutron and proton induced reactions with nitrogen and oxygen nuclei. It was first measured by Arnold and Al-Salih (1955) in rainwater samples collected at Chicago IL and

Lafayette IN (United States) between 1953 and 1954. Cruikshank et al. (1956) soon after reported on the presence of this radionuclide attached to aerosols in surface air. Since these pioneering efforts, numerous papers have been written on the production, concentration, distribution and application of 7Be in the environment. This review summarises the main

findings of a large number of these contributions and examines the current state of

knowledge surrounding 7Be.

2.2 Nuclear properties

7Be decays to lithium-7 (7Li) via electron capture and is the lightest radionuclide to

decay by this mechanism. The presently adopted half-life for the transmutation 7Be + e- Æ

7Li + ν is 53.22 ± 0.06 days (Tilley et al., 2002). The decay of 7Be can proceed either

directly to the 7Li ground state or to the first excited state in 7Li. The branching ratio to the first excited state is 10.44 ± 0.04 % (Tilley et al., 2002). This decay then proceeds to the 7Li

ground state by prompt gamma-ray emission with energy of approximately 477.6 keV. 7Be cannot transmute to 7Li by positron (β+) decay as this transition can be shown to be

excluded energetically (Evans, 1955).

2.3 Activity measurement

7Be activity is measured through gamma spectrometry. Early investigators utilised sodium iodide (NaI) detectors for analysis of precipitation and air samples. However, the relatively low resolution of these detectors and the presence of certain artificial radionuclides in the environment due to atmospheric nuclear weapon tests (e.g. antimony-

125, ruthenium-103 and rhodium-106) meant that the characteristic 477.6 keV peak from the decay of 7Be could not be distinguished. Chemical separations were required for recovery of a purified Be sample and often involved several coprecipitations of Be with iron hydroxides and other hydroxide phases (see, e.g., Arnold and Al-Salih, 1955;

Cruikshank et al., 1956; Goel et al., 1959). In recent decades germanium detectors have been widely used for measurement of 7Be activity. These detectors have much higher

energy resolution than their NaI counterparts and in most instances environmental samples

can be analysed directly for 7Be.

2.4 Cosmogenic production

7Be is born in an endothermic reaction when cosmic-ray-produced neutrons and protons disintegrate the atomic nucleus of atmospheric nitrogen and oxygen into lighter fragments in a process known as spallation. Production of this radionuclide is continuous, global in extent, and depends upon the flux of cosmic-ray-produced neutrons and protons traversing the atmosphere, the density of target nitrogen and oxygen nuclei, and an energy dependent production cross-section (Lal et al., 1958; Lal and Peters, 1962; Lal and Peters,

1967; Nagai et al., 2000). Its main production site is in the stratosphere (approx. 70 %), though it is also produced in some abundance within the troposphere (approx. 30 %). Lal and Peters (1962, 1967) calculated 7Be production rates in the atmosphere from available

experimental data on cosmic-ray-produced neutron and proton fluxes and spallation

reactions involving nitrogen and oxygen and reported a global average value of 810 atoms

m-2 s-1. This value has been adopted by the United Nations Scientific Committee on the

Effects of Atomic Radiation (UNSCEAR) for the purpose of reporting 7Be production rates in the atmosphere (UNSCEAR, 2000). Average production rates of 7Be in the atmosphere

have also been determined by several other investigators through a variety of methods and

values ranging from around 100 to 1100 atoms m-2 s-1 are reported in the literature (Arnold and Al-Salih, 1955; Benioff, 1956; Cruikshank et al., 1956; Lal et al., 1958; Lal et al.,

1960a; Masarik and Reedy, 1995; Masarik and Beer, 1999; Nagai et al., 2000; O’Brien,

1979; Yoshimori, 2005a; Yoshimori et al., 2003).

The work of Lal and Peters (1962, 1967) shows that 7Be production rates in the

atmosphere vary with latitude and altitude. For all levels of the atmosphere production rates

increase with increasing latitude. For all latitudes the rate of production decreases by

around two orders of magnitude between the lower stratosphere and the surface air. These

authors also report 7Be production rates to vary with the 11 year sunspot cycle. Lower rates of production are expected at solar maximum due to a decrease in the galactic cosmic-ray flux to Earth and vice-versa. Masarik and Beer (1999) used a computer model to investigate the dependency of cosmogenic radionuclide production rates on solar activity and geomagnetic field intensity. They found that for radionuclides formed through spallation reactions the production rate at solar minimum was around 1.15 times higher compared to the average over a complete solar cycle, and the rates at solar maximum were about 0.85 times the average rates. There is also evidence to suggest that solar energetic particle events may temporarily perturb 7Be production rates and concentrations in the atmosphere

(Masarik and Reedy, 1995; Phillips et al., 2001; Yoshimori et al., 2003).

2.5 Production through nuclear detonations

A number of investigators working during the weapons test era reported the

concentration of certain cosmogenic radionuclides present in the atmosphere (e.g. sodium-

22, phosphorous-32 and sulphur-35) to be at enhanced levels following the detonation of

above ground nuclear devices (Drevinsky et al., 1964; Goel et al., 1959; Lal et al., 1960b,

Lal et al., 1979). Production of 7Be through nuclear detonations is possible via several reactions, though the 6Li(d, n)7Be reaction is considered most probable (Drevinsky et al.,

1964). The constituents for this reaction are present only in the fusion charge of a

thermonuclear device. 7Be in precipitation and surface air was found by several

investigators not to be at elevated levels following periods of atmospheric nuclear testing

(Arnold and Al-Salih, 1955; Cruikshank et al., 1956; Goel et al., 1959; Lal et al., 1960b;

Rama Thor and Zutshi, 1958). However, measurements made on stratospheric air suggested

a possible artificial source. Bleichrodt (1962) reported a fivefold increase in 7Be concentrations in the stratosphere over the Netherlands following the start of the 1961

Soviet test series. This increase was seen to be in proportion to the amount of artificial strontium-89 also present in the stratosphere. A similar observation was again made after the 1962 Soviet test series (Bleichrodt and Van Abkoude, 1963a). Drevinsky et al. (1964) attempted to identify bomb-produced 7Be above the natural level in stratospheric air over the central United States by comparing measured activity concentrations with values obtained from theoretical calculations of its cosmogenic production. They noted a small increase above the theoretical range only during the period of the United States nuclear test series in the Pacific in 1962.

2.6 Activity size distribution

7Be quickly becomes an aerosol associated species. It is therefore susceptible to

the same transport and deposition processes governing the aerosol, making it a useful

atmospheric tracer (Brost et al., 1991; Koch et al., 1996; Koch and Rind, 1998; Rehfeld and

Heimann, 1995). The 7Be-bearing aerosol is thought to be generated through the process of attachment when 7Be in the form BeO or BeOH adsorbs electrostatically to atmospheric

dust particles (Arnold and Al-Salih, 1955; Lal and Peters, 1967). Transformation of this

aerosol as it traverses the atmosphere can occur through a number of physical, chemical and

meteorological processes (e.g. coagulation of ultra-fine particles, fog and cloud droplet

formation, evaporation and condensation, washout, rainout and sedimentation, and

contributions of dust storms and combustion products to the tropospheric aerosol mixture)

which determine the overall activity size distribution of 7Be on the surface air aerosol population (Winkler et al., 1998). Experimental measurements with cascade impactors (low flow-rate multistage size fractionating aerosol collection devices) indicate that 7Be-bearing

aerosols in surface air follow a unimodal lognormal size distribution and have an activity

median aerodynamic diameter between about 0.4 and 1.2 µm (Bondietti et al., 1988; El-

Hussein and Ahmed, 1994; Lange, 1994; Papastefanou and Ioannidou, 1995; Papastefanou

and Ioannidou, 1996; Winkler et al., 1998; Yu and Lee, 2002).

2.7 Atmospheric residence time and concentration

Mean tropospheric residence times of 7Be-bearing aerosols have been calculated

by Koch et al. (1996) from a general circulation model. They used the ratio between the

average 7Be content of the troposphere and 7Be depositional fluxes provided by the model for different latitudinal zones and reported a global average value of 21 days.

Comparatively shorter residence times (approx. 10–15 days) were determined for the tropics where higher rates of convective circulation and precipitation occur. In the dry subtropics and polar regions they determined mean tropospheric residence times to be relatively long (approx. 25–40 days). Mean residence times of 7Be-bearing aerosols in the

troposphere have also been calculated by several other investigators through a variety of

methods and values in the range from around 3 days to greater than 60 days are reported in

the literature (Bleichrodt, 1978; Ďurana et al., 1996; Lal et al., 1960b; Papastefanou, 2006;

Papastefanou and Ionnidou, 1995; Rangarajan and Eapen, 1990; Shapiro and Forbes-Resha,

1976; Rama Thor and Zutshi, 1958; Winkler et al., 1998; Yu and Lee, 2002).

Aerosol residence times in the stratosphere are much longer than for the

troposphere and a residence half-time of the order of 1 year has been reported (Staley,

1982; Thomas et al., 1970). Because this residence time is much longer than the 7Be half-

life, equilibrium between the rate of cosmogenic production and radioactive decay of this

radionuclide is assumed to exist within the stratosphere. Aircraft-based sampling of

stratospheric air indicates an average 7Be concentration in the lower stratosphere in the

range from approximately 150 to 200 mBq m-3 (Dutkiewicz and Husain, 1985; Jordan et al.,

2003; Kownacka, 2002). Owing to differences in residence time, as well as rate of production and loss between the stratosphere and troposphere, there is a fairly high 7Be concentration gradient between the lower stratosphere and the surface air. Jordan et al.

(2003) reported a mean 7Be concentration in upper tropospheric air of 83 mBq m-3, and

surface air concentrations averaging a few milli-becquerels per cubic metre are widely

reported in the literature (see, e.g., Brost et al., 1991).

2.8 Surface air concentrations

Surface air 7Be concentrations have been measured by a number of investigators at

different locations worldwide and over different time periods. Table 2.1 summarises a

number of recent and historical studies. Low average concentrations are reported at high

latitude sites, as well as at sites in the Pacific under the intertropical convergence zone.

Higher average concentrations generally occur at middle latitudes of the Northern

Hemisphere. Measurements of surface air 7Be concentration have also been made at a large number of sites in the Americas, as well as at selected sites in the Pacific, Africa and

Antarctica by the Environmental Measurements Laboratory (EML) through its Surface Air

Sampling Program. These data are presently available and show a range in average surface air 7Be concentrations from around 1 to 7 mBq m-3 (EML, 1999).

Concentrations of 7Be in surface air show characteristic time variations with

frequencies below the synoptic variability (Gerasopoulos et al., 2003; Koch and Mann,

1996; Talpos et al., 2005). Seasonal variations in the surface air concentration of this

radionuclide are widely reported in the literature (see, e.g., Al-Azmi et al., 2001; Azahra et

al., 2003; Cruikshank et al., 1956; Feely et al., 1989; Ioannidou et al., 2005; Parker, 1962;

Table 2.1. Average surface air concentrations of 7Be at different locations. Data is sorted in order of descending latitude.

Location Latitude 7Be Period Reference

(mBq m-3)

Alert, Canada 82°N 1.9 1990–1992 Dibb et al. (1994)

Sodankylä, Finland 67°N 2.5 1995–1997 Paatero and Hatakka (2000)

Grindsjön, Sweden 59°N 2.3 1972–2000 Kulan (2006)

Edinburgh, United Kingdom 56°N 2.5 2002–2003 Likuku (2006)

Roskilde, Denmark 55°N 1.0 1990–1993 Fogh et al. (1999) 10 Vilnius, Lithuania 54°N 3.1 1965–1969 Luyanas et al. (1970)

Chilton, United Kingdom 51°N 2.0 1999–2002 Daish et al. (2005)

Quillayute WA, United States 48°N 4.2 1976–1977 Crecelius (1981)

Bratislava, Slovakia 48°N 3.1 1978–1994 Ďurana et al (1996)

Munich, Germany 48°N 3.6 1983–1985 Hötzl and Winkler (1987)

Zugspitze, Germany 47°N 4.6 1996–1998 Gerasopoulos et al. (2001)

Sonnblick, Austria 47°N 5.3 1996–1998 Gerasopoulos et al. (2001)

Table 2.1 (continued).

Location Latitude 7Be Period Reference

(mBq m-3)

Jungfraujoch, Switzerland 46°N 7.0 1996–1999 Gerasopoulos et al. (2001)

Belgrade, Serbia-Montenegro 44°N 4.0 1991–1996 Todorovic et al. (1999)

Mt. Cimone, Italy 44°N 5.7 1996–1999 Gerasopoulos et al. (2001)

Thessaloniki, Greece 40°N 5.0 1987–2001 Ioannidou et al. (2005)

Palermo, Italy 38°N 5.1 1982–2002 Cannizzaro et al. (2004) 11 Granada, Spain 37°N 4.5 1993–2001 Azahra et al. (2003)

Malaga, Spain 36°N 4.6 1996–2001 Dueñas et al. (2004)

Osaka, Japan 34°N 6.7 1983–1997 Megumi et al. (2000)

Kuwait City, Kuwait 29°N 5.2 1994–1998 Al-Azmi et al. (2001)

El-Minia, Egypt 28°N 2.0 1998–1999 El-Hussein et al. (2001)

Midway, North Pacific 28°N 2.9 1985 Uematsu et al. (1994)

Cienfuegos, Cuba 22°N 4.1 1994–1998 Alonso Hernandez et al. (2004)

Table 2.1 (continued).

Location Latitude 7Be Period Reference

(mBq m-3)

Oahu HI, United States 21°N 3.1 1985 Uematsu et al. (1994)

Bombay, India 19°N 3.5 1965–1966 Rangarajan and Gopalakrishnan (1970)

Enewetak, North Pacific 11°N 1.7 1985 Uematsu et al. (1994)

Nauru, South Pacific 1°S 1.4 1985 Uematsu et al. (1994)

Funafuti, South Pacific 9°S 1.6 1985 Uematsu et al. (1994) 12 Samoa, South Pacific 14°S 2.3 1985 Uematsu et al. (1994)

Fiji, South Pacific 18°S 1.6 1999–2000 Garimella et al. (2003)

Rarotonga, South Pacific 21°S 3.0 1985 Uematsu et al. (1994)

New Caledonia, South Pacific 22°S 3.1 1985 Uematsu et al. (1994)

Norfolk Island, South Pacific 29°S 2.7 1985 Uematsu et al. (1994)

Aspendale VIC, Australia 38°S 2.7 1967–1968 Hicks (1969)

Hokitika, New Zealand 42°S 3.1 1985–1986 Harvey and Matthews (1989)

Thomas et al., 1970). Feely et al. (1989) found that seasonal variations showed the effects

of at least four factors: (1) stratosphere-to-troposphere exchange (STE); (2) vertical mixing

within the troposphere; (3) air mass transport from middle to high latitudes; and (4) rainfall.

STE is a means for highly concentrated 7Be air from the stratosphere to enter the troposphere and is generally most intense at middle latitudes during the spring season.

Mechanisms of STE, including tropopause folding, have been studied by several investigators (see, e.g., Appenzeller et al., 1996; Baray et al., 2000; Borman, 1980;

Danielsen, 1968; Danielsen and Mohnen, 1977; Jordan et al., 2003). Vertical mixing of air within the troposphere can occur through convective circulation. Solar heating of earth’s surface in the warmer months leads to an increase in temperature of the surface air. Cooler air sinks and displaces the warm surface air, bringing with it higher concentrations of 7Be from the upper troposphere. Feely et al. (1989) found the effects of STE and vertical mixing within the troposphere on seasonal variations of 7Be in surface air to be evident at middle latitudes, where surface air concentrations of this radionuclide typically show a spring- summer peak and an autumn-winter trough. Other investigators have also attributed the observed seasonality at middle latitude sites to a combination of these two factors (Azahra et al., 2003; Ďurana et al., 1996; Ioannidou et al., 2005). A distinct peak in surface air 7Be concentrations at Antarctica is reported to occur in the austral summer (Feely et al., 1989;

Lambert et al., 1990). Feely et al. (1989) found that variations in 7Be concentration at the

South Pole paralleled the variations in aerosol concentration with a presumed middle latitude source. Such air mass transport within the troposphere from middle to high latitudes is possible through the polar cell of global circulation. Washout by rainfall strips the troposphere of 7Be-bearing aerosols, decreasing the concentration of this radionuclide in the surface air. Negative correlations between surface air 7Be concentrations and rainfall were reported by Feely et al. (1989) for sites where a strong seasonal variation in the rainfall was observed. Overall, the timing and magnitude of seasonal variations in surface

air 7Be concentrations can change with geographical location depending on the contribution from the individual factors that cause these variations.

The solar modulation of cosmic-ray fluxes also causes low frequency variations in the concentration of 7Be in surface air. These variations are noticeable in measurements made over extended periods and show an anti-correlation with sunspot number (Aldahan et al., 2001; Cannizzaro et al., 2004; Gerasopoulos et al., 2003; Megumi et al., 2000;

Papastefanou and Ioannidou, 2004). Hötzl et al. (1991) and Megumi et al. (2000) reported maximum average concentrations of 7Be in surface air to be around 1.6 times higher than minimum average concentrations over a complete sunspot cycle at Munich-Neuherberg

(Germany) and Osaka (Japan), respectively. Koch and Mann (1996) used a Fourier transform analysis to identify patterns of significant temporal variability in surface air 7Be concentration measurements of the EML. They found that the amplitude of solar variations differed between different locations, with no apparent latitude or altitude trends. Also, highest concentrations of 7Be in surface air at the South Pole occurred along the descending

branch of the sunspot maximum, which they suggested may be the result of low-energy

solar particles being able to penetrate the weaker strength magnetic field above the Poles.

Synoptic-scale variations in surface air 7Be concentrations have been investigated by Zanis et al. (1999) and Gerasopoulos et al. (2001) at a number of alpine stations. They found episodes of high 7Be concentration to be associated with upper-level ridges and

episodes of low 7Be concentration to be associated with upper-level troughs. Downward air mass transport from the upper troposphere under anti-cyclonic conditions (upper-level ridge) and wet scavenging of 7Be-bearing aerosols under the influence of an upper-level

trough were reported by these authors to be the main processes controlling short-term (day-

to-day) variations in surface air 7Be concentrations. Yoshimoro (2005b) suggested that high weekly 7Be concentrations in the surface air at Tokyo (Japan) may be the result of

downward air mass transport in the troposphere caused by travelling anticyclones formed

between the ridge and trough of the Rossby waves (large-scale meanders of the jet stream).

2.9 Deposition

7Be-bearing aerosols can reach the ground under clear sky conditions through particle sedimentation (dry deposition), but are removed more efficiently from the troposphere by precipitation scavenging (wet deposition). Experimental measurements show that around 90 % or more of total 7Be deposition in temperate zones generally takes place through the wet deposition process (Benitez-Nelson and Buesseler, 1999; Brown et al., 1989; Fogh et al., 1999; Harvey and Matthews, 1989; Ioannidou and Papastefanou,

2006; McNeary and Baskaran, 2003; Todd et al., 1989; Wallbrink and Murray, 1994). Wet deposition of 7Be occurs through both below-cloud scavenging (washout) and in-cloud

scavenging (rainout) of its carrier aerosol during a precipitation event. Washout occurs

during the early stages and quickly cleanses the lower troposphere of 7Be-bearing aerosols.

Rainout delivers 7Be from within the cloud layer to Earth’s surface and is active throughout

the duration of a precipitation event. Serial sampling within individual storms shows the

specific activity of 7Be in precipitation to decrease sharply during the initial stages and then

remains fairly constant throughout the mid-to-latter stages, reflecting a change from

washout to rainout as the predominant wet deposition process (Ioannidou and Papastefanou,

2006; Ishikawa et al., 1995; Wallbrink and Murray, 1994).

Annual deposition of 7Be worldwide ranges from around 400 to 6500 Bq m-2 and appears to be dependent upon rainfall (Table 2.2). Low annual depositions of this radionuclide have been reported for sites in the Middle East and Mediterranean regions, as well as at East Antarctica. The highest annual deposition of 7Be to be reported (6350 Bq m-2) is for a high rainfall area on the South Island of New Zealand. Young and Silker

(1980) reported an average 7Be flux across the surface of the North Pacific and North

Atlantic oceans of 270 atoms m-2 s-1, which corresponds to an annual deposition of approximately 1290 Bq m-2. Wallbrink and Murray (1994) are the only investigators to so

far report on 7Be deposition in Australia. They measured an annual deposition of 1030 Bq

Table 2.2. Annual 7Be deposition and rainfall at different locations. Data is sorted in order of ascending annual rainfall.

Location Latitude 7Be deposition Rainfall Period Reference

(Bq m-2) (mm)

Damascus, Syria 33°N 528 153 1995–1996 Othman et al. (1998)

Malaga, Spain 36°N 412 308 1992–1999 Dueñas et al. (2002)

Thessaloniki, Greece 40°N 736 424 1987–1992 Ioannidou and Papastefanou (2006)

Roskilde, Denmark 55°N 738 564 1990–1993 Fogh et al. (1999)

Bavaria, Germany 49°N 990 616 1989 Bachhuber and Bunzl (1992) 16 Canberra, Australia 35°S 1030 660 1988–1989 Wallbrink and Murray (1994)

Bologna, Italy 44°N 1243 707 1990–1992 Bettoli et al. (1995)

Heidelberg, Germany 49°N 1250 810 1960–1961 Schumann and Stoeppler (1963)

Chilton, United Kingdom 51°N 898 822 1959–1961 Peirson (1963)

Rijswijk, Netherlands 52°N 1574 881 1961–1961 Bleichrodt and Van Abkoude (1963b)

Osaka, Japan 34°N 1409 965 1983–1997 Megumi et al. (2000)

Geneva, Switzerland 46°N 2095 966 1997–1998 Caillet et al. (2001)

Table 2.2 (continued).

Location Latitude 7Be deposition Rainfall Period Reference

(Bq m-2) (mm)

College Station TX, United States 30°N 2117 980 1990–1990 Baskaran et al. (1993)

Solomons MD, United States 38°N 2080 1112 1986–1987 Dibb (1989)

Fayetteville AR, United States 36°N 867 1156 1980–1984 Lee et al. (1985)

Galveston TX, United States 29°N 2451 1167 1989–1991 Baskaran et al. (1993)

Oak Ridge TN, United States 36°N 2017 1251 1982–1984 Olsen et al. (1985) 17 Stillpond MD, United States 39°N 2167 1260 1995–1996 Kim et al. (2000)

Norfolk VA, United States 36°N 2075 1313 1983–1984 Todd et al. (1989)

Milford Haven, United Kingdom 51°N 1618 1328 1959–1961 Peirson (1963)

New Haven CT, United States 41°N 3780 1390 1977–1978 Turekian et al. (1983)

Bermuda 32°N 2850 1700 1977–1978 Turekian et al. (1983)

Bombay, India 19°N 1262 2277 1955–1970 Lal et al. (1979)

Hokitika, New Zealand 42°S 6350 2634 1985–1986 Harvey and Matthews (1989)

Table 2.2 (continued).

Location Latitude 7Be deposition Rainfall Period Reference

(Bq m-2) (mm)

East Antarctica 70°S 700 n.a. n.a. Nijampurkar and Rao (1993)

North Pacific and Atlantic Oceans 0–60°N 1290 n.a. n.a. Young and Silker (1980)

m-2 at Canberra ACT for a corresponding rainfall of 660 mm. Zonal annual average 7Be deposition has been simulated in a global climate model by Brost et al. (1991). They found deposition minima to occur in relatively dry regions such as at high latitudes, across the deserts of northern Africa and the Middle East, and under the subtropical high pressure systems off the west coasts of the Americas, Africa and Australia. Deposition maxima were found to occur in regions of relatively high precipitation such as at middle latitudes and under the intertropical convergence zone, as well as over the oceans.

7Be depositional fluxes (i.e. activity deposited per unit area per unit time) at different locations show a high positive correlation with rainfall, and temporal variations in the depositional flux of this radionuclide generally follow the local rainfall pattern

(Bachhuber and Bunzl, 1992; Baskaran et al., 1993; Benitez-Nelson and Buesseler, 1999;

Caillet et al. 2001; Dueñas et al., 2002; Ioannidou and Papastefanou, 2006; Kim et al.,

2000; Knies et al., 1994; McNeary and Baskaran, 2003; Olsen et al., 1985; Othman et al.,

1998; Turekian et al., 1983). Seasonal changes in the 7Be budget of the troposphere may

also influence the depositional flux of this radionuclide. Dibb (1989) reported the bulk of

7Be deposition during 1986 and 1987 at Solomons MD (Unites States) to occur during spring and early summer. This was in spite of near-drought conditions in the spring of

1986. He found the timing of the observed peak in 7Be deposition to be in close agreement

with estimates of the seasonal injection of 7Be rich stratospheric air into the troposphere due to STE and concluded that the 7Be depositional flux closely reflected the tropospheric inventory of this radionuclide. Investigators working at other sites across the eastern United

States have also reported springtime maxima in 7Be depositional fluxes (Olsen et al., 1985;

Todd et al., 1989). A pronounced winter peak in the depositional flux of 7Be has been observed at a number of sites in Japan located along the coast of the Japan Sea (Narazaki et al., 2003; Yamamoto et al., 2006). Yamamoto et al. (2006) suggested that the flow of cold air masses containing high 7Be concentration from high latitudes, mixing and generation of

convection clouds over the Sea of Japan, and accompanying heavy winter snowfalls were

responsible for this observation.

The deposition velocity of 7Be-bearing aerosols can be calculated from the ratio of

the 7Be depositional flux to its concentration in surface air. Average deposition velocities of this radionuclide at different locations, including over the Pacific and Atlantic oceans, are reported to be in the range from 0.004 to 0.074 m s-1 (Crecelius, 1981; Fogh et al., 1999;

Harvey and Matthews, 1989; Papastefanou et al., 1995; Todd et al., 1989; Turekian et al.,

1983; Young and Silker, 1980). Since certain artificial radionuclides (e.g. strontium-90 and caesium-137) are also found attached predominantly to submicron-sized aerosols and are present mainly in the stratosphere, the deposition velocity of 7Be may be a useful parameter

in estimating the long-term deposition of radioactive pollutants from remote sources

(Harvey and Matthews, 1989). This may be especially applicable to Australia and the South

Pacific region, where the distance from major nuclear installations means that any

radioactive pollution is likely to be mixed throughout the troposphere after it descends from

the stratosphere.

2.10 Distribution in ocean waters

7Be in surface ocean waters has been studied in detail by Young and Silker (1980).

They mapped 7Be surface water concentrations and areal activity densities for large areas of

the Pacific and Atlantic oceans and report concentrations in the range from 1 to 12 Bq m-3

and total areal activity densities in the range from 80 to 600 Bq m-2. They found that concentrations and areal activity densities were higher in areas associated with high rainfall, and generally increased with latitude. Depth profiles of 7Be were also reported by these authors and showed that the vertical distribution of this radionuclide in surface ocean waters generally reflected the temperature profile of the water column. In areas where a strong thermocline was present within the surface 100 m, they found 7Be concentrations in

the mixed layer to be generally constant and then decrease rapidly below the thermocline.

In areas where the thermocline was at a depth of around 100 m or greater, they found 7Be concentrations to drop to very low values within the mixed layer itself. Concentrations and depth profiles of 7Be in surface ocean waters were also reported by Silker in a number of earlier studies (Silker et al., 1968; Silker, 1972a; Silker, 1972b).

2.11 Distribution in soils and grasses

The areal activity density of 7Be in surface soils and grasses is the net result of

deposition, mobilisation, and radioactive decay. Some areal activity densities of this

radionuclide in soils and grasses which have been reported in the literature are summarised

in Table 2.3. Soils with an overlying grass cover seem to maintain a higher 7Be areal

activity density than bare soils, indicating that surface vegetation is an effective trap for

7Be-bearing aerosols. Bettoli et al. (1995) found more than 50 % of the total 7Be areal

activity density to be present within the grass layer for a number of samples collected at

Bologna (Italy). Wallbrink and Murray (1996) reported local spatial variations in the areal

activity density of 7Be of around 20 % at an undisturbed site at Canberra ACT (Australia).

They attributed this finding to differences in effective fallout and mobilisation of 7Be over

the measurement site prior to its sorption to the soil grain or grass. Local spatial variability

in the areal activity density of other fallout radionuclides (e.g. caesium-137 and

unsupported lead-210) has also been reported for relatively stable sites (Huh and Su, 2004;

Wallbrink et al., 1994; Walling et al., 2003). Temporal variations in 7Be areal activity

densities at undisturbed sites are likely to occur where a seasonal rainfall pattern exists

(Wallbrink and Murray, 1996). The rate of accumulation of this radionuclide is likely to

exceed the rate of loss by decay during months of higher rainfall, while the 7Be half-life

(radioactive decay) will control the areal activity density during months of lower rainfall.

These spatial and temporal variations in areal activity density may affect the use of 7Be as a

tracer in soil movement studies, in particular the selection and sampling of a suitable

reference site.

Table 2.3. Total 7Be areal activity densities at undisturbed sites. Data is sorted in order of ascending sampling date.

Sampling location Latitude Sample analysed 7Be areal activity Reference

date density (Bq m-2)

07/1984 Oak Ridge TN, United States 36°N Grass + soil 673±22 Olsen et al. (1985)

01/1985 Wallops Is. VA, United States 38°N Vegetated marsh soil 673±48 Olsen et al. (1985)

01/1985 Wallops Is. VA, United States 38°N Unvegetated marsh soil 107±19 Olsen et al. (1985)

09/1988 Canberra ACT, Australia 35°S Grass + soil 202±57 Wallbrink and Murray (1996)

09/1988 Canberra ACT, Australia 35°S Bare soil 135±9 Wallbrink and Murray (1996) 22 05/1989 Canberra ACT, Australia 35°S Grass + soil 400±144 Wallbrink and Murray (1996)

05/1989 Canberra ACT, Australia 35°S Bare soil 156±42 Wallbrink and Murray (1996)

08/1989 Canberra ACT, Australia 35°S Grass + soil 205±105 Wallbrink and Murray (1996)

08/1989 Canberra ACT, Australia 35°S Bare soil 95±9 Wallbrink and Murray (1996)

01/1991 Bologna, Italy 44°N Grass + soil 198±9 Bettoli et al. (1995)

01/1991 Bologna, Italy 44°N Bare soil 125±8 Bettoli et al. (1995)

03/1991 Bologna, Italy 44°N Grass + soil 157±8 Bettoli et al. (1995)

Table 2.3 (continued).

Sampling location Latitude Sample analysed 7Be areal activity Reference

date density (Bq m-2)

03/1991 Bologna, Italy 44°N Bare soil 153±13 Bettoli et al. (1995)

05/1996 West–central Idaho, United States 44°N Grass + soil 139±22 Bonniwell et al. (1999)

01/1998 Crediton, United Kingdom 50°N Bare soil 512±10 Blake et al. (2002)

10/1998 Silverton Mill, United Kingdom 50°N Bare soil 283±26 Blake et al. (2002)

05/1999 Treynor IA, United States 41°N Bare soil 121±21 Wilson et al. (2003) 23 1997–2000 Chintiangang, Taiwan 25°N Grass + soil 3280±1738 Huh and Su (2004)

Because of its short half-life 7Be does not usually penetrate deep into the soil profile. The activity concentration of this radionuclide in the soil at undisturbed sites generally shows an exponential decrease with depth (Huh and Su, 2004; Wallbrink and

Murray, 1996, Wilson et al., 2003). Wallbrink and Murray (1996) found 7Be to penetrate no deeper than 20 mm in soils under various surface cover conditions and reported penetration half-depths (i.e. the depth at which the activity concentration decreases to half some initial value) for this radionuclide to be in the range from 0.4 to 3.7 mm. They also found 7Be concentration to be higher at greater depths in grass covered soils than in bare soils. Olsen et al. (1985) reported 7Be to a depth of 100 mm in unsaturated marsh soils at Wallops

Island VA (United States). They suggested that particles mobilised by infiltrating rainwater and transported through small cracks in the soil surface formed during relatively dry periods might account for this finding. Wallbrink and Murray (1996) concluded that 7Be penetration in soils was primarily controlled by physical properties such as vegetation cover, soil density, and structure. They also suggested that the incorporation of 7Be into

plant tissue, followed by redistribution along nutrient pathways within the root matrix or

bioturbation by soil fauna could increase the penetration depth of 7Be within the soil

profile.

2.12 Environmental applications

7Be is a useful radionuclide for studying atmospheric transport processes for several reasons: (1) its source is known and global in extent; (2) it has a relatively short half-life; (3) it attaches rapidly and indiscriminately to available aerosols; (4) it has a steep concentration gradient between the lower stratosphere and the surface air; (5) it is relatively simple and inexpensive to measure; and (6) there is a large amount of reliable data available for hypothesis testing. 7Be measurements have been used by a number of investigators to test and validate the results of general circulation models of the atmosphere (Brost et al.,

1991; Koch et al., 1996; Rehfeld and Heimann, 1995). The potential for using 7Be to study

atmospheric transport processes is particularly great when it is coupled with a nuclide with a contrasting source function and/or a different half-life. Raisbeck et al. (1981) and Jordan et al. (2003) demonstrated that the ratio of beryllium-10 (10Be) to 7Be can be used as a

probe of atmospheric transport processes, in particular STE. The source function of 10Be in the atmosphere is identical to that of 7Be, but its half-life is very much longer (approx.

1.5×106 years). In addition, both radionuclides attach to aerosols and are transported and

deposited in the same way. The production ratio of 10Be/7Be in the atmosphere is reported by Lal and Peters (1967) to be around 0.5. Levels of 7Be within an irradiated air mass will reach a relatively steady state within a period of approximately 1 year. However, the

10Be/7Be ratio will continue to increase over time due to production of 10Be. Aerosol

residence times in the stratosphere are sufficiently long for this to occur and consequently

the ratio of 10Be/7Be in stratospheric air is higher than in tropospheric air, making the

10Be/7Be ratio a sensitive indicator of air mass origin and age. The ratio of 10Be/7Be in the

troposphere is reset to its initial production value (i.e. approx. 0.5) following a precipitation

event. Rehfeld and Heimann (1995) and Koch and Rind (1998) simulated 10Be/7Be ratios in

general circulation models to investigate the seasonal and latitudinal changes in STE and air

mass transport within the atmosphere. Dibb et al. (1994) measured the 10Be/7Be ratio in surface air at Alert NT (Canada) to be nearly constant at 2.2 throughout the year and on the basis of this finding concluded that the stratosphere was an important source of Be isotopes in the Arctic troposphere year round.

7Be may be used in conjunction with lead-210 (210Pb) to determine whether the source of a scavenged air mass is oceanic versus continental and/or upper versus lower tropospheric (Baskaran et al., 1993; Benitez-Nelson and Buesseler, 1999; Koch et al.,

1996). 210Pb is a decay product of gaseous radon-222, which emanates primarily from the surface of continental land masses. Production rates of 7Be in the atmosphere increase with altitude and are independent of geography. Both radionuclides attach primarily to submicron-sized aerosols and are removed from the atmosphere through similar processes.

A relative increase in the 7Be/210Pb ratio can indicate either an upper tropospheric source or changes in the extent of continental versus oceanic air. At locations where a high correlation between 7Be and 210Pb depositional fluxes exists, these radionuclides cannot be used as two independent atmospheric tracers. Such a correlation has been observed mainly at continental sites and those close to the continental margin (Baskaran et al., 1993; Hirose et al., 2004; McNeary and Baskaran, 2003; Todd et al., 1989). Baskaran et al. (1993) found that 7Be and 210Pb could be used to independently trace air mass origin only at oceanic and a few coastal sites. Baskaran (1995) reported a seasonal variation in the 7Be/210Pb ratio at

two sites in Texas (United States) during 1990. He concluded that higher values in the

warm summer season were the result of vertical mixing (convective circulation) within the

troposphere, which forces 210Pb enriched (7Be depleted) surface air upward and carries 7Be enriched (210Pb depleted) air from the upper troposphere downward.

Past measurements of weapons produced (artificial) radionuclide concentrations in

surface air show similar seasonal variations as 7Be (Gustafson et al., 1961; Peirson, 1963;

Rangarajan and Gopalakrishnan, 1970; Schumann and Stoeppler, 1963; Thomas et al.,

1970). Artificial radionuclides which are retained in the troposphere following an above

ground nuclear detonation are deposited within a few weeks of their release due to the short

residence time of tropospheric aerosols. Their presence in the troposphere after this time is

due to STE. Alonso Hernandez et al. (2004) used the ratio of 7Be in surface air at

Cienfuegos (Cuba) to 7Be in surface air at Miami FL (United States), as well as caesium-

137 (137Cs) concentrations measured in surface air at Miami, to reconstruct the monthly

137Cs signal in surface air at Cienfuegos due to global fallout between 1957 and 1994. They

found the calculated 137Cs values for Cienfuegos to be in agreement with measured

concentrations reported for other sites in the Antilles and Central America during the same

epoch. Lee et al. (1985) used the ratio of strontium-90 (90Sr) to 7Be measured in precipitation samples to estimate the amount of excess 90Sr deposited at Fayetteville AR

(United States) from the 25th Chinese nuclear test explosion. They found that an areal

activity density of 90Sr of approximately 13 Bq m-2 was attributable to his event. These

investigators also used polonium-210 (210Po) to 7Be ratios to estimate the amount of excess

210Po deposited as a result of the May 1980 eruption of Mount St. Helens.

Measurement of 7Be areal activity density and its depth distribution in the soil has the potential to tell the degree to which the soil surface has been disturbed. Several investigators have recently used 7Be to document rates of short-term and event-based soil redistribution (Blake et al., 1999; Blake et al., 2002; Walling et al., 1999; Wilson et al.,

2003). A quantitative estimate of soil erosion is usually made by comparing the areal

activity density within the study area to that measured at a nearby undisturbed and stable

reference point. Wilson et al. (2003) used a 7Be inventory balance to estimate soil erosion due to runoff-producing thunderstorms occurring over three consecutive days at Treynor IA

(United States). The inventory balance method accounted for 7Be deposition, losses to erosion, and changes in areal activity density within the soil profile over the sampling period. They found that the erosion rate calculated from the 7Be inventory balance

(0.58±0.33 kg m-2) was in agreement with that determined from the sediment flux at the

outlet of the study area (0.59 kg m-2).

The different depth distributions of 7Be, 137Cs and unsupported 210Pb can be used

to relate suspended sediment to its original soil depth (Matisoff et al., 2002; Wallbrink et

al., 1999). 7Be is typically concentrated in the very upper layers, while 137Cs and unsupported 210Pb penetrate to deeper layers of the soil profile. Suspended sediment with relatively high concentrations of all three radionuclides is most likely indicative of a surface source, whereas sediment that is depleted in 7Be and unsupported 210Pb but maintains a

relatively high concentration of 137Cs is most likely derived from the subsurface layer.

Wallbrink and Murray (1993) used 7Be, 137Cs and unsupported 210Pb to distinguish between

erosion processes during run-off experiments in a field study at Goulburn NSW (Australia).

They found the concentration of all three radionuclides in the suspended sediment to be

initially high, but with the passage of time they observed a decrease in the concentrations of

7Be and unsupported 210Pb, while 137Cs concentration remained constant. They interpreted this finding as indicating a change from sheet erosion to minor rilling as the dominant erosion process.

2.13 Annual effective dose

The UNSCEAR reports the annual effective dose due to cosmogenic 7Be to be

0.03 micro-Sieverts (UNSCEAR, 2000). This represents around 0.001 % of the total annual

effective dose received due to all natural radiation sources. Therefore, cosmogenic 7Be does

not pose a radiological health risk to the population.

2.14 Summary and conclusions

7Be is a cosmogenic radionuclide produced in the stratosphere and troposphere when cosmic-ray-produced neutrons and protons disintegrate the atomic nucleus of atmospheric nitrogen and oxygen into lighter fragments. It decays by electron capture to 7Li

and has a half-life of approximately 53 days. A gamma-ray is emitted in around 10 % of all

7Be disintegrations and the activity of this radionuclide is measured through gamma spectrometry. 7Be quickly becomes an aerosol associated species and attaches mainly to

particles in the submicron size range. These aerosols have an average residence time in the

stratosphere and troposphere of approximately 1 year and 21 days, respectively. Because of

this difference in residence time, as well as differences in the rate of production and rate of

loss between the stratosphere and troposphere, there is a fairly high concentration gradient

of 7Be in the atmosphere. Concentrations have been measured at different levels of the

atmosphere and show a decrease of around two orders of magnitude between the lower

stratosphere and the surface air. Surface air concentrations of 7Be are around a few milli- becquerels per cubic metre on average and exhibit characteristic time variations with frequencies below the synoptic variability. Seasonal variations in surface air concentrations

of this radionuclide at different geographical locations are thought to be the manifestation

of a number of atmospheric processes, including stratosphere-to-troposphere exchange,

vertical mixing within the troposphere, air mass transport from middle to high latitudes, and

rainfall. Even lower frequency variations result from the solar modulation of the galactic

cosmic-ray flux to Earth over the 11 year period of the sunspot cycle. Depositional fluxes

of 7Be show a high positive correlation with rainfall, but are also dependent upon the

tropospheric inventory of this radionuclide. Deposition of 7Be occurs mainly through wet

scavenging of its carrier aerosol, which delivers 7Be to ocean and land surfaces. 7Be is

generally present in surface ocean waters, as well as in the upper layers of the soil and

overlying vegetation.

7Be has its applications in the atmospheric and earth sciences. It is a useful radionuclide for testing and validating general circulation models of the atmosphere, and when coupled with a radionuclide of different source function and/or half-life has the potential to provide valuable information on air mass origin and age. 7Be may also serve as

a useful tool to predict fallout of artificial radionuclides from remote sources due to the

similar source distributions of these radionuclides in the atmosphere. Estimates of short-

term and event-based soil erosion rates, as well as information on erosion processes, can

also be ascertained from 7Be measurements. Most applications of 7Be require a detailed

understanding of its distribution and behaviour within different environmental systems and

at different geographical locations. Such an understanding is still somewhat incomplete.

Therefore, further work is required if the full potential of this radionuclide as a tracer of

environmental processes is to be realised.

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CHAPTER THREE

VERTICAL DISTRIBUTIONS OF 210Pb EXCESS, 7Be AND 137Cs IN SELECTED

GRASS COVERED SOILS IN SOUTHEAST QUEENSLAND, AUSTRALIA

Che Doering a, Riaz Akber a, Henk Heijnis b

a International Laboratory for Air Quality and Health, School of Physical and Chemical

Sciences, Queensland University of Technology, 2 George Street, Brisbane, Qld. 4000,

Australia

b Environmental Radiochemistry Group, Australian Nuclear Science and Technology

Organisation, Private Mail Bag 1, Menai, NSW 2234, Australia

Journal of Environmental Radioactivity 87 (2006) 135–147

STATEMENT OF JOINT AUTHORSHIP

Title

Vertical distributions of 210Pb excess, 7Be and 137Cs in selected grass covered soils in

Southeast Queensland, Australia

Authors

Che Doering, Riaz Akber, Henk Heijnis

Che Doering (candidate)

Conducted field work, prepared samples for analysis, analysed and interpreted data, and wrote manuscript.

Riaz Akber

Original idea, conducted field work, analysed and interpreted data, and offered advice and editorial comments throughout the preparation of the manuscript.

Henk Heijnis

Provided laboratory facilities for sample preparation and analysis.

Abstract

210 7 137 Net accumulated areal activity densities and profiles of Pbex, Be and Cs in

the surface 10 cm of the soil are reported for eight sites in Southeast Queensland, Australia.

210 7 -2 Areal activity densities of Pbex and Be varied from 1080 to 4100 Bq m and from 176 to

778 Bq m-2 respectively. A significant (p<0.001) portion of the variance (R2>0.99) in their vertical distributions was explained by depth in the profile using an exponential function.

210 Around 85 % of accumulated Pbex was present in the surface 10 cm of the soil.

Beryllium-7 was mainly confined to the grass and surface 2 cm of the soil. Average

210 penetration half-depths of 3.6 ± 0.2 cm and 0.3 ± 0.1 cm were determined for Pbex and

7Be, respectively. Areal activity densities of global fallout 137Cs varied from 10 to 361 Bq

m-2. Its signal was well mixed within the surface 10 cm. Comparison of the measured 137Cs values to the estimated input value for the region (~490 Bq m-2) and profiling of a 1 m deep

soil core suggests a vertical migration of 137Cs over the past decades.

210 The paleo-radon activity flux determined from the Pbex areal activity density

(5.1 ± 0.9 mBq m-2 s-1) was not statistically different to that measured using activated charcoal cups (5.5 ± 0.4 mBq m-2 s-1), tending to suggest that Southeast Queensland is neither a net source nor a net sink of 210Pb-bearing aerosols.

210 7 137 Keywords: Pbex; Be; Cs; Soil; Areal activity density; Penetration half-depth; Radon

activity flux; Australia

3.1 Introduction

Dictated in part by the geomorphological and ecological consequences of human

development over the past decades, a number of nuclear techniques have been developed

for quantitative assessment of soil erosion and sedimentation rates. Among these, 137Cs has been extensively used to estimate rates of soil erosion (Ritchie and Ritchie, 2005). This technique compares the 137Cs areal activity density at an eroded site to a nearby undisturbed site to assess the extent of soil loss. Similar methods based on the measurement of 210Pb

210 226 7 excess ( Pbex) over Ra and Be have more recently been established (Zapata, 2003).

Caesium-137, 210Pb and 7Be are all highly particle reactive (see Matisoff et al.

(2002) and references therein). Their deposition to earth labels the land surface with a

unique and identifiable radiological fingerprint. Due to differences in their half-life and

fallout history, these radionuclides are suitable for estimating erosion rates over different

timescales. Fissiogenic 137Cs (half-life, 30.2 years) can provide a retrospective estimate of

land erosion rates over the past 30 to 40 years (Zapata, 2003), i.e. since the cessation of

large-scale nuclear weapons tests in the atmosphere, lithogenic 210Pb (half-life, 22.6 years)

offers a means of estimating erosion rates dating back 100 years (Walling and He, 1999;

Walling et al., 2003), and 7Be (half-life, 53.3 days) can provide an estimate of soil loss

associated with a single erosive event (Blake et al., 1999; Wilson et al., 2003).

Southeast Queensland presently has the highest population growth in Australia

(Australian Bureau of Statistics, 2005). Coupled to this growth, urban development has

been identified as a key factor which may accelerate erosion rates in Queensland

(Environmental Protection Agency, 2004). To lessen the risk of erosive degradation in

populous urban areas, legislative land management practices have been introduced. In the

State’s southeast and along much of the coastal stretch rainwater runoff poses the greatest

threat of erosion, especially during summer when high intensity rainfall usually occurs

(Environmental Protection Agency, 2004). For the purpose of monitoring rates of land

erosion in these areas, quantitative and reproducible methods are required. To highlight the

possibility of nuclear methods for future estimates of land erosion rates in Southeast

210 7 Queensland, in particular the potential usefulness of Pbex and Be, this paper examines

210 7 137 vertical distributions of Pbex, Be and Cs in selected grass covered soils in the region.

210 222 In addition, as the main source of Pbex in surface soils is likely to stem from Rn released to the atmosphere, 222Rn exhalation rates from the ground have also been

210 measured. Comparison of these exhalation rates with the Pbex areal activity density can provide additional information about the behaviour and fate of 210Pb in the environment.

3.2 Materials and methods

3.2.1 Soil sampling sites

Soil samples were collected from eight sites along a linear route intersecting the coastline of Southeast Queensland at Southport and extending inland approximately 80 km to Boonah (Figure 3.1). Sites selected for soil sampling were flat open areas having a uniform grass cover and set in a semi-natural environment showing no apparent signs of disturbance. The slope of the terrain at each site was measured at several points. Slopes of less than 2° were generally recorded, though at Clagiraba (Site 4) the slope of the land was up to 5° at some points. All sites were outside any land areas used for cattle grazing or other agricultural activities, i.e. the land had not been tilled. Occasionally the grass at these sites was clipped using mechanical slashers, but the clippings neither collected nor removed from the site. Visual inspection of the sites suggested no bioturbation of the soil from ant activity or animal burrows.

3.2.2 Soils

Soil texture varied from loamy sand at Southport (Site 1) to silt loam at

Tabragalba (Site 5). Physicochemical properties of the soils (bulk density, moisture content, quartz and clay mineral content, organic matter content, particle size distribution, pH and cation exchange capacity) were determined and are shown in Table 3.1.

27° S Australia

Queensland

Brisbane

0 25 50 km

28° S 8 7 6 5 4 3 21 Pacific Darling Downs Ocean

New South Wales

152° E 153° E 154° E 29° S

Figure 3.1. Location of the sampling sites and map of Australia (inset).

Table 3.1. Site location, annual rainfall rate, and physical and chemical properties of the soils (moisture content, composition and particle size

distribution are all expressed as percentage weight of the sample).

Site Locality Geographical position Rainfall Bulk Moisture Composition Particle size pH CEC

no. ratea density content (%) (%) (mmol

(mm y-1) (kg m-3) (%) kg-1)

Latitude (S) Longitude (E) Quartz Clay Organic <2 2-75 >75

mineralsb matter μm μm μm

1 Southport 27° 58′ 28″ 153° 25′ 02″ 1454(123) 1490 17 67.2 29.9 2.9 2.9 24.9 72.1 5.8 20.0

2 Ashmore 27° 58′ 56″ 153° 23′ 02″ n.a. 1390 33 69.9 21.1 9.0 5.2 50.3 44.5 5.4 24.1 49

3 Nerang 28° 00′ 00″ 153° 19′ 48″ 1458(117) 1590 27 30.1 61.5 8.4 12.4 55.8 31.7 6.1 29.7

4 Clagiraba 27° 59′ 42″ 153° 14′ 12″ n.a. 1700 24 82.8 12.7 4.5 5.6 44.7 49.7 5.0 16.7

5 Tabragalba 27° 59′ 44″ 153° 02′ 36″ 952(104) 1260 19 40.7 47.9 11.4 16.3 71.1 12.6 6.5 37.9

6 Beaudesert 27° 59′ 37″ 152° 58′ 26″ 912(117) 1450 11 54.8 37.3 7.9 2.3 31.2 65.8 6.2 32.8

7 Coulson 27° 57′ 42″ 152° 44′ 53″ n.a. 1550 10 53.6 39.9 6.5 3.2 33.8 63.0 6.0 26.8

8 Boonah 27° 59′ 37″ 152° 41′ 53″ 861(106) 1290 13 37.7 51.4 10.9 9.1 46.6 44.3 6.3 30.7

a Numbers in parentheses denote the number of years over which the average rainfall rate has been calculated.

b Clay minerals present in the soil include albite, microcline, kaolinite, illite and montmorillonite.

3.2.3 Soil sampling method

At each site, twelve soil cores of 5 cm diameter were collected from within a plot area of between 5 and 6 m2. Nine cores were taken to a depth of 5 cm and sectioned in 1 cm increments, and three cores taken between 5 and 10 cm depth. Soil from within the same depth interval was pooled to form a single sample. Grass growing at each site was clipped to the level of the topsoil and collected as a separate sample. All samples were collected in the penultimate week of May 2003, roughly corresponding to the end of the wet season in this subtropical climate. Additional soil samples were collected for analysis of physical and chemical properties.

3.2.4 Analytical technique

The soils were weighed to determine bulk density, dried at 90 °C for 48 h, and reweighed to determine moisture content. Samples were then crushed using a ring-mill grinder, packed into 65 mm plastic dishes, and sealed to prevent the escape of 222Rn (the gaseous precursor of 210Pb). An insufficient sample volume of grass existed to fill the prescribed geometry. Consequently a known mass of analytical grade anhydrous Na2CO3

was added to these samples prior to crushing. Once sealed in their geometry, all samples

were left for a period of at least three weeks to allow for 222Rn to acquire secular equilibrium with its parent 226Ra. Gamma emission following decay of short-lived 222Rn progeny radionuclides, 214Pb and 214Bi, were then used to calculate the activity of 226Ra present in the sample. Similarly, measured activities of 234Th and 234mPa represented 238U

activity. To accommodate for the detection of 7Be, a number of samples were also counted

during the ingrowth phase.

Sample analysis was performed at the Australian Nuclear Science and Technology

Organisation using a Canberra Industries Compton Suppression gamma-ray spectrometry

system. The system comprised an active NaI(Tl) suppression annulus, a NaI(Tl) plug

detector and a reverse electrode germanium detector all housed within an inert lead shield.

Detector efficiency was 12 % at 46.5 keV (210Pb), 5.7 % at 477.6 keV (7Be) and 2.5 % at

661.6 keV (137Cs). Sample count times varied between 1 and 4 days. Activities of 238U series radionuclides and 137Cs were determined by direct comparison with IAEA standard

reference materials. Beryllium-7 activities were determined in a similar manner, though

using soil and grass matrices spiked with a known amount of certified 7Be standard solution obtained from Brookhaven National Laboratory (United States). All radionuclide activities reported herein have been decay corrected to the date of sample collection. Quoted uncertainties represent the 1-sigma statistical error of sample counting.

3.2.5 Radon-222 exhalation measurements

Radon-222 activity flux measurements were made on three separate occasions at

Jimboomba (Site 9) located approximately 20 km north of the soil sampling route (Figure

3.1). On each occasion, twenty four activated charcoal cups of diameter 7 cm were planted at equidistantly spaced points within a grid area measuring 50 m by 30 m. Radon-222 exhaled from the ground was collected over a 1 week period. The physical characteristics of this site closely matched with the sites selected for soil sampling. The additional feature of this site was its accessibility over periods of several days to securely leave measurement equipment for 222Rn exhalation surveys.

Radon-222 activity was determined through the gamma emissions of its short-

lived progeny, 214Pb and 214Bi, which were measured using common NaI(Tl) gamma spectroscopy techniques. Each sample was counted for a fixed period of 20 minutes (live- time). A measure of the background count rate and an efficiency calibration of the system were performed immediately prior to and succeeding counting of the field samples. The calibration standard had an identical geometry to the field samples – that is activated charcoal spiked with a known amount of a certified 226Ra solution obtained from

Amersham International (United Kingdom) was sealed in a 7 cm diameter cup.

3.3 Results

210 7 3.3.1 Pbex and Be areal activity densities

210 7 Areal activity densities of Pbex and Be in the grass and superficial 10 cm of the

soil ranged from 1080 to 4100 Bq m-2 and from 176 to 778 Bq m-2 respectively (Table 3.2).

210 On average, only a small fraction of Pbex (0.7 ± 0.1 %) was present in the grass. A much

7 210 larger portion of Be (17.6 ± 2.4 %) was associated with this vegetal layer. Our Pbex

results are comparable to values reported by Pfitzner et al. (2004) for undisturbed soils in

northern Queensland (518 to 3411 Bq m-2). Values listed in Table 3.2 represent around 85

210 % of the accumulated Pbex signature at our measurement sites (see below). No previous

measurements of 7Be in Queensland soils are known to exist, and only a small number of

7Be soil studies have been conducted in Australia. Beryllium-7 areal activity densities ranging from 110 to 450 Bq m-2 have been reported for grass covered soils at Black

Mountain, Australian Capital Territory and ranging from 90 to 990 Bq m-2 in clear-felled and 260 to 620 Bq m-2 in undisturbed eucalypt forest soils are reported at St. Helens,

Tasmania (Wallbrink and Murray, 1996a).

210 7 Cumulative areal activity densities of Pbex and Be, normalised to unity at a

210 depth of 10 cm, are shown in Figure 3.2. At all sites the cumulative Pbex signal increases with soil depth, but with a decreasing contribution from the lower layers. This signal appears to extend beyond 10 cm depth. The depth-rate accumulation of 7Be is much more

210 rapid than that of Pbex. Beryllium-7 remains predominantly in the grass and surface 1 cm

of the soil. Only at Clagiraba (Site 4), Tabragalba (Site 5) and Coulson (Site 7) did we

detect its presence below 2 cm depth. Olsen et al. (1985) measured 7Be to a depth of 10 cm in unsaturated marsh soils in the eastern United States and suggested that particles mobilised by infiltrating rainwater may be transported through fissures in the soil surface formed during relatively dry periods. It has also been suggested that the incorporation of

7Be into plant tissue, followed by redistribution along nutrient pathways within the root

210 7 137 Table 3.2. Accumulated Pbex, Be and Cs areal activity density in the surface 10 cm and portion present in grass.

Site Locality Sampling date Areal activity density Fraction in grass

no. (d/m/y) (Bq m-2) (%)

210 7 137 210 7 137 Pbex Be Cs Pbex Be Cs

1 Southport 22/5/2003 2180 ± 240 474 ± 68 217 ± 23 0.3 ± 0.2 15.9 ± 2.3 <0.1

2 Ashmore 22/5/2003 4100 ± 310 778 ± 78 89 ± 19 0.3 ± 0.1 9.3 ± 1.3 1.1 ± 0.6

3 Nerang 20/5/2003 1780 ± 320 383 ± 75 10 ± 2 1.3 ± 0.3 22.6 ± 2.4 10.9 ± 3.8

4 Clagiraba 20/5/2003 1770 ± 210 406 ± 40 361 ± 29 0.8 ± 0.1 22.0 ± 2.3 0.4 ± 0.1 53 5 Tabragalba 20/5/2003 2030 ± 220 322 ± 50 273 ± 25 0.3 ± 0.2 26.3 ± 12.9 0.6 ± 0.2

6 Beaudesert 21/5/2003 1080 ± 160 229 ± 27 19 ± 4 1.0 ± 0.3 21.7 ± 3.1 <0.1

7 Coulson 21/5/2003 1450 ± 210 478 ± 72 61 ± 13 0.8 ± 0.2 6.7 ± 1.4 <0.1

8 Boonah 21/5/2003 2090 ± 260 176 ± 33 272 ± 25 0.5 ± 0.2 16.2 ± 3.7 <0.1

1.2

1.0

0.8 Southport 0.6 Ashmore Nerang activitydensity Clagiraba ex 0.4 Tabragalba (relative scale)

Pb Beaudesert Coulson 210 0.2 Boonah Eq. (1) 0.0 0246810

Soil depth (cm)

1.2

1.0

0.8

0.6

0.4 (relative scale) Be activityBe density 7 0.2

0.0 0246810

Soil depth (cm)

210 7 Figure 3.2. Pbex (top) and Be (bottom) cumulative areal activity density profiles normalised to unity at 10 cm depth. The solid line indicates the best fitting exponential given by Equation 3.1 to the data. Error in individual data points is typically 10 %.

matrix or bioturbation by soil fauna could lead to the transportation of 7Be to greater soil

depths (Wallbrink and Murray, 1996a).

210 7 Both Pbex and Be cumulative areal activity profiles exhibit an exponential

dependence with depth (x) which can be expressed as:

⎡ ⎛ ln 2 ⎞⎤ ⎜ ⎟ Equation 3.1 A = Agrass + Asoil ⎢1− exp⎜− x ⋅ ⎟⎥ ⎣ ⎝ Ph ⎠⎦ where Agrass and Asoil are the contribution from the grass and soil respectively to the accumulated areal activity density (A), and Ph is the penetration half-depth of the radionuclide under consideration. Penetration half-depth is taken to be the depth at which the accumulated areal activity density is expected to decrease by a factor of two.

210 7 We have fitted Equation 3.1 to average Pbex and Be values at each depth

interval (Figure 3.2). Statistical analysis indicates that a significant (p<0.001) portion of the

2 210 7 variance (R >0.99) in Pbex and Be areal activity profiles can be explained by soil depth when Equation 3.1 is applied to the data. This analysis was performed using the software package DataFit (Version 8.0) obtained from Oakdale Engineering. Penetration half-depths

210 7 of 3.6 ± 0.2 cm and 0.3 ± 0.1 cm were determined for Pbex and Be, respectively. At the

210 sites selected for this study, roughly 85 % of Pbex activity was concentrated in the upper

210 10 cm of the soil. Further to this, Equation 3.1 also predicts that 95 % of Pbex will be present in the surface 15 cm and 99 % present in the surface 25 cm of the soil. These results agree with the findings of Matthews and Potipin (1985), who reported more than 90 % of

210 Pbex in the surface 15 cm and a maximum penetration depth of 25 cm in New Zealand

7 soils. Our result for Be compares favourably with Ph values reported by Wallbrink and

Murray (1996a) for eucalypt forest soils (0.30 to 0.37 cm) at St. Helens, Tasmania, but is somewhat greater than the Ph reported by the same authors for alluvial bare soils (0.057 to

0.141 cm) at Black Mountain, Australian Capital Territory.

210 Among all sites studied, areal activity densities of Pbex were highest at

Southport (Site 1) and Ashmore (Site 2), i.e. nearest the coast. This was in opposition to our

210 expectation that coastal sites would exhibit a lower Pbex activity than those further inland due to a dilution of 222Rn from mixing with ocean air. Water bodies emit little 222Rn

(Wilkening and Clements, 1975) and therefore do not contribute significantly to deposited

210 7 Pbex radioactivity. Areal activity densities of Be were also higher at these sites. Because

of its cosmogenic origin 7Be production rates are independent of geography and its

atmospheric concentration over a small area is expected to be more or less the same.

Therefore factors causing spatial variability in 7Be areal activity density will be 7Be deposition rate and redistribution of the land surface. Since we have specifically chosen sites where no apparent perturbations have occurred, variations in the measured 7Be areal

activity density should result from variations in 7Be deposition rate alone. Rainfall is known

to be an efficient scavenger of both 7Be- and 210Pb-bearing aerosols from the atmosphere

(Martin, 2003; Turekian et al., 1983; Wallbrink and Murray, 1994). Our sampling sites are

separated by a series of small mountain ranges. Those sites towards the west of the ranges

(Tabragalba (Site 5), Beaudesert (Site 6), Coulson (Site 7) and Boonah (Site 8)) are located

in a region of traditionally lower rainfall than those nearer the coast (Table 3.1). A

significant (p<0.05) positive correlation (R>0.98) between 7Be areal activity density and

total rainfall in the 12 month period preceding sample collection was observed. But the

210 correlation between Pbex areal activity density and long-term annual average rainfall was

210 poor (R=0.29). To substantiate this apparent lack of relationship between Pbex and rainfall a greater number of sampling sites is required.

210 222 3.3.2 Accumulation of Pbex on the land surface and Rn activity fluxes

Assuming the deposition to earth of a particular radionuclide to be constant in

time, an estimate of its accumulation rate (B) on the land surface can be made from its areal

activity density according to Equation 3.2.

λA B = Equation 3.2 1− exp()− λt

In this equation λ is the disintegration constant, A is the areal activity density, and t is the

time over which the activity has accumulated. In the situation where the land surface has

remained undisturbed for a time period much longer than the half-life of the isotope

involved (i.e. t >> T1/2), Equation 3.2 reduces to

B = λA Equation 3.3

210 7 Using Equation 3.3, we have calculated the average accumulation rate of Pbex and Be across our measurement sites as being 73 ± 11 Bq m-2 y-1 and 1.9 ± 0.3 kBq m-2 y-1,

respectively. However, it is well known that the deposition rate of 210Pb and 7Be varies with

rainfall (Fogh et al., 1999; Winkler and Rosner, 2000). In Southeast Queensland the

distribution of the annual rainfall follows a seasonal pattern – around 66 % of the yearly

rainfall occurs between November and April (inclusive) and 34 % falls in the period from

May to October. In the case of 210Pb, the seasonal influence of the rainfall on its deposition rate is likely to be smoothed over the accumulation period (~100 years). The half-life of

7Be, on the other hand, is considerably shorter than the temporal variations in the annual rainfall pattern and as a result its areal activity density is expected to display a seasonal variation. Hence, 7Be areal activity densities given in Table 3.2 should be viewed as representing end-of-wet season values for the region.

If on a regional scale an equilibrium exists between 222Rn exhaled from the ground

and that deposited back in the form of 210Pb, and this fallout is the dominant source

contributing to its excess, then an estimate of the average 222Rn activity flux (φ) over the

210 previous 100 years can be made from the Pbex areal activity density (APbex) as

ϕ = λRn APbex Equation 3.4

222 where λRn is the Rn disintegration constant. Using Equation 3.4 and correcting the

210 measured Pbex signal to its terminal depth, the paleo-radon activity flux has been calculated as 5.1 ± 0.9 mBq m-2 s-1. Radon-222 exhalation surveys performed on three

separate occasions yielded activity fluxes of 5.8 ± 0.3, 6.4 ± 0.2 and 2.0 ± 0.2 mBq m-2 s-1,

respectively. The first two measurements were made under rainfall free conditions and the

third taken during a period of persistent rainfall. Long-term meteorological records indicate

that in the vicinity of Jimboomba (Site 9) rainfall occurs on approximately 25 % of all days

in any given year. From this, a rainfall weighted average 222Rn activity flux of 5.5 ± 0.4

mBq m-2 s-1 has been determined. This value is not statistically different to the paleo-radon

210 activity flux determined on the basis of the measured Pbex areal activity density and tends to suggest that Southeast Queensland is neither a net source nor a net sink of 210Pb-bearing aerosols. Southeast Queensland’s climate is classified as subtropical (Environmental

Protection Agency, 2004). Temperatures are warm to mild and the weather is generally controlled by the passage of non-aggressive high- and low-pressure systems with a hint of tropical influence resulting in higher rainfall during summer. Near surface air mixing through eddy currents and the quiescent nature of the weather pattern are most likely responsible for maintaining a balanced atmospheric 210Pb budget across the region.

While good agreement has been found between our retrospective and direct measurements, there exist a number of complicating factors which can affect paleo-radon activity flux calculations using Equation 3.4. Such factors not only include the net influx

(or out-flux) of 210Pb-bearing aerosols to (or from) a region, but also soil properties, e.g. moisture and porosity, which can affect 222Rn retention in surface soils (Tanner, 1964;

210 Tanner, 1980) and consequently the Pbex areal activity density and profile. It could also

be argued that an excess of 210Pb activity over 226Ra in the soil may arise from a loss of

226Ra. Because of the half-lives involved, it is expected that 226Ra (half-life, 1600 years)

will be in secular equilibrium with its long-lived ancestor 238U (half-life, 4.5×109 years).

Figure 3.3 shows areal activity densities of 226Ra and 210Pb plotted against that of 238U. In general 226Ra values are less than those of 238U (i.e. fall below the line of equality)

suggesting that with time some 226Ra loss has occurred. The 210Pb values, however, are in

excess of both 226Ra and 238U, indicating that a loss of 226Ra alone cannot justify the values

210 226 of Pbex which we have measured in the soil. A weak correlation (R=0.60) between Ra

1000

) -2 800

600

400

Pb activity density (Bq m (Bq activityPb density 210 226 200 Ra Ra, 210 Pb 226 1 : 1 0 0 200 400 600 800 1000

238U activity density (Bq m-2)

Figure 3.3. 226Ra and 210Pb areal activity density plotted against 238U. The solid indicates the line of equality. Errors in 226Ra, 210Pb and 238U are typically 5, 10 and 10 %, respectively.

210 areal activity density and clay mineral content was observed. However, Pbex areal

activity density shows no strong proportionate dependency with any physicochemical

properties listed in Table 3.1.

3.3.3 Fallout of 137Cs on the Southeast Queensland landscape

In Australia, fallout of radioactive debris from above ground nuclear weapons tests

began in the early 1950’s. Between October 1952 and September 1958 the British

conducted a series of nuclear tests at sites on the Australian continent (Marlinga and Emu

Field, South Australia) and also at nearby Trimouille, Christmas and Malden Islands. The

accumulated total 137Cs input at Brisbane, Queensland (Figure 3.1) between 1954 and the

end of 1978 due to global nuclear testing has been estimated as 488 Bq m-2 (Longmore et al., 1983). At Swan Creek, located in the Darling Downs area of Southeast Queensland

(Figure 3.1), Longmore et al. (1983) report the average 137Cs areal activity density in

undisturbed soils to be 495 Bq m-2. For the purpose of comparison these values have been

decay corrected to the end of May 2003, corresponding to the soil sampling period of the

present study. As our sampling sites are situated between Brisbane and the Darling Downs,

and the number and yield of atmospheric nuclear tests since 1978 has been relatively low, it

does not seem unreasonable to assume the accumulated total 137Cs input at sites along our sampling route to be around 490 Bq m-2.

Levels of 137Cs which we have measured in the surface 10 cm range from 10 to

361 Bq m-2 (Table 3.2). These values are at least one order of magnitude less than the

210 corresponding Pbex activity density at each site. Caesium-137 areal activity in grass

varied from <1 to ~2 Bq m-2. At most sites the 137Cs signature was well mixed within the surface 10 cm of the soil (Figure 3.4). There was a significant (p<0.001) linear correlation

(R>0.99) between the incremental areal activity density and soil depth. Nerang (Site 3) and

Beaudesert (Site 6) were excluded from this analysis due to an abnormal behaviour. The

137Cs areal activity density at these sites was lowest among all sites and retained only within

1.2

1.0

0.8

0.6

Cs activityCs density 0.4 (relative scale) 137 0.2

0.0 0246810

Soil depth (cm)

Figure 3.4. 137Cs cumulative areal activity density profiles normalised to unity at 10 cm depth. The site key is the same as that shown in Figure 3.2. The solid line represents the best fitting linear function to the data. The dotted lines indicate the 95 % confidence region.

Error in individual points is typically 10 %.

210 the surface 2 to 5 cm of the soil. Yet the Pbex signal at these sites exhibited a similar

trend to that observed at other sites (Figure 3.2), suggesting that some surface soil may

137 have been removed from these areas following the cessation of Cs deposition. A Ph for

137Cs was not determined as the nature of its profile was not exponential, at least within the surface 10 cm. Analysis of a 1 m deep soil core taken from Jimboomba (Site 9) suggests that the peak in the 137Cs signature presently resides at a depth of between 10 and 20 cm, and that at depths greater than 30 cm its activity is below the lower limit of detection.

A comparison of our 137Cs areal activity densities to other values reported in the

literature is not offered since we have not quantitatively measured the 137Cs signal in its

entirety. However, it is apparent from the profile of the deeper soil core and the reduced

areal activity density in the surface 10 cm relative to the estimated input value that a

vertical migration of 137Cs has occurred. Such a downward movement of 137Cs is not uncommon. Schuller et al. (2004) reported the convection velocity of fallout 137Cs from

global nuclear weapons tests to vary from 0.03 to 0.11 cm y-1 in soils from South Patagonia,

Chile. The vertical migration velocity of Chernobyl derived 137Cs in soils of Western

Macedonia, Greece is reported to be between 0.1 and 0.3 cm y-1 (Arapis and Karandinos,

2004). Caesium-137 sorbs strongly to most soils, particularly to clay minerals. Its downward movement is largely governed by the physical transport of 137Cs-bearing

particles from the surface to the subsoil layers, though certain physicochemical properties

of individual soils can also affect it mobility (Livens and Loveland, 1988).

210 7 3.3.4 Natural radionuclide signatures ( Pbex and Be) for use in erosion studies

As mentioned, nuclear methods developed to examine soil loss by erosion have

been dominated by the 137Cs technique. This technique can be used to estimate land erosion rates over the past 30 to 40 years. However, 137Cs areal activity densities in the Austral

Pacific region have not been replenished since the cessation of above ground nuclear weapon tests. In this region, no significant increase in 137Cs activity in either surface air

(Matthews, 1995) or soil (Schuller et al., 1993) was observed following the Chernobyl

accident. Also, original 137Cs fallouts in the Southern Hemisphere were far less intense than

in the Northern Hemisphere (UNSCEAR, 2000). Forgoing any future weapons test era or

serious nuclear accidents, the 137Cs signal will wane due to both its physical half-life and

dispersion in the soil, thereby restricting its use in future land erosion estimates. If nuclear

methods are to continue to be used as a quantitative means to measure soil erosion, then

suitable and sustainable alternatives to 137Cs are required.

Lead-210 and 7Be both have a strong affinity for most soils (Matisoff et al., 2002).

But unlike 137Cs their areal activity densities are continuously replenished through natural

production processes. Applications of these isotopes for studies of land erosion are now

emerging in the literature. For example, Walling and He (1999) and Walling et al. (2003)

210 used Pbex areal activity densities and profiles to estimate rates and patterns of soil erosion

210 on cultivated land. In addition, the results of Walling et al. (2003) demonstrate that Pbex

can be used as an alternative radiotracer to 137Cs for examining erosive soil loss. The work of Wallbrink and Murray (1996b) indicates that a more sensitive method for investigating

210 137 soil erosion rates is to consider inventory ratios of Pbex to Cs as opposed to either of these signatures alone. Also, Blake et al. (1999) and Wilson et al. (2003) have shown that

7Be can be used to quantify rates of water-induced erosion at the event-scale.

The few previous studies of soil erosion and sedimentation rates in Southeast

Queensland have focused their attention on the measurement of 137Cs (Longmore et al.,

210 1983; Wallbrink, 2004). In this region, no consideration has yet been given to either Pbex

or 7Be for quantitative assessment of land erosion rates. The results of the present study

210 7 137 demonstrate that Pbex, Be and Cs are all present in the surface 10 cm of Southeast

Queensland soil’s and that all three can be simultaneously measured through standard gamma spectrometric techniques. Given the areal activity profiles which we have measured and taking into consideration the source of each isotope, i.e. natural and replenished (210Pb

7 137 210 and Be) versus anthropogenic and non-replenished ( Cs), it would appear that Pbex and

7Be show great promise to be used either individually, together or in conjunction with 137Cs for future studies of soil erosion in Southeast Queensland.

3.4 Conclusions

210 7 137 Accumulated areal activity densities and profiles of Pbex, Be and Cs in grass covered soils and 222Rn activity fluxes from the ground have been measured at selected sites in Southeast Queensland, Australia. The main findings of this study are summarised in the following points:

210 7 • Areal activity profiles of Pbex and Be in the surface 10 cm of undisturbed soils

show an exponential depth dependence and 137Cs a linear dependence

210 7 • Average penetration half-depths of Pbex and Be were determined to be 3.6 ± 0.2

cm and 0.3 ± 0.1 cm, respectively

• Comparison of paleo and present 222Rn activity fluxes suggest a balanced

atmospheric 210Pb-bearing aerosol budget over Southeast Queensland

210 7 • The vertical distribution and activity of Pbex and Be indicates that these natural

and replenished radionuclides may provide a suitable and sustainable alternative to

fissiogenic 137Cs for future studies of decadal and event-based soil redistribution

rates in the region

Acknowledgements

The authors gratefully acknowledge the financial assistance provided to them by the Australian Institute of Nuclear Science and Engineering in the undertaking of this study

(AINSE grant no. 03/084). We would also like to thank Miss Lindsay Churchley for carrying out the 222Rn exhalation surveys, Mr. Tony Raftery for the quartz and clay mineral

analysis, Mr. Les Dawes for analysis of additional physical and chemical properties of the

soils, and Mr. Jaya Dharmasiri for preparation of 7Be and 226Ra calibration standards.

Rainfall data used in this study was generously supplied by the Australian Government

Bureau of Meteorology.

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Martin, P., 2003. Uranium and thorium series radionuclides in rainwater over several

tropical storms. Journal of Environmental Radioactivity 65, 1–18.

Matisoff, G., Bonniwell, E.C., Whiting, P.J., 2002. Soil erosion and sediment sources in an

Ohio watershed using beryllium-7, cesium-137, and lead-210. Journal of

Environmental Quality 31, 54–61.

Matthews, K.M., 1995. Measurements of residual traces of 137Cs in the atmosphere in New

Zealand. Journal of Environmental Radioactivity 27, 221–229.

Matthews, K.M., Potipin, K., 1985. Extraction of fallout 210Pb from soils and its distribution

in soil profiles. Journal of Environmental Radioactivity 2, 319–331.

Olsen, C.R., Larsen, I.L., Lowry, P.D., Cutshall, N.H., Todd, J.F., Wong, G.T.F., Casey,

W.H., 1985. Atmospheric fluxes and marsh-soil inventories of 7Be and 210Pb.

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estuarine and marine sediment in the central region of the Great Barrier Reef

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102.

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Environmental Radioactivity 76, 67–80.

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Water Resources Research 32, 467–476.

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Tillage Research 69, 3–13.

CHAPTER FOUR

BERYLLIUM-7 IN NEAR-SURFACE AIR AND DEPOSITION AT BRISBANE,

AUSTRALIA

Che Doering and Riaz Akber

Radiological Laboratory, School of Physical and Chemical Sciences, Queensland

University of Technology, 2 George Street, Brisbane, Qld. 4000, Australia

Journal of Environmental Radioactivity (submitted 11 October 2006)

STATEMENT OF JOINT AUTHORSHIP

Title

Beryllium-7 in near-surface air and deposition at Brisbane, Australia

Authors

Che Doering, Riaz Akber

Che Doering (candidate)

Original idea, collected and prepared samples for analysis, analysed samples, analysed and interpreted data, and wrote manuscript.

Riaz Akber

Assisted with sample collection, assisted with data analysis and interpretation, and offered advice and editorial comments throughout the preparation of the manuscript.

Abstract

Measurements of 7Be concentrations in near-surface air and 7Be deposition were carried out at Brisbane, Australia. Concentrations of 7Be in near-surface air measured over

4 years show seasonal variations with values above the annual mean occurring mainly in

the spring and summer months of each year. These higher concentrations coincide with the

expected influx of stratospheric air to the planetary boundary layer in early spring and

higher rates of convective circulation within the troposphere during summer. 7Be deposition measurements over 3 years show seasonal variations similar to the seasonal rainfall pattern.

There is a statistically significant (p<0.001) linear relationship between monthly 7Be deposition and rainfall amount. This relationship is used to calculate the net cumulative 7Be areal activity density.

Keywords: 7Be; atmospheric radioactivity; deposition; areal activity density; Australia

4.1 Introduction

Beryllium-7 (7Be) is one of a family of cosmogenic radionuclides produced by the continuous bombardment of earth’s atmosphere by cosmic radiation (Lal and Peters, 1967).

It is born in an endothermic reaction when cosmic-ray particles disintegrate the atomic nucleus of atmospheric nitrogen and oxygen in a process known as spallation. The main production site of this radionuclide is in the lower stratosphere at a height of around 20 km.

Aircraft and ground based measurements indicate that there is a steep vertical concentration gradient of 7Be within the atmosphere: concentrations decrease by around two orders of magnitude between the lower stratosphere and the surface air (Dibb et al., 2003;

Dutkiewicz and Husain, 1985).

7Be attaches predominantly to aerosols in the submicron size range (Papastefanou and Ioannidou, 1995). Therefore, it is susceptible to the same transport and depositional processes governing these aerosols, making it a useful atmospheric tracer (Brost et al.,

1991). 7Be labelled aerosols can reach the ground under clear sky conditions through

particle sedimentation (dry deposition), but are removed more efficiently from the

atmosphere by precipitation scavenging (wet deposition). Wet deposition occurs through

below cloud scavenging (washout) and in cloud scavenging (rainout) of the carrier aerosol.

We are interested in studying the atmospheric cycle of 7Be in the troposphere in

the Austral Pacific region and also the potential for this radionuclide to quantify rates of

short-term soil erosion. The present study was carried out at Brisbane (Australia) and deals

with the deposition and concentration in near-surface air of cosmogenically produced 7Be.

It examines the temporal variations in each of these quantities and attempts to identify the processes that cause these variations. Finally, the net cumulative 7Be inventory (areal activity density) is calculated and compared with values measured in composite grass and soil samples.

4.2 Sampling and analytical techniques

A high volume air sampler (Ecotech, model 2000) and cellulose filter (Whatman,

grade 41) were used to collect aerosol samples from near-surface air. The sampler operated

on a six day cycle (24 hour sampling) and was set up at a height of about 30 m above the

ground level on the roof of the Electrical and Electronic Systems Engineering building at

the Queensland University of Technology, which is located in Brisbane, Australia, at a

latitude of 27° 29′ South and a longitude of 153° 2′ East. The air flow rate through the filter

was about 1.17 m3 min-1. The sampler operated from January 2002 until December 2005.

7Be activity was measured using a well-type high purity germanium detector (Canberra), which detected the characteristic gamma-ray emission (energy, 477.6 keV) associated with the transitional electron capture decay of this radionuclide (branching ratio, 0.1044). The counting efficiency of the detector was determined to be 15.6 % using a certified 7Be standard obtained from Brookhaven National Laboratory. The uncertainty in individual measurements of 7Be concentration in near-surface air was generally less than 10 %.

Integrated monthly deposition samples were collected using a purpose built

stainless steel funnel that had an effective surface area of approximately 0.57 m2. The collector was set up directly adjacent to the high volume air sampler. The surface of the collector was exposed to the atmosphere continuously, thus collecting both wet and dry

(total) deposition. Rainwater was stored in a polyethylene collection drum positioned directly beneath the collector. At the end of each month, the collector surface and interior walls of the collection drum were washed with a 1 % nitric acid (HNO3) solution and the wash added to the sample. The sample was transported to the laboratory in clean

7 polyethylene buckets and acidified to 1 % HNO3 to prevent any loss of Be atoms by adsorption. It was then concentrated by evaporating to a volume of 1 litre (Marinelli

Beaker). The sample was analysed for 7Be activity using a coaxial-type high purity germanium detector (Ortec). The counting efficiency of the detector was 1.2 % relative to activity in the sample. Results were decay corrected over the month long sampling period

using a half-life of 53.22 days. The uncertainty in monthly measurements was usually less

than 10 %. Measurements were made between January 2004 and December 2006.

The monthly rainfall was obtained from the nearest Bureau of Meteorology

weather station (BOM Station 40913; WMO Station 94576), which was located

approximately 1 km from the sampling site.

4.3 Results and discussion

4.3.1 7Be concentrations in near-surface air

The results of near-surface air 7Be concentration measurements are summarised in

Table 4.1. Concentrations which we have measured are similar to those measured

elsewhere at similar latitude (comparative data for a number of sampling sites is available

from the Surface Air Sampling Program measurements database

http://www.eml.st.dhs.gov/databases/sasp).

7Be concentrations in surface air are sensitive to the phase of the 11-year sunspot

cycle (Koch and Mann, 1996). Therefore, in order to study the behaviour of 7Be in near-

surface air over several years, we have normalised the measured data. Figure 4.1 shows the

percent deviation of monthly average 7Be concentrations away from the annual mean. The

normalised data shows seasonal variations with concentrations above the annual mean

occurring mainly in the spring and summer months (September–February) of each year.

Such variations are a widely recognised phenomenon and are said to be the manifestation of

several atmospheric processes, including stratosphere to troposphere exchange, vertical

transport within the troposphere and rainfall (Feely et al., 1989). The increase in 7Be concentration during the early spring at our sampling site coincides with the circulation of stratospheric air into the planetary boundary layer based on observations of strontium-90

(90Sr) in surface air at southern latitudes (Rehfeld and Heimann, 1995). 90Sr is an artificial radionuclide present mainly in the stratosphere except immediately after an above ground nuclear detonation. The higher concentrations of 7Be in near-surface air throughout the late

Table 4.1. Summary of near-surface air 7Be concentration measurements at Brisbane during

the years 2002–2005.

Year Range Mean ± σ

(mBq m-3) (mBq m-3)

2002 1.7–8.1 4.6 ± 1.5

2003 0.6–7.6 4.1 ± 1.5

2004 1.7–8.8 5.2 ± 1.8

2005 0.8–10.3 5.6 ± 1.8

50% 2002 2003 2004 2005

25%

0% JFMAMJJASONDJFMAMJ JASONDJFMAMJ JASONDJFMAMJJASOND

Deviation from mean annual -25%

-50%

Figure 4.1. Percent deviation of monthly average 7Be concentrations from the annual mean

at Brisbane during the years 2002–2005. Uncertainty in 7Be concentrations is generally less

than 10 %.

spring and summer are most likely sustained by higher rates of convective circulation,

bringing 7Be rich air from the upper to the lower troposphere and into the surface air.

Rainfall does not seem to have a controlling influence over the seasonal behaviour of 7Be in

near-surface air at our sampling site.

4.3.2 7Be deposition

Table 4.2 summarises the results of the 7Be deposition measurements. Monthly

values of 7Be deposition and rainfall are shown in Figure 4.2. The temporal pattern in 7Be deposition is seasonal and closely matches the temporal pattern in rainfall.

Figure 4.3 shows 7Be deposition plotted against rainfall for each month of the study with the linear least squares regression line representing the empirical relationship

Y = (0.91 ± 0.10)X + (35 ± 9) where Y and X represent the monthly 7Be deposition (Bq m-2) and rainfall (mm), respectively. The coefficient of determination (R2) for this relationship indicated that 72 % of the variance in 7Be deposition was explained by the variance in rainfall, which was significant at a confidence level of 99.9 % (p<0.001). In contrast, less than 1 % of the variance in 7Be deposition was accounted for by the variance in 7Be concentration in near- surface air. This finding suggests that the deposition of 7Be is not constrained by its availability in near-surface air, at least on a monthly timescale.

The y-axis intercept of the linear fit shown in Figure 4.3 suggests a monthly dry

deposition component of 35 ± 9 Bq m-2. However, this is around four times higher than the

measured deposition of 9 ± 3 Bq m-2 in July 2004 when only 0.6 mm of rainfall was recorded. The latter (measured) value suggests that dry deposition of 7Be at our sampling

site constitutes around 10 % of the total deposition of this radionuclide on an annual basis.

The total deposition velocity of 7Be labelled aerosols was calculated as the ratio of

the monthly deposition to the monthly average concentration in near-surface air and ranged

from 0.75 to 25 mm s-1 with a mean of 6.8 mm s-1. By comparison, Young and Silker

Table 4.2. Summary of 7Be deposition measurements at Brisbane during the years 2004–

2006.

Year Monthly range Annual deposition Annual rainfall

(Bq m-2) (Bq m-2) (mm)

2004 9–236 1070 ± 100 1056.6

2005 33–222 1164 ± 92 718.0

2006 10–226 1362 ± 100 795.6

300

)

-2 200

Rainfall (mm) 100 Be (Bqdeposition Be m 7

0 JFMAMJ JA SOND JFMAMJ JA SOND JFMAMJ JA SOND

2004 2005 2006

Figure 4.2. Monthly 7Be deposition (columns) and rainfall (solid circles) at Brisbane during the years 2004–2006. Uncertainty in monthly 7Be deposition measurements is generally

less than 10 %. The dashed line shows the trend in rainfall.

300

) 200 -2

Be deposition (Bq Be m 7 100

Y = (0.91 ± 0.10)X + (35 ± 9) R2 = 0.72

0 0 100 200 300 Rainfall (mm)

Figure 4.3. Monthly 7Be deposition (Y) plotted against monthly rainfall (X). The solid line represents the linear least squares regression line.

(1980) determined the average deposition velocity for the standing crop of 7Be over the

South Pacific Ocean to be 9.0 mm s-1 at latitudes between 20°S and 30° S. Changes in the

deposition velocity at our sampling site were most closely linked to changes in the monthly

deposition amount, which itself was dependent upon the rainfall.

4.3.3 Net cumulative 7Be areal activity density

The net cumulative 7Be areal activity density was calculated from available

monthly rainfall data using the relationship shown in Figure 4.3 by allowing the predicted

areal concentration to decay exponentially and then summing with the concentration from

the next month, and so on (Wallbrink and Murray, 1994). Figure 4.4 shows the calculated

net cumulative 7Be areal activity density and corresponding monthly rainfall at our

sampling site during the years 2000–2006. The 7Be decay curve shows that the areal

activity density will be representative after about 12 months, i.e. when 7Be activity deposited prior to January 2000 has decayed to less than 1 % of its original value. After 12 months the areal activity density shows distinct seasonal variations that are related to the seasonal pattern in rainfall. The areal activity density increases sharply during the maximum rainfall periods which typically occur in the austral summer at our sampling site.

The values are then controlled by the 7Be half-life and subsequent lower intensity rainfall.

The calculated average areal activity density for a period of 6 years (2001–2006) was 310

Bq m-2. The maximum and minimum values during this period were 180 and 540 Bq m-2, respectively. These variations suggest that the net cumulative 7Be areal activity density at a measurement site can vary substantially depending upon the time at which samples are taken and local rainfall patterns.

The calculated net cumulative areal activity density was checked on one occasion

(April 2005) against that measured in composite grass and soil samples taken to a depth of

20 mm from an adjacent site. The average 7Be areal activity density of 356 ± 38 Bq m-2

from these samples was around 30 % higher than the calculated areal activity density of 270

Table 4.3. Comparison of measured and calculated net cumulative 7Be areal activity

densities for May 2003.

Location 7Be areal activity density (Bq m-2)

Measured Calculated

Southport & Ashmore 616 ± 52a 680 ± 89

Nerang 383 ± 75 550 ± 75

Beaudesert 229 ± 27 260 ± 44

Boonah 176 ± 33 250 ± 44

a Mean value for two sites.

600

500 ) -2 400

300

Rainfall (mm) 200

Areal activityAreal density (Bqm Be decayBe curve (relative scale) 7 100

0 JA JOJAJOJA JOJA JOJA JOJA JOJA JO 2000 2001 2002 2003 2004 2005 2006

Figure 4.4. Calculated net cumulative 7Be areal activity density (columns) and measured

rainfall (solid circles) at Brisbane during the years 2000–2006. The solid line is the decay

curve for 7Be activity deposited before January 2000. The dashed line shows the trend in

rainfall.

± 46 Bq m-2. It is interesting to note that the site from which the samples were taken used an automated sprinkler system to water the grounds during dry spells. The system effectively simulated rainfall in the surface 5–10 m of the air and has probably added to the total 7Be deposition locally through the washout process, though the actual contribution is not known. Net cumulative areal activity densities were also calculated for several more sites in the region where we have previously measured 7Be in composite grass and soil

samples (Doering et al., 2006). Spray irrigation did not occur at these sites. Soil sampling

was performed during May 2003. Rainfall data for each site was obtained from the nearest

Bureau of Meteorology weather station. The results are summarised in Table 4.3 and

indicate that the measured and calculated areal activity densities generally agree within

statistical uncertainties. However, in each case the calculated value is somewhat higher,

suggesting the possibility of overestimation and/or some loss of 7Be from the soil sampling

site.

4.4 Conclusion

7Be in near surface air was measured over a 4 year period and 7Be deposition

measured over a 3 year period at Brisbane, Australia (lat. 27° 29′ S, long. 153° 2′ E).

Concentrations of 7Be in near-surface air showed seasonal variations with values above the

annual mean occurring mainly in the spring and summer months of each year. These

variations were attributed to the injection of stratospheric air into the surface air in early

spring and increased convective circulation within the troposphere during summer. Levels

of 7Be in near-surface air did not appear to control the amount of deposition of this radionuclide. Rather, rainfall was found to be an excellent predictor of 7Be deposition with

a statistically significant (p<0.001) linear relationship shown to exist. Precipitation

scavenging was the dominant removal mechanism of 7Be from the troposphere with dry

deposition estimated to comprise around 10 % of the total deposition during any single

year.

The net cumulative 7Be areal activity density was calculated from the deposition and showed seasonal variations that were influenced by the seasonal rainfall pattern. The average areal activity density measured in composite grass and soil samples taken from a nearby site was found to be higher than the calculated value. It was suggested that washout of 7Be at this site may have been increased by the use of an automated sprinkler system. At

other sites in the region where 7Be in grass and soil has previously been measured, the

calculated and measured areal activity densities agreed within statistical uncertainty in most

cases.

References

Brost, R.A., Feichter, J., Heimann, M., 1991. Three-dimensional simulation of 7Be in a

global climate model. Journal of Geophysical Research 96 (D12), 22423-22445.

Dibb, J.E., Talbot, R.W., Scheuer, E., Seid, G., DeBell, L., Lefer, B., Ridley, B., 2003.

Stratospheric influence on the northern North American free troposphere during

TOPSE: 7Be as a stratospheric tracer. Journal of Geophysical Research 108 (D4),

1–8.

Doering, C., Akber, R., Heijnis, H., 2006. Vertical distributions of 210Pb excess, 7Be and

137Cs in selected grass covered soils in Southeast Queensland, Australia. Journal of

Environmental Radioactivity 87, 135–147.

Dutkiewicz, V.A., Husain, L., 1985. Stratospheric and tropospheric components of 7Be in

surface air. Journal of Geophysical Research 90, 5783-5788.

Feely, H.W., Larsen, R.J., Sanderson, C.G., 1989. Factors that cause seasonal variations in

beryllium-7 concentrations in surface air. Journal of Environmental Radioactivity

9, 223-249.

Koch, D.M., Mann, M.E., 1996. Spatial and temporal variability of 7Be surface

concentrations. Tellus 48B, 387-396.

Lal, D., Peters, B., 1967. Cosmic ray produced radioactivity on the Earth, in: Sitte, K. (Ed.),

Encyclopedia of Physics. Springer-Verlag, New York, pp. 551-612.

Papastefanou, C., Ioannidou, A., 1995. Aerodynamic size association of 7Be in ambient

aerosols. Journal of Environmental Radioactivity 26, 273–282.

Rehfeld, S., Heimann, M., 1995. Three dimensional atmospheric transport simulation of the

radioactive tracers 210Pb, 7Be, 10Be, and 90Sr. Journal of Geophysical Research

100, 26141-26161.

Wallbrink, P.J., Murray, A.S., 1994. Fallout of 7Be in South Eastern Australia. Journal of

Environmental Radioactivity 25, 213–228.

Young, J.A., Silker, W.B., 1980. Aerosol deposition velocities on the Pacific and Atlantic

Oceans calculated from 7Be measurements. Earth and Planetary Science Letters

50, 92–104.

CHAPTER FIVE

DESCRIBING THE ANNUAL CYCLIC BEHAVIOUR OF 7Be CONCENTRATIONS IN

SURFACE AIR IN OCEANIA

Che Doering and Riaz Akber

Radiological Laboratory, School of Physical and Chemical Sciences, Queensland

University of Technology, 2 George Street, Brisbane, Qld. 4000, Australia

Journal of Environmental Radioactivity (submitted 13 April 2007)

STATEMENT OF JOINT AUTHORSHIP

Title

Describing the annual cyclic behaviour of 7Be concentrations in surface air in Oceania

Authors

Che Doering, Riaz Akber

Che Doering (candidate)

Original idea, collected samples and supplementary data, analysed samples, analysed and interpreted data, and wrote manuscript.

Riaz Akber

Assisted with sample collection, assisted with data analysis and interpretation, and offered advice and editorial comments throughout the preparation of the manuscript.

Abstract

Surface air concentrations of 7Be at a number of stations in Oceania show a distinct annual cycle. We apply a sinusoidal model to describe this cycle. The results of model fitting show that peak 7Be concentrations in surface air occur during early spring at tropical latitudes and during mid-to-late summer at middle latitudes. Comparison with available 90Sr surface air data for the southern hemisphere indicates that stratosphere-to-

troposphere exchange is an active atmospheric process controlling the 7Be annual cycle throughout the Oceania region. Vertical transport of air within the troposphere also seems to influence the observed annual cycle. Seasonality in rainfall is not thought to control the annual cyclic behaviour of 7Be in surface air.

Keywords: 7Be; Atmospheric radioactivity; Surface air; Annual cycle; Oceania

5.1 Introduction

Beryllium-7 (7Be) is a cosmogenic radionuclide produced in the stratosphere and troposphere when cosmic-ray-produced neutrons and protons disintegrate the atomic nucleus of atmospheric nitrogen and oxygen into lighter fragments (Lal and Peters, 1967).

It decays by electron capture to lithium-7 and has a half-life of approximately 53 days. 7Be is carried in the atmosphere attached predominantly to submicron-sized aerosols and is a useful radionuclide for studying atmospheric transport processes (Brost et al., 1991; Jordan et al., 2003). Concentrations of 7Be in surface air show characteristic time variations with

frequencies below the synoptic variability (Koch and Mann, 1996). The well reported

annual cycle is said to be the manifestation of several meteorological factors (Feely et al.,

1989). There have been few studies of 7Be in surface air reported for sites in Oceania

(Garimella et al., 2003; Hicks and Goodman, 1977; Uematsu et al., 1994). Most measurements carried out in the region are of a monitoring nature only, and little interpretation of the data has been performed. To better understand the annual cycle of 7Be in surface air and to identify factors that may be controlling this behaviour, we present an analysis of 7Be surface air concentrations for several stations in the Oceania region.

5.2 Data and methods of analysis

5.2.1 Data sources

7Be data used in this study was obtained from our own measurements carried out at Brisbane (Australia), the Environmental Measurements Laboratory Surface Air Sampling

Program database (EML, 1999), measurements performed by the National Radiation

Laboratory of New Zealand (Matthews, 1996), the University of the South Pacific

(Garimella, personal communication), and measurements performed as part of the Sea-Air

Exchange program (Uematsu, personal communication). The location of each sampling station is shown in Figure 5.1 and the time periods over which the measurements were made are given in Table 5.1.

Equator 1

3 4 5 Tropic of Capricorn

6 30° S 7

9 11 10 12 13

150° E 180° E, W 150° W

Figure 5.1. Map of Oceania showing location of 7Be measurement stations.

Table 5.1. Location of sampling stations, measurement periods, mean 7Be concentration in surface air, and long-term annual average rainfall.

No.a Station Geographical location Measurement period 7Be (mean ± σ) Rainfall

(mBq m-3) (mm)

Latitude Longitude

1 Nauru 0.5°S 167.0°E 1985–1986 1.4 ± 0.3 2097

2 Funafuti 8.5°S 179.2°W 1986–1987 1.8 ± 0.4 3398

3 American Samoa 14.3°S 170.6°W 1976–1999 2.5 ± 0.9 3093

4 Fiji 18.2°S 178.5°E 1999–2000 1.5 ± 0.7 3041 88 5 Cook Islands 21.3°S 159.8°W 1987–1995 3.0 ± 0.4 1838

6 Brisbane (Australia) 27.5°S 153.0°E 2002–2005 4.9 ± 1.2 1186

7 Norfolk Island 29.0°S 168.0°E 1983–1999 4.5 ± 1.1 1308

8 Kaitaia (New Zealand) 35.1°S 173.3°E 1987–1995 3.2 ± 0.4 1337

9 Cape Grim (Australia) 40.7°S 144.8°E 1982–1996 2.9 ± 0.9 1079

10 Lower Hutt (New Zealand) 41.2°S 174.9°E 1987–1999 2.9 ± 0.8 1249

11 Hokitika (New Zealand) 42.7°S 171.0°E 1987–1995 2.4 ± 0.2 2876

Table 5.1 (continued).

No.a Station Geographical location Measurement period 7Be (mean ± σ) Rainfall

(mBq m-3) (mm)

Latitude Longitude

12 Chatham Island 43.9°S 176.0°W 1983–1996 3.0 ± 1.0 864

13 Invercargill (New Zealand) 46.4°S 168.4°E 1983–1999 2.4 ± 0.8 1115

aRefers to the station number shown in Figure 5.1.

5.2.2 Measurement technique

The sampling and analytical techniques for most stations have been previously

reported in the literature (see, e.g., Feely et al., 1989; Garimella et al., 2003; Matthews,

1996; Uematsu et al., 1994). We only provide a description of our own methods.

A high volume air sampler (Ecotech, model 2000) and cellulose filter (Whatman,

grade 41) were used for aerosol collection. Air was drawn through the filter at a rate of

approximately 1.17 m3 min-1 over a 24 hour period. Samples were collected at 6 day

intervals. 7Be activity collected on the filter was measured through the characteristic gamma-ray emission associated with the transitional electron capture decay of this radionuclide (branching ratio, approx. 10 %). A quarter section of the filter was pressed into a small pellet and counted on a Canberra well-type high purity germanium detector. The

7Be counting efficiency of the detector was determined using a certified 7Be standard purchased from Brookhaven National Laboratory (United States). The concentration of 7Be in surface air was calculated using the half-life, branching ratio, counting efficiency, sampled air volume, and time delay in sampling and measurement. Monthly average concentrations were calculated using the arithmetic mean. The overall uncertainty in individual measurements was generally less than 10 %.

5.2.3 Data treatment

Measurements at different stations were made over different time periods (Table

5.1). Koch and Mann (1996) showed that concentrations of 7Be in surface air are sensitive to the phase of the sunspot cycle. Lower concentrations typically occur at sunspot maximum due to a decrease in the galactic cosmic-ray flux to Earth and vice-versa. This is a global phenomenon. In addition, Koch and Mann (1996) identified a cyclic pattern having a periodicity of around 2 to 3 years. They suggested that these variations were caused by rainfall anomalies during El Niño/Southern Oscillation (ENSO). To minimise the effects of low frequency solar and ENSO variations we have normalised the concentration data.

Normalised data Annual cycle

Un-normalised data Annual cycle Solar cycle

ENSO cycle scale) (relative density Spectral

01234 Frequency (years-1)

Figure 5.2. Frequency representation of normalised (top panel) and un-normalised (bottom panel) 7Be concentration data for the monitoring station at American Samoa.

Also, as not all detector systems were inter-calibrated, normalising the data may help to

reduce any inconsistencies between the measurement laboratories. The annual mean

concentration was subtracted from the monthly mean concentration of the corresponding

year and then divided by the annual mean to produce a set of monthly deviations from the

annual mean concentration for each year. Figure 5.2 shows the frequency representation of

the normalised and un-normalised concentration data for the monitoring station at

American Samoa, where measurements were made over a period of 24 years. It can be seen

that normalising the concentration data effectively removes the low frequency solar and

ENSO variations.

We wish to describe the annual cycle in surface air 7Be concentrations at each station using a sinusoidal model similar to that developed by Junge (1963) for describing seasonal changes in tropospheric ozone concentration. For this purpose we have synthesized an average year by calculating the mean of monthly deviations for each station.

The model used to describe these deviations is given by Equation 5.1. In this equation, y is the percent deviation of 7Be concentration away from the annual mean at time t. The

amplitude of the oscillation is given by y0. The phase, φ, indicates the time in months

relative to the start of the calendar year at which peak 7Be concentrations in surface air are predicted to occur. The period of oscillation, T, is assumed to be 12 months.

y = y0 cos[2π/T (t - φ)] Equation 5.1

5.3 Results and discussion

5.3.1 Results of sinusoidal model

The results of Equation 5.1 applied to the mean of monthly deviations at each station are presented in Table 5.2 in order of ascending latitude. For each station the analysis was performed at a confidence level of 95 %. To assess the predictability and correctness of Equation 5.1 to describe the behaviour of 7Be in surface air, we consider the

Table 5.2. Summary of results of sinusoidal model fitting.

2 Station Latitude Amplitude, y0 Phase, φ R P

(%) (month) (%) (%)

Nauru 0.5°S 27 ± 3 9.0 ± 0.2 90 99

Funafuti 8.5°S 19 ± 5 8.4 ± 0.5 59 82

American Samoa 14.3°S 26 ± 3 8.3 ± 0.2 91 99

Fiji 18.2°S 52 ± 10 8.4 ± 0.4 73 97

Cook Islands 21.3°S 21 ± 3 9.8 ± 0.3 82 99

Brisbane 27.5°S 17 ± 4 11.0 ± 0.4 68 94

Norfolk Island 29.0°S 17 ± 1 0.5 ± 0.2 93 99

Kaitaia 35.1°S 28 ± 2 1.1 ± 0.1 97 99

Cape Grim 40.7°S 18 ± 3 1.3 ± 0.3 82 99

Lower Hutt 41.2°S 32 ± 2 1.3 ± 0.1 95 99

Hokitika 42.7°S 23 ± 2 1.5 ± 0.2 90 99

Chatham Island 43.9°S 33 ± 3 1.3 ± 0.2 94 99

Invercargill 46.4°S 25 ± 3 1.0 ± 0.2 89 99

coefficient of determination and the power of the fit. The coefficient of determination (R2)

is the ratio of the variation explained by the model to the total variation. It is a measure of

the certainty in making predictions from a particular model. The power (P) is the

probability that the model correctly describes the relationship of the variables and is

affected by the level of significance at which the model is tested. Values of R2 and P for each station are given in Table 5.2. In general, the model shows good statistical rigor. The amount of variation explained by the model is in the range 59–97 % and the probability that the model correctly describes the relationship of the variables is in the range 82–99 %. It therefore seems that factors controlling the annual cycle of 7Be in surface air produce a

sinusoidal behaviour at most stations in Oceania.

The phase of the annual cycle predicted by Equation 5.1 varies between the

stations (Table 5.2). Between tropical latitudes and middle latitudes there appears to be a

delay of up to 5 months in the occurrence of peak 7Be concentrations in surface air. Peak

concentrations occur in early spring at tropical latitudes, during early summer at subtropical

latitudes, and during mid-to-late summer at middle latitudes. Hicks and Goodman (1977)

reported the annual peak in surface air 7Be concentrations to propagate poleward at a rate of approximately 1000 km month-1 at stations setup along the 150th meridian east between 9°S

and 43°S. Our own analysis suggests that over the whole of the Oceania region such a shift

mainly occurs between 20°S and 35°S. This finding may be explained by a poleward

migration of folds in the southern hemisphere tropopause as suggested by Matthews (1996).

However, there may also be a difference in the relative importance and timing of the

atmospheric processes controlling the annual cycle between the different stations.

5.3.2 Atmospheric processes controlling the 7Be annual cycle

Koch et al. (1996) reported the mean residence time of 7Be-bearing aerosols in the troposphere to be around 3 weeks. Aerosol residence times in the stratosphere are much longer than for the troposphere and a residence half-time of the order of 1 year has been

reported (Thomas et al., 1970). Because the stratospheric residence time is much longer

than the 7Be half-life, equilibrium between the rate of cosmogenic production and radioactive decay of this radionuclide is assumed to exist within the stratosphere. Jordan et al. (2003) reported the average concentration of 7Be in the lower stratosphere to be around

185 mBq m-3. Owing to differences in residence time, as well as rate of production and rate

of loss between the stratosphere and troposphere, there is a fairly high 7Be concentration gradient between the lower stratosphere and the surface air. Average concentrations of 7Be in surface air are typically a few milli-becquerels per cubic metre (see, e.g., Brost et al.,

1991). Elevated levels of 7Be in surface air have been used in the past to indicate an upper

atmospheric source (Husain et al., 1977; Viezee and Singh, 1980). However, concentrations

of 7Be in surface air can vary in response to a number of atmospheric processes. Feely et al.

(1989) reported that the annual cyclic behaviour of 7Be in surface air showed the effects of

stratosphere-to-troposphere exchange (STE), vertical and horizontal transport of air within

the troposphere, and rainfall.

STE is a means for concentrated 7Be air from the stratosphere to enter the troposphere. Artificial radionuclides released to the atmosphere by above ground nuclear weapon tests can be used to indicate the presence of stratospheric air in the troposphere (Lal and Suess, 1968). We can attempt to gauge the effect of STE on the 7Be annual cycle by

comparing the 7Be results with surface air measurements of artificial radionuclides reported

in the literature. Figure 5.3 shows a latitude-time isopleth diagram of the zonal averaged

ratio of the mean of monthly strontium-90 (90Sr) concentrations in surface air for the period

1964–1968 reported by Rehfeld and Heimann (1995). A similar spatiotemporal trend to that of 7Be in surface air in the Oceania region is apparent in the 90Sr data presented in this figure. 90Sr concentrations in the southern tropics show a peak in the early spring season.

There is a progressive shift in phase of this peak towards the summer months at latitudes greater than about 20°S. Peak concentrations of 90Sr in surface air at southern mid-latitudes

can be seen to occur in the mid-to-late summer season. It therefore seems that STE is an

Figure 5.3. Latitude-time isopleth diagram of the zonal averaged ratio of the mean of monthly 90Sr concentrations in surface air. From Rehfeld and Heimann (1995).

active atmospheric process controlling the 7Be annual cycle at stations in the Oceania

region. As an example of the similar behaviour of both 7Be and 90Sr in surface air, Figure

5.4 shows a plot of mean of monthly 7Be deviations for a number of the measurement

stations and mean of monthly 90Sr deviations at the equivalent latitude. The peak to trough

ratio of the 7Be cycle is less than that of the 90Sr cycle. 90Sr in surface air indicates the subsidence of stratospheric air. 7Be on the other hand is produced in both the stratosphere

and the troposphere.

Vertical transport of air between the upper and lower troposphere can occur

through convective circulation. Solar heating of Earth’s surface in the warmer months leads

to an increase in temperature of the surface air. Cooler air from above sinks and displaces

the warm surface air, bringing with it high concentrations of 7Be from the upper troposphere. 7Be concentrations in the upper troposphere are the result of in-situ production as well as STE. Maximum STE in the southern hemisphere is reported to occur in mid- winter (Appenzeller et al., 1996). Stratospheric air that enters the troposphere must therefore remain suspended in the upper troposphere for some time before descending to the surface air. At stations located in the tropics, vertical mixing within the troposphere is probably quite rapid due to the perpetually warm surface air temperatures above the West

Pacific Warm Pool. However, stations located at more temperate latitudes experience more distinct warm and cool seasons. Our results show that peak concentrations of 7Be in surface air at middle latitude stations in Oceania occur in the summer season when surface air temperatures are also high (Table 5.2). This peak in 7Be coincides with the peak in 90Sr

concentrations in surface air at these latitudes. It therefore seems that, in addition to STE,

vertical transport within the troposphere is an active atmospheric process which influences

the annual cycle of 7Be in surface air.

Washout by rainfall removes 7Be-bearing aerosols from the lower troposphere and

decreases the concentration of this radionuclide in surface air. In Oceania, low mean 7Be concentrations generally occur at stations were the annual rainfall is high (Table 5.1). Most

75% American Samoa (14.3°S) 50%

25%

-25%

-50%

-75% jan feb mar apr may jun jul aug sep oct nov dec

75% Brisbane (27.5°S)

50%

25%

-25%

-50%

-75% jan feb mar apr may jun jul aug sep oct nov dec

75% Cape Grim (40.7°S)

50%

25%

-25%

-50%

-75% jan feb mar apr may jun jul aug sep oct nov dec

Figure 5.4. Mean of monthly 7Be deviations (filled circles) at American Samoa (top panel),

Brisbane (middle panel) and Cape Grim (bottom panel), and mean of monthly 90Sr deviations (open circles) at the equivalent latitude.

Table 5.3. Values of linear correlation coefficient (R) and Spearman correlation coefficient

(ρ) between mean of monthly 7Be deviations and long-term mean of monthly rainfall.

Station Latitude Data pairs R ρ

Nauru 0.5°S 12 -0.64 -0.66a

Funafuti 8.5°S 12 -0.79 -0.50

American Samoa 14.3°S 12 -0.88 -0.80a

Fiji 18.2°S 12 -0.87 -0.87a

Cook Islands 21.3°S 12 -0.57 -0.61a

Brisbane 27.5°S 12 0.04 0.22

Norfolk Island 29.0°S 12 -0.86 -0.85a

Kaitaia 35.1°S 12 -0.90 -0.84a

Cape Grim 40.7°S 12 -0.94 -0.90a

Lower Hutt 41.2°S 12 -0.89 -0.93a

Hokitika 42.7°S 12 -0.23 -0.06

Chatham Island 43.9°S 12 -0.79 -0.85a

Invercargill 46.4°S 12 0.05 0.06 aValue of ρ is significant (p<0.05).

stations experience a seasonal rainfall pattern. Feely et al. (1989) suggested that the rainfall

affecting an air mass prior to its arrival at a sampling site may be better represented by the

long-term mean of monthly rainfall for the site rather than the actual amount of rain falling

in any particular month. This is because an approaching air mass has a good likelihood of

passing through bands of rain producing clouds whether or not such bands happen to

produce rainfall at the sampling site itself. We have calculated the linear correlation

coefficient (R) and Spearman correlation coefficient (ρ) between mean of monthly 7Be and

long-term mean of monthly rainfall at each station. Values of R and ρ are given in Table

5.3. The Spearman correlation coefficient is a non-parametric measure of the correlation

between the variables and does not assume the association to be linear. The calculated value

of ρ is significant (p<0.05) for a number of the stations. This finding suggests that the

seasonal pattern in rainfall may also be influencing the 7Be annual cycle. However, the

seasonal pattern in rainfall is perhaps more coincidental with the timing of other processes

(e.g. STE and vertical transport within the troposphere) rather than causal. Stations at

Lower Hutt and Hokitika are located within close proximity but experience different annual

rainfall patterns. Lower Hutt shows a distinct seasonal rainfall pattern, whereas rainfall at

Hokitika is distributed fairly evenly throughout the year. Nevertheless, both stations show a

distinct and in phase 7Be annual cycle. Also, the correlation between mean of monthly 7Be and mean of monthly rainfall at Invercargill is poor, but a distinct cyclic behaviour of 7Be in surface air is observed.

5.4 Conclusions

We have reported on the annual cyclic behaviour of 7Be concentrations in surface

air at a number of stations in the Oceania region. The observed annual cycle of 7Be was well approximated by a sinusoidal model at most stations. The results of model fitting suggested a delay of up to 5 months in the occurrence of peak concentrations between tropical and middle latitudes. It appeared that stratosphere-to-troposphere exchange and

100

vertical transport of air from the upper to the lower troposphere were active atmospheric

processes controlling the 7Be annual cycle. A significant (p<0.05) negative correlation

between mean of monthly 7Be and long-term mean of monthly rainfall was found to exist at a number of the stations. However, seasonality in rainfall was considered to be coincidental with the timing of other atmospheric processes rather than a mechanistic cause of the annual cyclic behaviour of 7Be concentrations in surface air.

Acknowledgements

Contributions of 7Be data from Professor Mitsuo Uematsu of the University of

Tokyo (Japan) and Professor Sitaram Garimella of the University of the South Pacific (Fiji) were warmly welcomed and are gratefully acknowledged.

References

Appenzeller, C., Holton, J.R., Rosenlof, K.H., 1996. Seasonal variation of mass transport

across the tropopause. Journal of Geophysical Research 101(D10), 15071–15078.

Brost, R.A., Feichter, J., Heimann, M., 1991. Three-dimensional simulation of 7Be in a

global climate model. Journal of Geophysical Research 96(D12), 22423–22445.

EML, 1999. Surface air sampling program database.

http://www.eml.st.dhs.gov/databases/sasp.

Feely, H.W., Larsen, R.J., Sanderson, C.G., 1989. Factors that cause seasonal variations in

beryllium-7 concentrations in surface air. Journal of Environmental Radioactivity

9, 223–249.

Garimella, S., Koshy, K., Singh, S., 2003. Concentration of 7Be in surface air at Suva, Fiji.

South Pacific Journal of Natural Science 21, 15–19.

Hicks, B.B., Goodman, H.S., 1977. Seasonal and latitudinal variations of atmospheric

radioactivity along Australia’s east coast (150°E longitude). Tellus 29, 182–188.

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Husain, L., Coffey, P.E., Meyers, R.E., Cederwall, R.T., 1977. Ozone transport from

stratosphere to troposphere. Geophysical Research Letters 4(9), 363–365.

Jordan, C.E., Dibb, J.E., Finkel, R.C., 2003. 10Be/7Be tracer of atmospheric transport and

stratosphere-troposphere exchange. Journal of Geophysical Research 108(D8), 3-

1–3-14.

Junge, C.E., 1963. Studies of global exchange processes in the atmosphere by natural and

artificial tracers. Journal of Geophysical Research 68, 3849–3856.

Koch, D.M., Jacob, D.J., Graustein, W.C., 1996. Vertical transport of tropospheric aerosols

as indicated by 7Be and 210Pb in a chemical tracer model. Journal of Geophysical

Research 101(D13), 18651–18666.

Koch, D.M., Mann, M.E., 1996. Spatial and temporal variability of 7Be surface

concetrations. Tellus 48B, 387–396.

Lal, D., Peters, B., 1967. Cosmic ray produced radioactivity on the Earth, in: Sitte, K. (Ed.),

Encyclopedia of Physics. Springer-Verlag, New York, pp. 551–612.

Lal, D., Suess, H.E., 1968. The radioactivity of the atmosphere and hydrosphere. Annual

Review of Nuclear Science 18, 407–434.

Matthews, K.M., 1996. Recent observations of atmospheric radioactivity in New Zealand

and Rarotonga, in: Akber, R.A., Martin, P. (Eds.), SPERA96: Radioactivity in the

environment. South Pacific Environmental Radioactivity Association,

Christchurch, pp. 72–87.

Rehfeld, S., Heimann, M., 1995. Three dimensional atmospheric transport simulation of the

radioactive tracers 210Pb, 7Be, 10Be, and 90Sr. Journal of Geophysical Research

100(D12), 26141–26161.

Thomas, C.W., Young, J.A., Wogman, N.A., Perkins, R.W., 1970. The measurement and

behaviour of airborne radionuclides since 1962, in: Gould, R.F. (Ed.),

Radionuclides in the Environment. American Chemical Society, Washington D.C.,

pp. 158–172.

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Uematsu, M., Duce, R.A., Prospero, J.M., 1994. Atmosphere beryllium-7 concentrations

over the Pacific Ocean. Geophysical Research Letters 21(7), 561–564.

Viezee, W., Singh, H.B., 1980. The distribution of beryllium-7 in the troposphere:

implications on stratospheric/tropospheric air exchange. Geophysical Research

Letters 7(10), 805–808.

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CHAPTER SIX

CONCLUDING REMARKS

6.1 Summary and conclusions

The research presented in this thesis advances our knowledge of the distribution and behaviour of 7Be in the natural environment, particularly within the Southeast

Queensland region of Australia. It has investigated the vertical distribution of 7Be in grass covered soils and identified factors controlling the areal activity density of this radionuclide. It has also investigated the atmospheric cycle of 7Be in the troposphere through studies of 7Be deposition and concentration in surface air.

Areal activity density profiles of 7Be in grass covered soils were obtained for a number of sites in Southeast Queensland. 7Be was found mainly in grass and the surface 1 cm of the soil. Grass was found to be a good sink for 7Be-bearing aerosols, with around 18

% of the total areal activity density of this radionuclide being present in this vegetal layer

on average. The average penetration half-depth of 7Be in the soil was determined to be 0.3

± 0.1 cm. The measured vertical distribution of 7Be within the soil suggested that this

radionuclide may be suitable for studying erosion and redistribution of surface soils in the

Southeast Queensland region. A high positive correlation existed between 7Be areal activity density and rainfall amount during the 12 month period preceding sample collection. This finding suggested that 7Be areal activity densities were subject to temporal variations depending upon local rainfall conditions.

Monthly depositional fluxes of 7Be were measured over 3 consecutive years at

Brisbane. The observed temporal pattern in 7Be deposition was seasonal and closely matched the temporal pattern in rainfall. A high positive correlation between monthly 7Be deposition and monthly rainfall existed and suggested that wet scavenging was the dominant removal mechanism of 7Be-bearing aerosols from the lower troposphere. Dry

104

deposition of 7Be-bearing aerosols was estimated to constitute around 10 % of total annual deposition of this radionuclide on average. There was a poor correlation between monthly

7Be deposition and mean monthly 7Be concentration in surface air. This finding suggested

that the deposition of 7Be was not constrained by its availability in surface air on a monthly timescale.

A historical record of 7Be areal activity density extending over several years was

generated from results of the deposition measurements. This record showed that 7Be areal activity densities at Brisbane varied seasonally and followed the local rainfall pattern. The areal activity density was found to rise sharply during months of high intensity rainfall and then decrease at a slower rate in subsequent months governed mainly by the 7Be half-life and lower intensity rainfall. The results suggested that 7Be areal activity density could potentially vary by up to a factor of 3 depending on the time when samples were collected and local rainfall conditions. Comparison of calculated and measured 7Be areal activity

densities showed agreement within statistical uncertainties for a number of sites in

Southeast Queensland.

Concentrations of 7Be in surface air measured over four consecutive years at

Brisbane showed an annual cyclic behaviour. An analysis of these measurements together with 7Be data for a number of additional sites in the Oceania region was performed. The annual cycle of 7Be in surface air at each site was described using a sinusoidal model. The proportion of the variance explained by the model was in the range 59–97 % and the probability that the model correctly described the relationship of the variables was in the range 82–99 %. The results showed that there was a delay of up to 5 months in the occurrence of peak 7Be concentrations in surface air between tropical and middle latitudes.

Comparison of the 7Be results with available 90Sr surface air observations for the Southern

Hemisphere showed a similar spatiotemporal trend. This finding suggested that stratosphere-to-troposphere exchange was an active atmospheric process controlling the annual cyclic behaviour of 7Be in surface air at sites in the Oceania region. Vertical

105

transport of air from the upper to lower troposphere was also identified as influencing the

observed 7Be annual cycle.

6.2 Future directions

This thesis has presented measurements and analysis of the distribution and

behaviour of 7Be in a number of natural systems. It adds to the current 7Be knowledge base and provides a foundation for further study. The next step in research is to use this knowledge to develop 7Be nuclear techniques for the evaluation of environmental problems. One area in particular is the use of 7Be areal activity densities to investigate

erosion and redistribution of surface soils in the Southeast Queensland region. Soil erosion

is recognised by the State Environmental Protection Agency as a major factor contributing

to land degradation. Erosion in Southeast Queensland is thought to be occurring at an

accelerated rate due to high rates population growth and urban development. Land

management strategies for controlling this loss will require quantitative methods of

assessing erosion rates and processes. Nuclear techniques may be useful for this purpose.

Because of its relatively short half-life, measurement of 7Be has the potential to examine

rates of short-term soil redistribution or erosion associated with a single event. The results

of this research show that 7Be is present mainly in the surface layers of the soil and can be measured simultaneously with other radionuclides through established gamma spectrometric techniques. The factors identified as controlling 7Be areal activity densities

will add significantly to the future development of nuclear techniques for quantitative

assessment of soil erosion and redistribution in the region.

106