10 Weather and Climate (1988) 8: 10-22

ASSESSING AIR POLLUTION POTENTIAL AT A LARGE INDUSTRIAL SITE - A CLIMATOLOGY OF DEEP INVERSIONS, MIXING HEIGHTS AND ATMOSPHERIC STABILITY AT

B. B. Fitzharris Department of Geography, University of , C. M. Cosgrove Department of Earth Sciences, University of Virginia, Charlottesville, USA J. B. Symon Department of Geography, University of Newcastle, Newcastle, NSW, Australia

ABSTRACT Temperature stratification of the atmosphere at Aramoana, , has been studied in detail during a one year period from April 1981 to March 1982. Information from thermographs, an acoustic sounder, airsonde and tethersonde ascents was used to describe the frequency and nature of deep ground-based and elevated inversions, mixing heights, and stagnation episodes. A method that requires wind speed and net radiation data was used to assess atmospheric stability. Ground-based inversions extending up to 80 m and 230 m occurred for 25% and 13% of the time, respectively, but were mostly weak and short lived. Elevated inversions were frequent at all times of the year but again were generally weak with gradients of less than 2' C/100 m. Average hourly mixing heights varied between 554 m (at 0200 hours) and 657 m (at noon). There was little seasonal, but large day to day variation. Over the measurement year mixing heights fell to below 200 m and 300 m for 14% and 23% of the time respectively, but these low values did not often combine with up-harbour winds. Stagnation episodes were rare, with only three occurrences during the measurement year. Atmospheric stability would not restrict dispersion for 69% of the time, but would do so 18% of the time.

to an extensive area of unoccupied flat land. It is INTRODUCTION also relatively close to cheap sources of hydro- Aramoana, located 19 km northeast of Dun- electric power. Over the last 30 years Aramoana edin, has long been considered by the Otago has been proposed as a site for a number of large Harbour Board and Otago Regional Develop- industries including a steel mill, nuclear and coal ment Council as land suitable for development by fired power stations, a copper smelter and more a large scale industry. It is one of the few places in recently for an aluminium smelter. The latter the where deep water ships could proceeded as far as an environmental impact load and unload cargoes in sheltered waters next report (South Pacific Aluminium and Otago Assessing Air Polution Potential 11

Harbour Board, 1981) and subsequently a detail- Pacific Ocean ed investigation aimed at providing a meteor- ological assessment of Aramoana and its likely air dispersion (Fitzharris and Edwards, 1982). 1 0 1 Measurements for the meteorological investig- scale km ation were made at Aramoana by the Depart- ments of Geography and Physics of the Univers- ity of Otago under contract to South Pacific Aluminium. Part of the research is presented in this paper which details the climatology of deep inversions observed at Aramoana over a one year period referred to herein as the 'measurement year' (April 1981 - March 1982). Deep inversions are defined as those whose tops are above 80 m. These deep inversions may be either ground- based or elevated. The paper also discusses mixing height, atmospheric stability and stag- nation episodes A description of shallow ground- based inversions (less than 50 m) is provided by • thermograph 010m anemometer Edwards and Isaac (1984a). (0;) land above 150 m • acoustic sounder radiation instruments

In general terms, a number of important Fig. 1.• Location map showing Aramoana in relation to the Otago characteristics of emissions from any large indus- Harbour and the siting of instruments (A 50 m tower was located near try will have a bearing on the resulting air the acoustic sounder). pollution at and about the proposed site. Oke SITE DESCRIPTION (1978) reviews these, pointing out that the rate of emission and the physical and chemical nature of Aramoana is located on flat land 2 km west of the emissions are central to the likely pollutant the entrance to Otago Harbour (Fig. 1). Any large loading. The shape of the emission area, duration industry is likely to be situated near the base of of releases and the effective height at which cliffs and steep hills which rise up to 230 m above pollutants are injected are also key factors, but the site. These hills shelter the site from the west these all will vary, depending on the nature of the and northwest, but it is exposed to the major large industrial plant. northeast and southwest wind patterns of the area (Fitzharris and Cosgrove, 1980). A biologically After release, the dispersion of emissions is important salt marsh lies immediately to the east controlled by the state of the atmosphere, and of the industrial site. The small township of together with the topography, its climatology Aramoana (population 50) is also nearby although determines the air pollution potential of a site. many of the houses are used as holiday homes. Two main factors are important — first the wind, Otago Harbour occupies a 2-6 km wide trough and second the temperature stratification. The that extends southwest from Aramoana to Port wind regime at Aramoana has been described by Chalmers (8 km distant) and Dunedin. The hills Edwards and Isaac (1984b) from detailed meas- surrounding the harbour are generally 200 m to urements near the site and on a 50 m high tower. 300 m high but reach 676 m at Mt Cargill. In a second paper we report on spatial variations in the wind regime in the Upper Otago Harbour Although no long term wind data exist for the using tetroons (Fitzharris et al., 1988). The site, the wind regime may be inferred from data temperature stratification, which is dealt with collected during the measurement year (Edwards here, is important to the dispersion of pollutants and Isaac, 1984b), combined with data from because it defines the atmospheric stability. This Taiaroa Heads, 2 km to the northeast (Fitzharris in turn is a major control on the intensity of and Cosgrove, 1980). This information indicates thermal turbulence (or buoyancy) and the depth that winds are predominantly from the southwest of the surface mixed layer. Together with mech- (1/4 of the time), and from the north or northeast anical turbulence and wind shear induced by air (1/3 of the time). Mean wind speed is close to 2/3 flow over irregular terrain, these features regulate of that at Taiaroa or 5 ms-1 (Edwards and Isaac, the upward and horizontal dispersion of pollut- 1984b). This ratio however is higher for the more ants and hence their likely concentration. critical light winds and calms. 12 Assessing Air Polution Potential The radiation regime is dominated by frequent hourly intervals. Inversions were assumed to occurrences of partly cloudy conditions so that exist when the temperature difference between sunshine hours are less than 1800 hours/year and the higher and lower sites was positive. Occasion- amongst some of the lowest in the country al malfunctions with one of the thermographs (Fitzharris et al., 1982a). During the measure- meant that data was missing for 357 hours or 4% ment year, incoming solar radiation (I“) had a of the time over the measurement year. maximum of 30 MJm-2 day-1 in December, with The N.Z. Meteorological Service calibrated minimums of less than 2 NIJm-2 day-' in June. and issued the thermographs for the project. On Net longwave radion (1,*) showed little seasonal their receipt in Dunedin, the timing of the clocks change, varying from 0 to -9 MJin-2day-' during and calibrations were re-checked. Calibrations the measurement year. Net radiation (Q*) had were made in the field 2-3 times a week by its highest value of 18 MJni-2 day-' in December comparing the chart traces with temperatures with values ranging between -±2MJm-2 day-1 for measured by precision maximum and minimum most of June and July. Measured albedo for the thermometers, also housed in the screens. Errors rushes and low grasses that typify the vegetation were estimated at ±0.5° C. Comparison of ridge of the industrial site, was 0.23. temperatures at 230 m with those in the free Average annual rainfall at Aramoana is est- atmosphere obtained from airsonde and tether- imated at 660 mm with 107 wet days. Tem- sonde ascents indicate a good relationship, except eratures are generally mild with a mean daily that below about 8.0° C, ridge top temperatures maximum of 13.6° C and a mean daily minimum are sometimes cooler than in the air. This appears of 7.0' C (Fitzharris et al., 1982b). to occur because of weak radiation inversions which form above the flat topped ridge on calm, Long term records from nearby climate stations ( (1967-80) and Mussel- clear nights. Temperatures less than 8.0° C oc- curred for 6% of the time during which inversion burgh (1947-80)) were compared with Aramoana data to assess the representativeness of the frequency and strength could be underestimated. measurement year. All climatic variables were Spatial surveys of temperature at Aramoana on about average except minimum air temperature clear nights indicate that the coolest air in the area and total sunshine hours. Minimum air temper- tends to pond near the site of the lower thermo- atures for the measurement year indicate that graph. Therefore this site is considered suitable there were probably fewer air and ground frosts for monitoring inversion frequency. Comparison than expected in the long term. Thus the fre- KEY ft. quency of inversions measured over the year may 200 01900 NZ% be less than the long term climatological average. • 2200 NZST Sunshine hours were 16% below average indicat- 180 — 0000 NZST ing cloudier than usual conditions. This suggests 0 0200 NZST 160 • 0415 NZST that night-time longwave radiation losses could Ls 0600 NZST have been less than average and the frequency of unstable conditions over the measurement year 140 may also be less than the long term climatological average. Wind speeds and wind directions ob- 120 served at Aramoana during the measurement 100 year are described by Edwards and Isaac (1984b) as being typical of their long term averages. 80

60 METHODS THERMOGRAPH MEASUREMENTS 40

Thermographs installed in Stevenson screens 20 at standard height above the ground measured the temperature during the measurement year 2 3 4 between Aramoana (elevation 1 m) and the ridge TEMPERATURE 1°C) to the west (elevation 230 m), as an index of Fig. 2: Temperature profiles of the lower atmosphere at Aramoana deeper inversions (see FLO). Charts were on a clear night during anticyclonic conditions (17-18 September changed weekly and temperatures extracted at 1981). Assessing Air Polution Potential 13 of temperatures between Aramoana and the DETERMINATION OF MIXING HEIGHT AND OTHER FEATURES FROM ACOUSTIC SOUNDER western ridge provide but an index of inversion CHARTS frequency; the relationship of this index with actual inversion frequency depends on factors Mixing height (sometimes called mixing such as depth of inversion, changing temperature depth) is the height above the surface to which gradients, and inversion strength. Typical tem- emissions will extend, primarily through the perature profiles during inversion conditions action of atmospheric turbulence (Newman, (Fig. 2) suggest however, that the temperature 1976). This measure is in wide spread use in difference between 1 m and 230 m does provide a North America and is required as input into reasonable estimate of the occurrence and many computer dispersion models to simulate strength of deep inversions. Furthermore, there ground level concentrations of pollutants. is a good relationship between climatology of Where surface based inversions occurred, mix- inversions obtained from the thermograph data ing depth was taken as the top of the ground- and that obtained from acoustic sounder records. based echo shown in the acoustic sounder chart (usually 100 m to 300 m). As suggested by Russell and Uthe (1978) and Jones (1985), this procedure was also followed even when layered echoes, which indicated elevated inversions, were seen above the ground based layer. When no surface ACOUSTIC SOUNDER MEASUREMENTS based inversion was present, but elevated inver- A monostatic acoustic sounder, located at sions occurred, mixing depth was initially taken Aramoana (Fig. 1) measured inversion occur- as the height to the top of the echo. This rence during the measurement year. The acoustic procedure allows for some mixing within the sounder consists of two functional units. The first elevated inversion layer. An alternative inter- is an antenna which is used both as a transmitter pretation is to take the mixing depth as the height and as a receiver. As a transmitter, it converts a to the base of such an echo. The echo of an 1600 Hz electrical signal into a beam of sound elevated inversion is usually a band 60 m to 150 m energy that is propagated upwards into the thick, so the latter procedure would place the atmosphere. As a receiver, it detects echoes mixing depth that much lower. As this is the more reflected back from regions of thermal variations conservative procedure, mixing heights were in the atmosphere and converts them into elec- decreased by the mean monthly thickness of the trical signals. The second unit is a transceiver lowest echo to conform with the second method display with a time - height - intensity chart of interpretation. recorder. Sound scattered back from small scale Where multiple, elevated inversions were pre- temperature variations within the lowest 1000 m sent, but there was no surface based inversion, of the atmosphere is recorded on a continuous the mixing height was taken to the lowest layer, chart that displays the intensity of the return provided it was strong and persistent. Fainter, echo. A number of meteorological phenomena transitory echoes, which occasionally occurred can be observed by acoustic sounders (see Cronen- below this chosen layer were ignored, as the wett et al., 1971), but they are particularly useful airsonde and tethersonde calibrations usually for defining the presence, height and duration of indicated them to be weak and near isothermal. inversions and mixing heights (Wyckoff et al., When no surface or elevated inversions were 1973). Because of interference, no signal can be evident, mixing height was set to 1000 m, the interpreted from the lowest 50 m or so of the upper height range of the sounder. It is possible atmosphere, so that acoustic sounders detect only that this procedure could introduce an error if the deep inversions. Those shallow ones close to the mixing height was actually below the minimum ground were measured by temperature sensors range of the acoustic sounder (50-80 m), as might on a 50 m high tower and are discussed in happen during shallow nocturnal katabatic air Edwards and Isaac (1984a). flows (see Jones 1985). By comparing the fre- At various times, 71 airsonde or tethersonde quency of ground-based inversions as observed ascents were made at Aramoana in order to between 5 and 50 m on the tower (Edwards and calibrate the acoustic sounder chart records. In Isaac 1984a) with that observed on the sounder, it is estimated this error occurred no more than 7% this way commonly occurring and special features could be identified on the charts. of the time. 14 Assessing Air Poluti on Potential For one quarter of the time, information from DETERMINATION OF ATMOSPHERIC STABILITY the acoustic sounder was obscured by noise of Atmospheric stability is assessed using a vari- rain and strong wind, but these conditions do not ation of the classical Pasquill-Gifford-Turner often accompany inversions. When strong winds method, using 10 m wind speed (Edwards and blew 20 knots or 10 ms-1), heavy striations Isaac, 1984b) and net radiation data, both occurred on the chart. Echos were obscured measured at hourly intervals at Aramoana. Net during rainfall because of the noise of drops radiation was obtained with a CSIRO Swissteco hitting the receiver causing very dark echoes on net pyrradiometer one metre above a short grass the chart. surface. The scheme of Williamson and Kren- To check that inversions and hourly mixing mayer (1980), which gives stability class as a heights obtained from the acoustic sounder re- function of these parameters, was used directly. cord were reliable, comparisons were made with The scheme was developed for Missouri, U.S.A. information from the airsonde and tethersonde (latitude 38'N) where incoming solar radiation profiles, with inversion data from the thermo- levels are 10% higher and winter nights are graphs, and with inversions measured on the shorter than those measured at Aramoana. Stab- nearby 50 m tower. These calibration procedures ility is classified into 7 categories or classes enabled the recognition of the following types of ranging from A (highly unstable) to G (extremely echo between 80 m and 1000 m above ground: stable) with category D representing neutral elevated inversions, multiple elevated inversions, conditions. isothermal layers, ascending and descending in- Some more generalised indication of atmos- versions, surface based thermal plumes caused by pheric stability may be inferred from loss of net heating of the ground, occurrence of rain, and radiation at night, and from the strength of strong winds. These interpretations of the acous- incoming solar radiation during the day (Oke tic sounder traces were discussed with staff of the 1978). The latter was measured hourly at Ara- N. Z. Meteorological Service, other users at the moana by an upward facing Kipp and Zonen University of Canterbury, and by consulting pyranometer. Calibrations that were supplied by examples given in Hewson (1976), Kaimal et al. the manufacturers for radiation sensors were (1976), Russell and Uthe (1978), and Jones checked in the field by comparing their outputs (1985). On this basis, the acoustic sounder charts with those of reference instruments maintained were interpreted and read systematically at hour- for this purpose by the ly intervals with the elevation of various features noted to within ± 20 m. (Fitzharris et al., 1982a).

STAGNATION PERIODS When inversions are strong and persistent and winds are light, poor ventilation and dispersion is often the result (Oke, 1978). These conditions are RESULTS sometimes called stagnation periods or episodes. DEEP INVERSIONS IDENTIFIED FROM McCormick and Holzworth (1976) list the follow- THERMOGRAPHS ing criteria for stagnation periods which define a Deep inversions measured between Aramoana high meteorological potential for air pollution: and the western ridge as obtained from thermo- (a) mixing heights <1500 m. graph records, occurred for 13% of the time (b) average wind speeds< 4 ms-1, during the measurement year (Table 1), and were (c) no significant occurrence of precipitation, most frequent in autumn and winter. The major- and ity of inversions form at night, especially in (d) persistence of any of the above for at least 2 spring and winter. Summer inversions form at days. any time, while autumn inversions form mainly These criteria were applied to Aramoana for in the afternoon and evening. Most inversions are the measurement year, except that in (a) mixing short lived (Fig. 3) although a few can last more height was set at <1000 m, the maximum height than 24 hours especially in autumn and winter. for which information could be obtained from the The majority of spring and summer inversions acoustic sounder. (86%) lasted for -< 6 hours. Inversion strengths tend to be weak with two thirds having a temper- ature difference of < 2' C (Fig. 4). Assessing Air Polution Potential 15

_— 175

150

125

a iso

1-ds 75 155 2 50

25

9 10 II 12 13 14 15 16 17 18 19 20 21 22 23 >-24 l'0 20 30 40 50 60 70 80 90 100 11,0 DURATION (hours) STRENGTH (Al '6/230m)

Fig. 3: Frequency distribution of inversion periods as measured by Fig. 4: Frequency distribution of inversion strength between Ara- temperatures at Aramoana and the western ridge (April 1981 - moana and the western ridge (April 1981 - March 1982). March 1982).

TABLE 1: INVERSIONS MEASURED BETWEEN maps show that they are mainly produced during ARAMOANA AND THE WESTERN RIDGE (GIVEN periods when anticyclones are over or near New AS % OF TIME); DATA FOR TIWAI POINT IS SHOWN Zealand. A few are produced when depressions FOR COMPARISON lie to the east of Aramoana, or in the tar north. Aramoana Tiwai Point Most inversions of this strength are shortlived Month 1981-82 1972-79 with only 5 lasting for more than four hours during the course of the measurement year. Wind April 4 25 speeds tend to be <2ms-1 during strong deep May 32 28 inversions, but can exceed this if an elevated inversion is involved. On over half the 88 hours June 29 40 28 35 with strong inversions during the measurement July year, winds were calm, otherwise there was a August 8 36 10 24 tendency for wind to blow from the southwest or September northwest quarter suggesting katabatic flow October 11 13 down the western ridge. November 7 12 December 4 15 With sustained inversions (i.e. persistence 13 11 2 hours) and near calm conditions, a cloud of January emissions could build up near an industrial plant. February 8 17 When the wind subsequently strengthens, this March 9 18 cloud will move away and may give brief episodes of high concentrations downwind, in much the Season same way that Clarkson (1981) observed the Autumn 15 24 behaviour of tracer gas in the Otago Harbour Winter 22 37 after it was released from Aramoana. Over the 9 16 measurement year, there were 12 episodes when Spring strong deep inversions persisted for more than Summer 9 14 two hours. Six of these were associated with Year 13 23 winds from the north, three with winds from the southwest, two with winds from the west and one Source: Tiwai Point data from Bromley (1973) and N.Z. with winds from the east. No deep inversion Aluminium smelters (1980). however, remained strong for more than 11 hours, so a major fumigation seems unlikely. The fact that a large industry is likely to have a "hot" During the year, strong deep inversions (de- plume which would penetrate any shallow in- fined here as 6° C/230 m) occurred for 88 hours version and then loft above it as it moved or 1% of the time. Analysis of surface synoptic downstream, is a further mitigating factor. 16 Assessing Air Polution Potential INVERSIONS IDENTIFIED BY THE ACOUSTIC Otago Harbour does not always extend to the SOUNDER height of the western ridge. The sounder con- (a) Summary Analysis of the Sounder Record firmed that deep inversions tend to form more at night especially in winter and spring. The record displayed on the acoustic sounder chart for Aramoana is complicated and rapidly changing when compared with those reported for TABLE 2: FREQUENCY AND DEPTH OF GROUND- many other sites (eg Fitzharris et al., 1983, BASED INVERSIONS AS MEASURED BY THE Surridge, 1979a, 1979b, 1980). Many features are ACOUSTIC SOUNDER present, from the surface up to 1000 m, but they often tend to be transient. Before a detailed Month Occurrence Depth (m) analysis is made, some more general findings are (% of time) outlined: >80 m > 120 m Mean Std. Dev. 1. Surface inversions can extend from the April 1981 12 6 109 29 ground up to heights between 100 m and 460 May 31 26 154 59 m. They appear to be formed by radiational 21 134 46 cooling, cold air ponding, and advection of June 29 warmer marine air across a cooler land surface July 53 51 185 35 (Fitzharris, 1981). August 23 19 143 40 2. These surface inversions fluctuate in depth, September 32 23 130 34 and frequently almost break down, as shown October 18 15 138 34 by "holes" in the chart record. These holes November 5 2 117 31 occur semi-periodically at intervals of about December 11 6 120 27 90 minutes. 25 23 145 40 3. Elevated inversions are common between 200 January 1982 February 37 11 116 37 m and 800 m, especially during up harbour March 20 12 121 44 winds (that is those from the north or north- east), when they represent the boundary Season between cooler marine air, and warmer, drier air above. They then correspond with the Autumn 21 15 128 elevations of cloud tops seen over the Penin- Winter 35 30 154 sula and Mt. Cargill during many up-harbour Spring 18 13 128 winds. Other elevated inversions seem to be Summer 24 13 127 caused by subsidence within anticyclones that influence the area. Year 25 18 134 4. Elevated inversions and isothermal layers, 100 m to 400 m thick, sometimes descend, or rise from ground level to elevations above 1000 m, over periods of 8 hours or more. 5. Thermal plumes representing unstable, rising Two-thirds of inversions with depth 80 m air formed by solar heating of the Aramoana lasted for less than two hours (Fig. 5). Deep land platform, extend up to 500 m above the inversions persisting for more than 6 hours were surface. rare, but a very few ground-based inversions lasted for more than 24 hours. Ground-based (b) Ground-based Inversions (depth> 80 m) inversions are deepest (mean depth 154 m) and The acoustic sounder recognised a higher most persistent in winter. percentage of ground-based inversions (25%, see A detailed analysis of ground-based inversions Table 2) than the thermograph on the western 120 m indicates that their frequency is ridge (13%). This occurred because the latter highest in winter, and lowest in spring and cannot detect some inversions that are too shallow summer (Table 2). Ground-based inversions to reach the ridge top, and because the frequency extending to above 120 m occurred for 18% of the of inversion occurrence decreases with height. time. Similar inversions at Tiwai Point (measured There is a strong tendency for deep inversions to between temperature sensors at 5 and 128 m) have their tops at 100 m to 140 m. Therefore, the occurred for 23% of the time from 1972 - 1979 depth of cold air trapped in the trough of the (N.Z. Aluminium Smelters 1980) (see Table 1). Assessing Air Polution Potential 17 210

180

150 25

izo 20 c,'a 90 15 a 60 cr 1

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•<1 1 Z 3 4 6 7 8 9 10 11 12 13 14 15 16 17 18 19 2021 02 2324 74 0 1 Z 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 ZO 71 22 23 Z DURATION (hours) DURATION (HOURS) Fig. 5: Frequency distribution of duration of ground-based inversions Fig. 6: Frequency distribution of duration of elevated inversions 80 m (April 1981 - March 1982) as measured by acousttc 80 m (April 1981 - March 1982) as measured by acoustic sounder. sounder. (c) Elevated Inversions autumn. Single and multiple events occur with similar frequency. As suggested by Fitzharris and Cosgrove (1980), elevated inversions are frequent at all Elevated inversions have bases that are largely above 240 m with the average height at almost 400 times of the year above Aramoana (Table 3). m. Bases tend to be lowest in autumn, but within They are slightly more common in autumn, and less common in summer, but overall are present any event, the inversion base sometimes varies for one-third of the time. While elevated inver- over a wide range of levels often undulating with a marked wave like motion. Few elevated inver- sions average 11 hours in duration, individual sions ascend from the surface, but about one third events vary widely (Fig. 6). Seasonally they last descend down to a ground-based inversion. an average of 12.7 hours in spring but 9.8 hours in About half show no distinct tendency to ascend or descend. The modal thickness of the elevated TABLE 3: ELEVATED INVERSIONS AS MEASURED inversion echo averages 115 m, but is also variable BY THE ACOUSTIC SOUNDER (Table 3). Echoes tend to be thicker in spring, No. of % of Mean Mean when more complicated patterns are frequently Month Periods time height of thickness recorded. The majority of elevated echoes inver- base (m) (m) sions (38%) occur with up-harbour winds, but also occur with down-harbour and cross-harbour April 1981 25 24 344 114 winds, and during conditions when wind direc- May 24 47 370 103 tion changes. June 24 38 360 93 From analysis of vertical soundings of the July 28 30 371 99 lower atmosphere at Aramoana (Fitzharris et al., August 19 29 454 113 1982c), elevated inversions are often weak, with September 30 29 399 129 temperature gradients of less than 2° C/100 m October 19 35 405 144 and changes in the water vapour mixing ratio of November 20 48 392 130 less than lgkg-1. Fewer than 7% of elevated December 24 36 422 123 inversions sampled by airsondes or tethersonde January 1982 16 27 425 115 had temperature differences of more than V C. These inversions represented the boundary sep- February 15 19 361 109 32 34 413 104 arating the cooler moister air advected over the March Aramoana land platform from the ocean and Season drier, warmer air aloft, either within a northwest flow that had passed over land to the west, or that Autumn 81 35 376 107 had subsided within a nearby anticyclone. Winter 71 32 395 102 Spring 69 38 399 134 MIXING HEIGHTS Summer 55 27 403 116 For 27% of the time, mixing height was in the range of 300 m to 600 m above the surface, which 33 393 115 Year 276 largely reflects the influence of elevated inver- 18 Assessing Air Polution Potential + + + +11•+ + + + + -H- + 1000 + + + ++ + -H- + ++ + + + -HH+ --e

800

Cr%

+ + 600

+ + + + +4- + + + -Hi- +I- + + + + ++ 400 ++ + + + + + + + + + + + + + ++ + + + + + + + + + 200 + -H- +.4. + + +4- + + + + + + + + -IH- + + + + + + + + ++ + + + 41- + + +

c) APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR Fig. 7: Mixing height (m) at 0900 hours (Values at 1000 m represent the upper limit of measurement of the acoustic sounder). sions. Mixing height fell to below 300 m for 23% situation which usually produces tongues of fog and to less than 200 m for 14% of the time, with moving up the harbour as far as this latter figure in close agreement with the 13% and even Dunedin. deep inversion frequency obtained independent- Mixing heights greater than 500 m occur for ly using the thermographs (Table 1). From the 25% of the time and with a variety of wind acoustic sounder analysis, it is apparent that low directions. In these conditions, there is likely to mixing heights at Aramoana are caused by form- be little restriction of vertical dispersion. Mixing ation of nocturnal ground-based inversions, and heights during strong winds (10 ms-'), which occasionally by descent of elevated inversions to occur for a further 15% of the time, are not the surface or to ground-based inversions near known, but dispersion is also unlikely to be the surface. The lowest mixing heights assessed restricted unless the plume is blowing directly from the acoustic sounder record occur in sum- toward nearby terrain. mer. They are frequently below 200 m (25% of the time) or 300 m (35% of the time). Day to day Average hourly mixing heights over the year mixing height however, is extremely variable (see are lowest at 0200 hours (mean, 554 m) and Fig. 7) and tends to over-ride any seasonal or highest at noon (mean, 675 m). The spring and diurnal pattern. summer diurnal patterns are similar to this, but in autumn the mixing height is highest at 1000 The combination of up-harbour winds and hours (mean, 720 m) and lowest at 1800 hours shallow mixing heights is critical in assessing the (mean, 574 m). In winter the highest mixing air pollution potential of the site, as these con- heights tend to occur in early evening (1800 hours ditions could restrict dispersion. This condition mean, 700 m) and the lowest in early morning occurs for 14% and 9% of the time for mixing (0400 hours mean, 544 m). Early morning mixing heights below 300 m and 200 m, respectively. heights average more than 550 m, which is higher Up-harbour winds and mixing heights below 100 than data reported for much of the interior m are rare (less than 2% of the time) and is a U.S.A. (McCormick and Holzworth, 1976). Asses sing Air Polution Potential 19

STAGNATION CONDITIONS excess of 60 per year in some mountain States. It appears therefore that at Aramoana wind speeds Stagnation days numbered 19 during the are sufficiently high, mixing heights are seldom measurement year with most in summer (7) and persistently low, and rainfall is frequent enough winter (6), followed by autumn and spring (3 to ensure that stagnation periods are relatively each). There were only three stagnation episodes rare and shortlived. 2 consecutive stagnation days) during the year, with two in December 1981 and one in February 1982. No stagnation period lasted more ATMOSPHERIC STABILITY than three days. This corresponds to a total of 7 The percentage frequency of stability categ- "stagnation episode days", which is low com- ories for various wind directions is called a pared with values given by McCormick and stardata set. At Aramoana, the stardata set de- Holzworth (1976 their Fig. 8) for the U.S.A. rived from net radiation and wind speed data where they range from 0-10 per year in the shows neutral conditions over half the time, and Midwest, to 20-40 along the Pacific Coast and in stable conditions for just under one third of the

TABLE 4: FREQUENCY OF STABILITY CLASS BY WIND DIRECTION CALCULATED FROM HOURLY WIND SPEED AND NET RADIATION DATA ( n=7630)

Wind Pasquill Stability Category CA frequency) direction A B C D E F G F+G E+F+G

Calms 0.0 0.2 0.3 0.3 0.0 0.0 1.9 1.9 1.9 0.0 0.1 1.5 10.9 4.0 1.4 1.4 2.8 6.7 NE 0.1 0.8 3.2 12.4 1.8 0.8 1.1 1.9 3.6 0.0 0.3 0.8 1.1 0.2 0.2 0.6 0.9 1.0 SE 0.0 0.2 0.5 0.9 0.2 0.1 0.3 0.4 0.6 0.1 0.2 0.4 3.3 0.3 0.3 0.4 0.7 1.1 SW 0.1 0.6 3.0 17.3 2.6 1.0 1.5 2.6 5.1 0.0 0.1 0.8 5.5 2.6 1.4 2.2 3.6 6.2 NW 0.0 0.2 0.7 2.6 1.6 1.1 2.1 3.3 4.9

Totals 0.4 2.9 11.2 54.3 13.3 6.4 11.5 17.9 31.2

TABLE 5: MONTHLY STABILITY FREQUENCY CALCULATED FROM HOURLY WIND SPEED AND NET RADIATION DATA

Stability Class (% frequency) Month A B C D E F F+G E+F+G

April 1981 2 4 64 14 8 8 16 30 May 3 7 48 11 7 24 31 42 June 1 5 62 9 8 15 23 32 July 1 6 49 15 9 20 29 44 August 1 4 64 18 5 8 13 31 September 1 6 58 20 6 9 15 35 October 1 3 15 47 18 6 10 16 November 4 22 54 8 5 7 12 20 December 6 15 57 8 6 8 14 22 January 1982 1 4 19 50 12 5 9 14 36 February 1 5 16 52 10 7 9 16 36 March 1 3 14 52 17 6 7 13 30 20 Assessing Air Polution Potential time (Table 4). For 69% of the time atmospheric (Wratt and Holmes, 1984a), and Ohaaki (Wratt stability would not restrict dispersion (i.e. cate- and Holmes, 1984b) heights to the top of ground- gories A + B + C + D), but would markedly do so based inversions generally range between 100 m (F + G) for 18%, with some lesser limitation (E) and 350 m. This type of inversion at Aramoana for a further 13% of the time. appears to be not as deep, extending to less than 200 m on average, although elevated inversions Category D occurs frequently with both up- are an important consideration within a few harbour and down-harbour winds. Major limit- hundred metres above this height. At Cromwell, ations to dispersion (F +G) usually occur with located in a deep intermontane basin, ground- calms or katabatic flows from the western ridge as based inversions measured from June to October indicated by west, northwest and north flows. 1980 with an acoustic sounder occurred for more Up-harbour winds are accompanied by stable or than 40% of the time, and extended to 180 m to very stable atmospheric conditions for 15% of the 500 m above the surface (Fitzharris et al., 1983). time. Atmospheric stability would tend to impose Very marked multiple layering on the chart major limitations to dispersion mainly in May, echoes indicated they were more intense than at June and July (Table 5). During the growing the coastal Aramoana location. In mid-winter season, when pollutants may damage grass and many lasted for much of the day and often broke crops, such conditions prevail for less than 17% of up for only a few hours in mid-afternoon, whereas the time. Atmospheric stability would not restrict at Aramoana inversions seldom persisted for dispersion during more than 70% of the growing more than six hours and usually dissipated well season. before noon. Hourly analyses of incoming solar radiation and net radiation showed that both are strongly Direct comparison of ground-based inversion influenced by the frequent occurrence of partly frequencies among different locations in New cloudy conditions typical of the climatic regime Zealand poses considerable difficulties. Some of southern coastal . Strong insol- sites use towers, others compare temperature ation (here defined as 400 Wm-2) occurred for readings at valley bottoms with those at hill top only 14% of the time. Q* was less than zero for 9% level, and some rely on the more qualitative of daily totals, mainly in the period April to record from acoustic sounders. Towers provide September. During these days, no thermal turb- precise data but are usually too short to detect the ulence can be expected, and little is likely when it top of inversions, and temperature sensors have is positive but small (Oke 1978), say (r< 2 been at inconsistent heights. Hill measurements Mjm-2day-1(a further 27% of days). Consequent- may not detect inversions which are shallow ly, on about one third of days the radiation regime relative to the hill height and may be different suggests some degree of atmospheric stability from free air temperatures. Acoustic sounders do when dispersion must rely on mechanical turb- not detect inversions that are shallower than ulence created by wind. about 50 m. At Aramoana, all three methods were used. Inversion frequencies over the measure- An analysis of hourly values of Q*<- 0, which ment year were 32% of the time for tower occurred for 52% of the time over the measur- measurements between 5 and 50 m (Edwards and ement year, showed that half ranged between 0 Isaac (1984a), 25% of the time for sounder and -30 Wm-2 and three quarters between 0 and measurements from 50 to 80 m, 18% of the time -60 Wm-2 and less than 0.1% of the time for sounder measurements from 50 to 120 m, and than -130 Wm-2 and less than 0.1% of the time 13% of the time for hill and valley measurements was it lower than -100 Wm-2. Therefore the loss between 1 and 230 m. These results appear to be of radiation at night is usually small in this consistent, with inversion frequency decreasing cloudy, humid location and would appear to be logarithmically with height. insufficient to develop strong surface inversions, except on rare, clear winter nights which occur The hill thermograph data (1 to 230 m) and about six times a year at Aramoana. acoustic sounder data (50 to 120 m) have been analysed in a similar way to temperature dif- DISCUSSION ferences measured on the stack at Tiwai Point (5 A number of other studies have measured to 128 m, as in Bromley 1973). While noting the inversions in New Zealand. At Lower Hutt problems of direct comparison discussed above, (Thompson, 1973), Upper Hutt (Wratt et al., it appears that the climatology of deep inversions 1984), South Auckland (de Lisle, 1966), Motunui at Aramoana is not much different to that at Assessing Air Polution Potential 21

Tiwai Point (23% of the time), where there is a 230 m occurred for 13% of the time. Inversion large aluminium smelter. There is a suggestion frequency decreased logarithmically with height. that the frequency of deep inversions at Ara- Most deep inversions were weak and short lived. moana is higher in spring and summer, but less in Strong deep inversions ( >6' C/230 m) occurred autumn and winter. During winter, Aramoana for 88 hours or 1% of the time. Those lasting more inversions are largely a nocturnal phenomena, than two hours were followed by up-harbour whereas they tend to persist during the day at winds on only seven occasions over the measure- Tiwai Point. Thus inversions at Tiwai Point tend ment year. During these conditions dispersion to be of longer duration. from an industrial plant could be limited, de- There are several reasons for a relatively low pending upon plume rise relative to the inversion frequency of thermal stratification at Aramoana: height. the topography does not favour ponding of cold Elevated inversions at Aramoana represent the air because it can leak away towards the ocean; the boundary separating cooler, moister marine air frequent passage of frontal systems ensures that near the surface, and slightly warmer, drier air the wind usually blows and the lower atmosphere aloft that has passed over land to the west. They is mixed; the proximity of the sea ensures that are frequent at all times of the year, and have advected air will initially be no cooler than the sea bases generally above 240 m with an average surface temperature; and finally, frequent clouds height of 400 m. Most of these inversions are prevent strong night-time radiation losses. weak, with temperature gradients of less than The results from the analyses of atmospheric 2° C/100 m and mixing ration changes of less stability, mixing heights, and stagnation episodes than 1 gkg-1. Approximately one third of elevated all indicate that Aramoana has some advantages inversions descend to a ground-based inversion. for dispersion of pollutants. However, the terrain Average hourly mixing heights vary diurnally in the area includes hills, a harbour trough, and between 554 m and 657 m. Lowest mixing coastlines and is so complex that this assessment heights tend to occur in summer, but variations in has to be qualified. For example, highest concen- day to day mixing height are large and dominate trations of emissions from a tall stack can occur over diurnal or seasonal patterns. The combin- under very unstable conditions (class A or B) if ation of up-harbour winds and mixing heights material is brought down to ground level in below 200 m, which will result in restricted convective or lee eddy down-drafts. On the other dispersion at Aramoana, is relatively infrequent, hand, if a tall stack emits a plume above an occurring 9% of the time. inversion, it will loft away and never reach the surface. If the emissions are close to the ground, Atmospheric stability, as determined from net then they could be trapped by a higher inversion radiation and wind data, indicates that it would and the walls of the harbour trough. Further not restrict dispersion for 69% of the time, but complications arise if the industrial source re- would do so for 18% of the time with some lesser limitation for a further 13%. There were seven leases sufficient heat to modify or even break up inversions. Dispersion can still be unsatisfactory stagnation episode days over the measurement even under neutral stability (class D) when a year, which is a low number compared with those strong wind persistently blows a plume directly quoted for much of the U.S.A. onto a hill. Thus a full assessment of dispersion from Aramoana needs to include information on local wind flows about the harbour trough and on engineering design of any proposed industry. ACKNOWLEDGEMENTS Funding for this research was provided by South Pacific Aluminium Ltd, with some logist- ical assistance from the Otago Harbour Board. We gratefully acknowledge the grant from the CONCLUSIONS New Zealand Meteorological Service to assist Over a one year period at Aramoana deep with editing, and also the wind data made surface inversions that extended up to 80 m and available by Dr P. J. Edwards. Mason Stretch 120 m elevation occurred for 25% and 18% of the performed much of the final preparation of the time respectively, while those extending up to manuscript. 22 Assessing Air Polution Potential

REFERENCES Thompson, D.C., 1973: Air pollution meteorology of the Lower Hutt Valley Wellington, N.Z. Meteorological Ser- Clarkson, T.S., 1981: An experimental evaluation of air vice Technical Note 223, 38pp. pollution potential at Aramaoana. N.Z. Meteorological Williamson, H.J. and Krenmayer, KR., 1980: Analysis of the Service Technical Note 246, 33pp. relationship between Turner's stability classifications and Cronenwett, W.T., Walker, GB., and Inman, R.L., 1972: wind speed and direct measurements of net radiation, Acoustic sounding of meteorological phenomenon in the Proceedings, 2nd. Joint Conference on Application of Air planetary boundary layer,lournal of Applied Meteorology Pollution, Meteorology American Meteorological Society, 11, 1351-55. 777-780. de Lisle, J.F., 1966: Temperature inversions over South Wratt, D.S. and Homes, L.F., 1984a: Measurements on a 30 Auckland. N.Z. Meteorological Service Technical Note 164, on tower at the New Zealand Synthetic fuels corporation 5PP• plant at Motunui with applications to atmospheric dis- Fitzharris, B.B. and Edwards, P.j., 1982: Meteorological persion. N.Z. Meteorological Service Scientific Report 7, measurements for the second aluminium smelter at 7Opp. Aramoana. Proceedings Tenth N.Z. Geography Conference, Wratt, D.S. and Homes, L.F., 1984b: Measurements on a 70 Wellington, 1981: 199-201. m tower at Ohaaki with applications to air pollution Fitzharris, RB., Symon, TB. and Cosgrove, CM., 1988: meteorology. N.Z. Meteorological Service Scientific Report Assessing air pollution potential at a large industrial site 9, 89pp. —Wind in the Otago Harbour as assessed by tetroons. Wratt, D.S., Salinger, M.j., Clarkson, T.S., Imrie, B.W., Paper in preparation. Bromley, A.M. and Lechner, I.S., 1984: Airflow and Fitzharris, B.B., Turner, A. and McKinley, W., 1983: Cold pollution dispersion in a valley, THe Upper Hutt study. season inversion frequencies as measured by acoustic N.Z. Meteorological Service Scientific Report 4, 66pp. sounder in the Cromwell basin. N.Z. Journal of Science 26, Wyckoff, RJ., Beran, D.W., and Hall, F.F., 1973: A com- 307-313. parison of the low-level radiosonde and the acoustic echo Hewson, E.W., 1976: Meteorological Measurements, in Stern sounder for monitoring atmospheric stability, Journal of A.C. (editor) Air Pollution, Academic Press, Vol I, Applied Meteorology 12, 1196-1204. 563-642. Jones, D.E., 1985. Mixing depth in the Latrobe Valley, Clean Air 19, 49-51. Kaimal, J.C., Wyngaard, J.C., Haugen, D.A., Cote, OR., and Izumi, F., 1976: Turbulence structure in the con- vective boundary layer, journal of Atmospheric Science 33: REFERENCE IS ALSO MADE TO THE FOLLOWING 2152-69. UNPUBLISHED MATERIAL: Lusk, CB., 1967: Temperature inversions over Taieri. N.Z. Meteorological Service Technical Note 169, 4pp. Bromley, A.M., 1973: Temperature inversions at Tiwai Point McCormick, R.A. and Holzworth, G.C., 1976: Air pollution 1972. N.Z. Meteorological Service Report. climatology, in Stern, A.C. (editor) Air Pollution, Aca- Edwards P.J. and Isaac P., 1984a: Ground-based inversions at demic Press Vol I, 643-700. Aramoana. Report from Physics Department, University of N.Z. Aluminium Smelters, 1980: Expansion of Tiwai Point Otago to N.Z. Meteorological Service. Report 4. Aluminium smelter to Three Pot-lines — Environmental Edwards P.J. and Isaac P., 1984b: Surface airflow in the Statement, 71pp. Lower Otago Harbour. Report from Physics Department, Oke, T.R., 1978: Boundary Layer Climates, Methuen and Co, University of Otago to N.Z. Meteorological Service. Report 372pp. 3. Russell, P.B. and Uthe E.E., 1978: Sodar network measure- Fitzharris, B.B., 1981: Advection temperature profiles and ments of regional mixing depth and stability patterns for possible plume behaviour at Aramoana. University of an air quality model, in Fourth Symposium on Meteor- Otago Department of Geography, Discussion paper. ological Observations and Instrumentation Boulder, Color- Fitzharris, B.B. and Cosgrove, CM., 1980: An assessment of ado, American Meteorological Society: 490-497. air pollution dispersion at Aramoana. University of Otago South Pacific Aluminium and Otago Harbour Board., 1981: Business Development Centre Report. Proposed aluminium smelter at Aramoana — Environ- Fitzharris, B.B., Cosgrove, C.M. and Symon, j.B., 1982a: mental Impact Report, 284pp. The radiation climatology of Aramoana. University of Surridge, A.D., 1979a. Preliminary experiments with a Otago Department of Geography, Discussion paper. mobile acoustic sounder in Cromwell during May 1979. Fitzharris, JIB., Cosgrove, CM. and Symon, 1982b: DSIR Physics and Engineering Laboratory Report 648, Rainfall and temperature at Aramoana. University of 36pp. Otago Department of Geography, Dicussion paper. Surridge, A.D., 1979'u: Acoustic sounder operation in Christ- Fitzharris, 13.B., Cosgrove, CM. and Symcm, 1.B., 1982c: church New Zealand, in 1978. N.Z. Journal of Science 22, Vertical soundings of the lower atmosphere at Aramoana. 77-86. University of Otago, Department of Geography, Discussion Surridge, A.D., 1980: Examples of wind shear and temper- paper. ature inversion surfaces over Christchurch. N.Z. Journal Jones, MT., 1981: Computer modelling study of the air of Science 23, 283-288. dispersion from an aluminium smelter at Aramoana, Unpublished N.Z. Department of Health Report.