Dust Deposition on the Taieri Plain

Weather and Climate 22, 29-40 (2002)

A.M. Bromleyl, M.J. Harveyl, J.A. Renwickl and J. Shulmeister2

1National Institute of Water and Atmospheric Research, Wellington, New Zealand 2Research School of Earth Sciences, Victoria University of Wellington, Wellington, New Zealand

Abstract Atmospheric dustfall has been collected from the Taieri Plains in Otago, New Zealand for two years from October 1998 to provide a regional source of information on present-day rates of dust deposition; calibrating current rates with longer-term rates estimated from sediment cores collected in the region will assist in obtaining better estimates of paeleoclimate. With an accompanying study of size distribution estimates of paeleowind strength can be obtained and, in turn , related to likely pressure gradients over New Zealand at that time and the most likely synoptic regimes prevalent for that era. This study shows that the average dustfall on the Taieri Plain varies from 3kg/ha/30 days to nearly 65kg/ha/30 days, with a mean of 16.66kg/ha/30 days. The soluble content accounts for almost 70% of the total. The seasonal distribution shows a summer peak and autumn minimum. Dust deposition increases with increased rainfall due to dust particles being washed out from the atmosphere. There is a positive correlation between total dust flux and synoptic situations that produce strong southwesterly and northeasterly flows over the region.

1. Introduction The source, entrainment, transportation and deposition of aeolian dust are of increasing interest and importance to the global community. Dust generation results from aridification and mirrors the effects of climatic change and human impact on a region. Aeolian dust is defined as soluble and insoluble particulate originating from the entrainment of soil into the atmosphere. Atmospheric dustfall occurs continuously throughout the natural and urban environment. The dust is produced from a range of natural sources... sea salt, pollens, soil erosion...and human activities... earthworks, vehicles, unsealed roads, and various industrial and domestic activities. Deposited particulates are the proportion of dust that settles out of the atmosphere by gravity or by frictional contact with surfaces. Rates of dust deposition can be monitored quantitatively using passive dust deposition traps. Monitoring is carried out using the horizontal deposit gauge method described in the Draft International Standard ISO/DIS 4222.2. The National Institute of Water and Atmospheric Research Ltd (NIWA) has carried out dust deposition monitoring on the Taieri Plain near Lake Waihola from October 1998 (see Fig. 1). The aims of the study are to provide present-day calibration data for analysis of sediment cores obtained by Victoria University of Wellington and to

Corresponding author: A.M. Bromley, NIWA, P.O. Box 14-901, Killoirnie, Wellington, New Zealand. Email: [email protected] 30 Weather and Climate, Volume 22

unedin

Dunedin Airport

Berwick • Henley Farm • Henley School Lake Waipari Pacific Ocean • VValhola

Lake Wai hola 1 1 1 0 5 10 km

Figure 1: Map showing Taieri Plain sampling sites. determine current relationships between windspeed and particle deposition Comparing current rates of dust influx with long-term rates estimated from drill cores at selected sites will enable better estimates of paleoclimate. With an accompanying study of size distribution, information on paleowind strength can be obtained from grain sizes of mineral components in the cores, which in turn will enable an understanding of the likely pressure gradients over the New Zealand area at that time in the past, and the most likely synoptic regimes prevalent for that era.

2. Measurement of Dust Deposition The total flux (FT) consists of both insoluble (F1) and soluble (Fs) components. FT F1 + Fs The insoluble flux consists of dry deposition of mineral dust aerosols (Fm) and insoluble dust suspended in rain (Flu) e.g. particulate removed in rainout. ' FDI + The soluble flux can be broken down further into dry deposition of salt aerosol (FDs), mainly of marine origin and the wet deposition of those salts dissolved from mineral dust in rain (Fws). Fs = FDS + FWS Bromley, Harvey, Renwick, Shulmeister: Dust Deposition Taieri Plain 31

In this paper the word "dust" is used to describe any windblown or suspended aerosol of mineral or salt origin. The total dust deposition (FT) was collected in the gauges; the insoluble (F1) and soluble (Fs) components were separated later in the laboratory. Four passive horizontal deposition gauges were installed in early October 1998. Two sites, Henley Farm and Henley School, are on the flat plain just north of Lake Waihola; Waihola Farm site is approximately 100ams1 up Ferry Hill (overlooking the SH1 bridge over the Taieri River), and the fourth site, Berwick Farm, is at 60ams1 (close to Trig 0 No2) overlooking Lake Waipori (see Fig. 1). Each gauge is securely bolted to a post with the gauge aperture level and at 1.5m above ground level to minimise "splash" from the immediate surface contaminating the sample. The gauges are designed to measure total deposition. Dry particles are trapped in the gauge by a small quantity of distilled water. The water sample, containing the dust, is collected at the end of each month and analysed at NIWA in Wellington. The water sample is passed through a screen (1mm square apertures as per ISO 2194) to remove extraneous matter (insects, large leaves etc), filtered and the residue weighed to give a mass of insoluble material. The total amount of water-soluble material in the screened sample is determined by taking an aliquot of the liquid filtrate and slowly evaporating off the water; the remaining residue is weighed to obtain the mass of dry material and this is adjusted in proportion of the aliquot sample to the total amount of filtrate. The results are expressed in either grams per square metre, or kilograms per hectare, per 30 days. The Taieri Plain was chosen as a potentially valuable site for the dust measurements as it is relatively uncontaminated by industrial or large urban activities and the data is therefore a good example of New Zealand rural background dust accumulation. Victoria University of Wellington has also drilled a calibrating sediment core close to Henley School in the centre of the study area.

3. Climate and Weather The low-lying Taieri Plain extends 25 km southwest of Dunedin City (45.9S, 170.5E) and averages about 7km across. Lakes Waipori and Waihola are at the southern extent of the plain (see Fig. 2). To the east of the plain is a belt of hills separating it from the sea; these are 7 to 12 km across and rise to 105-450m. To the west and north the terrain rises steeply from the inland edge of the plain to a general level of 300m, with the highest point reaching nearly 900m about 10km further inland. The steep hilly area north of Dunedin City provides some sheltering from the east-northeast (see Fig. 1). The alignment of the ranges plays a considerable part in channeling the surface winds in a west-southwest and east-northeast direction. The prevailing directions over the plain are southwest and northeast. Because the area is surrounded by hills, cool nighttime air tends to stagnate at the surface and consequendy there is a high frequency of calms. With wind speeds above 2.5m/s, winds blow 47 percent of the time between 32 Weather and Climate, Volume 22

Figure 2. Looking east across Lakes Waipori and Waihola from the Berwick collection site. The dust collector is in the right foreground.

200' and 270', and 30 percent of the time between 020' and 090'. Sustained winds over 15m/s are infrequent (0.3 percent of the time) and usually occur from the southwest or northwest. Because of the line of hills between the plain and the coast, the sea breeze is fairly weak, confined to summer and is evident only in weak gradients. Nevertheless this relatively weak sea breeze can penetrate well to the western boundaries of the Taieri Plain. It is typically south-southeast at 2.5 to 5 m/s, commencing in late morning and dying out in the early evening. When there is a weak east or northeast gradient flow the sea breeze is augmented and northeasterlies up to 7.5m/s occur during the afternoon. Rain falls on average on about 160 days of the year, mostly from fronts moving up the east coast over Southland and Otago. Rainfall is at a maximum in late spring and early summer, with a pronounced secondary maximum in June, probably due to a winter increase in frontal activity in the southwesterly flow

4. Results Summary statistics of total, insoluble and soluble dustfall in kg/ha/30 days from all four sites are presented in Table 1 and time series showing the monthly dustfall is shown in Figure 3. A summary of meteorological data for the same period recorded at Dunedin Airport, 8 km north of the study area, is given in Table 2. Bromley, Harvey, Renwick, Shulmeister: Dust Deposition Taieri Plain 33

A remarkable feature seen in Table l is the strong agreement of the dust flux between all four sites, indicating that dustfall is fairly consistent over the study area. The average amount of dust deposited over the area (obtained as a simple average of the results from the four gauges) was 16.66kg/ha/30 days. The temporal variation of dust deposition was high at all sites, ranging from just under 3kg/ha/30 days (Waihola Farm, Table 2) to nearly 65kg/ha/30 days (Berwick Farm, Table 2). These numbers compare well with dustfall data collected in the vicinity of Macraes gold mine in northern East Otago, by the Department of Geography, Otago University; there average dustfalls have varied from 17.7kg/ha/30 days to 35.6 kg/ha/30

Table 1: Statistics for the total dustfall and the insoluble and soluble components at the four Taieri gauge sites.

Henley Farm Henley School Waihola Farm Berwick Farm

Total all components: Mean 16.09 15.14 16.89 18.52 Standard Deviation 7.46 7.36 8.50 13.27 Sample Variance 55.59 54.18 72.17 176.08 Range 28.90 33.29 38.95 59.92 Minimum 4.86 4.90 2.69 4.77 Maximum 33.76 38.19 41.64 64.69 Sum 370.18 348.28 371.61 425.99

Insoluble component: Mean 6.07 5.05 5.22 5.51 Standard Deviation 4.52 4.10 4.52 3.72 Sample Variance 20.44 16.80 20.44 13.87 Range 14.78 14.57 15.41 12.81 Minimum 1.24 1.10 0.56 1.01 Maximum 16.02 15.67 15.97 13.82 Sum 109.34 90.85 88.77 99.24

Soluble component: Mean 10.93 10.87 12.05 10.92 Standard Deviation 5.72 5.80 6.66 5.97 Sample Variance 32.73 33.61 44.35 35.68 Range 20.45 20.68 24.37 21.47 Minimum 1.94 2.31 1.30 1.26 Maximum 22.39 22.99 25.67 22.73 Sum 251.42 250.08 265.09 251.20 34 Weather and Climate, Volume 22

days. (Otago University Consulting, 1991). The large peak in total dustfall at Berwick Farm in July 2000 (Figure 3) consisted of insoluble pollen, blown from a large pine forest immediately west of the gauge. This did not occur in 1999, but the rainfall at that time was considerably higher, (62.6mm over 16 raindays) tending to wash any pollen out of the trees and atmosphere close to its source. July 2000 was much drier, (24.4mm over only 7 raindays) allowing the drier pollen to be more readily blown further from the forest and into the area of the collecting gauge. Table 3 shows the soluble dustfall as percentage of the total dust deposition. The soluble content of the dustfall is dominant, accounting on average for 69% of total dustfall over the four sites. The seasonal distribution of dustfall is shown in Figure 4. Total dustfall peaks in summer and is at its lowest in autumn; both soluble and insoluble contents follow this pattern. The soluble content is assumed to be mainly sea salt particles and would be expected to increase in summer when sea breezes are more common.

Table 2. Meteorological data from Dunedin Airport, Momona (45.91S, 170.5E)

Date Hourly mean %Easterly %Westerly Rainfall Raindays windspeed wind wind (mm) >0.1mm (ms-1) direction direction

02.10.98-30.10.98 4.6 22 38 64.0 20 30.10.98-20.11.98 3.3 30 28 8.2 4 20.11.98-08.01.99 3.9 33 29 100.4 13 08.01.99-03.02.99 3.4 28 23 28.6 11 03.02.99-28.02.99 3.0 33 15 8.0 4 28.02.99-09.04.99 3.7 30 27 78.0 12 09.04.99-10.05.99 3.3 21 31 45.4 15 10.05.99-31.05.99 2.6 19 18 0.8 2 31.05.99-06.07.99 3.3 18 37 62.6 16 06.07.99-26.08.99 3.1 20 31 68.8 27 26.08.99-29.09.99 3.1 19 27 52.6 15 29.09.99-08.11.99 3.4 30 26 45.0 22 08.11.99-26.11.99 3.2 21 26 58.0 15 26.11.99-22.12.99 3.6 24 30 56.6 17 22.12.99-25.01.00 2.8 30 18 131.4 17 25.01.00-28.02.00 3.8 24 37 102.0 16 28.02.00-31.03.00 2.8 23 24 87.8 9 31.03.00-01.05.00 3.5 22 51 37.0 11 01.05.00-31.05.00 2.5 25 17 42.0 9 Bromley, Harvey, Renwick, Shulmeister: Dust Deposition Taieri Plain 35 • 40 • 30 (a) Total Dustfall, Taieri

70 60 50

HenleyFarm --(3) — — — -Henley School WaiholaFarm 20 , a —Berwick Farm 10 irkk-%7Iamlayr 0 cbc5 05c5 cg) cb0) c3c) 0- (<0'0' \.0 coe9 c)0( \.,0'

Month

(b) Insoluble Dustfall,Taleri

70 60 50 HenleyFarm 40 — — — -Henley School 30 Waihola Farm Berwick Farm 20 10 0 op op op) op op c9 oP 0 p OC>' 'g

Month

50 HenleyFarm C13 40 — — — -Henley School 30 WaiholaFarm BerwickFarm 20

10 0 05 ) 0)0) 0)03 c QP 9) 06' .g <

Month

Figure 3: Time series of dustfall at four sites on the Taieri Plains. (a) Total dustfall, (b)Insoluble dustfall, (c) Soluble dustfall. 36 Weather and Climate, Volume 22

• Total dustfall o Insoluble o Soluble 5

Spring Summer Autumn Winter Season

Figure 4: Seasonal distribution of dustfall on the Taieri Plain (based on the average of dustfall at the four sites).

4.1 Relation of Dust Deposition to Weather. Dust entrainment and deposition depends on the interaction of many parameters. Entrainment depends on daily and seasonal meteorological conditions that can generate turbulence capable of raising dust and the fluid threshold friction velocities of exposed dust grains, which are the critical speeds that exposed grains of dust will start to move as the velocity of the near-surface airstream increases. The threshold velocities are dependent on size, moisture content, degree of crusting and type and amount of vegetation of the source region. (Bagnold, 1941). Deposition rates depend on the rate of supply from the source, the dust size distribution, on rainfall that washes particles out of suspension in the atmosphere and controls soil moisture levels, and on the wind flow. At any given site the deposition flux is dependent on climatic conditions both at the source area and in the deposition area. Relation of dust deposition to precipitation: Rainfall and deposited dust levels exhibit a positive correlation. Comparison of variation in monthly dust flux to precipitation is shown in Figure 5; it can be seen that in broad terms total dust deposition

Table 3: Soluble dustfall as a percentage of the total dust deposition for each of the four Sites.

Henley Farm 69% Henley School 74% Waihola Farm 69% Berwick Farm 62% Mean of all four sites 69% Bromley, Harvey, Renwick, Shulmeister: Dust Deposition Taieri Plain 37

140 40 120 35 30 rt___ 100 25 (J) 1:3 80 NM Co 20 7:3 60 IllituiliM11111111111 Ct "(T3 15 -c f2 40 H 1111111R1111W1111•111111/ 10 20 IIM11011111111111N11111 5 0 IMITI11111111111 o-) 0 C, CD 0 CD C. 6) Co D 0 2 < Z Month Rainfall (rnm) —Mean total dustfall

Figure 5: Monthly dust deposition and monthly rainfall increases with increased rainfall. A degree of correlation is expected because the wet deposition flux depends on the amount of rain and concentration of dust in that rain. Rain is one mechanism to deposit dust particles from the atmosphere; dust particles are scavenged from the atmosphere by rain droplets either through sweepout or as a result of having served as condensation nuclei, and fall to the ground with the drops, as opposed to gravitational settling and dry deposition. Correlation coefficients of simple linear regression support the interpretation that rainfall is significantly related to dust deposition on the Taieri Plain (Figure 6). From these figures it appears that during periods of low rainfall the total dry deposition flux on the Taieri Plain is about 10kg/ha/30 days (3kg insoluble and 7 kg soluble). Relationship of dust to wind: The seasonality and strength of surface winds are important to the generation of dust. Particle movement by the wind may be initiated by several mechanisms, acting alone or together; these mechanisms include drag on the particle by the windstream, aerodynamic lift, impacts by other rolling particles and mechanical disturbance. Particle size is also a factor. Wind data from Dunedin Airport (8km north of the study area) have been analysed with the windspeed classes set at various fluid threshold friction velocities; the speeds chosen (2, 6, 10 and 14 m.s-1) correspond to movement of particles with equivalent diameter less than 10, 10.1 to 25, 25.1 to 50 and greater than 50 pm. At speeds below 2 m.s-1 it is assumed no grains would become airborne. A moderate positive correlation between winds from the easterly quarter and soluble dustfall (R2 = 0.33) suggests that the soluble content is mainly salts of marine origin. However, 38 Weather and Climate, Volume 22

0 0 50 100 150 Rainfall (mm) (a)

— 12 I-1), -0 -M 10 _O -0 CY-) 8 o _0 c CT) 6 — 4

2 0 0 50 100 150 (b) Rainfall (mm)

50 100 150

Figure 6: Relationship between rainfall and deposited dust, for (a) total dust, (b) insoluble dust and (c) soluble dust. Solid line is the linear regression line and R is the regression coefficient. Bromley, Harvey, Renwick, Shulmeister: Dust Deposition Taieri Plain 39 in general, comparisons show little relation between measured surface winds and dust flux. Relation of Monthly Dust Flux to Synoptic Situation: Kidson (1994a and 1994b) has listed occurrence the frequencies of characteristic "synoptic types" common for the New Zealand region, (Table 4). Correlation with total dust flux shows a positive relationship with situations that produce strong southwesterly or northeasterly flows over the region, i.e. TSW (cyclonic trough) and NE & R (ridge., low to north). The insoluble dust flux exhibits a significant correlation (R2 = 0.36) with southwesterly flows (SW & TSW), possibly due to the entrainment of more insoluble particles from the drier inland areas of Central Otago. The soluble dust flux correlates well (R2 = 0.35) with situations producing NE flows onto the plain (NE & R), with a corresponding increase in salt particles of marine origin. Situations giving westerly flows over the plain exhibit an anticorrelation (R2 = -0.35) with soluble flux.

5. Summary Atmospheric dustfall on the Taieri Plain south of Dunedin has been systematically collected for two years. Passive dust traps have been exposed at four locations near Lake Waihola and data collected at approximately 30-day intervals. Both the insoluble and soluble portion of the dust has been measured. The natural regional dustfall varies from just under 3kg/ha/30 days to nearly 65kg/ha/30 days; the mean is 16.66 kg/ha/30 days. The soluble content is the dominant influence, accounting for 69% of average total dustfall. Total dustfall increases in summer, mainly due to the increased presence of soluble sea salt, and has a minimum in autumn. Dustfall appears to be closely linked with rainfall, probably as a result of scavenging of particulates through washout. To date there appears to be little correlation between

Table 4. The synoptic categories defined by Kidson (1994a,b) for the New Zealand region.

TSW Trough (cyclonic) southwesterly Trough SW Southwesterly NE Northeasterly Ridge (low to north) HW High to west HE High to east Westerly HNW High to northwest 40 Weather and Climate, Volume 22 dustfall and surface wind, other than an increase in soluble dust content with increased frequency of onshore (easterly quarter) winds. When comparing dustfall with synoptic situations there are some correlations; the insoluble content increases with southwesterly flows over the region, and the soluble content increases with situations that produce onshore flows. More data collection is required before firmer relationships between dustfall and weather parameters can be made although it is encouraging that even data collected over a relatively short period is indicating positive correlations with different types of synoptic situations.

Acknowledgements The authors wish to thank Henley School Board of Trustees and the owners of Henley Farm, Waihola Farm and Berwick Farm for permission to install the dust gauges on their respective properties, and to Kim Currie (NIWA, Dunedin) for the monthly data collections. This work was supported by the New Zealand Foundation for Research Science and Technology as a contribution to PGSF Contract VIC509 "Regional Paleoclimates and Climate Modelling".

References Bagnold, 1941. The physics of blown sand and Kidson, J.W., 1994b. The relation of New desert dunes. Methuen, London, 241pp. Zealand daily and monthly weather patterns Kidson, J.W., 1994a. An automated procedure for to synoptic weather types. Ink J. Climate!., 14, the identification of synoptic types applied to 723-737. the New Zealand region. Int. J. Climatol, 14, Otago University Consulting, 1991. Report to 711-721. Macraes Mining Company 1991.

Submitted to Weather and Climate, 31 July 2001 Revised, 23 May 2003.