A T M O S P H E R I C A C I D I F I C A T I O N IN T H E ASIAN R E G I O N G. P. AYERS*

Division of Atmospheric Research, CSIRO, PB 1, Mordialloc, Vic. 3195, Australia (Received November 1990) Abstract. Atmospheric acidification in the Asian region is discussed from the perspectives of currently available regional measurements, and the knowledge now available from several decades of acidic deposition research in the northern mid-latitudes. The main conclusions emerge: (1) that there is insufficient information currently available to enable a quantitative assessment of the present state or future potential for atmospheric acidification across the whole region; and (2) that within the limitations imposed by (1) the possibility of future acidification in certain areas cannot be ruled out if economic development and energy use on a per capita basis evolve to the levels of the major industrial countries. These two conclusions point to the need for systematic, multidisciplinary studies covering the whole region. The studies should assess quantitatively the current levels of acidic and alkaline emissions (both natural and anthropogenic) to the atmosphere, identify the relevant chemical transformations and transport/deposition pathways in the regional atmosphere, and assess the susceptibility of regional plants, soils and groundwaters to acidification.

I. Introduction An awareness of the phenomenon of acidic deposition, popularly known as 'acid rain', entered the public consciousness in the early 1970s, most notably in Stockholm in 1972 at the UN Conference on the Human Environment. Here the work of the Swedish chemist Oden was publicised: his analysis of precipitation composition data from more than 150 sites of the European Air Chemistry network led him to conclude that there had been a significant acidification of European precipitation over the period 1956 to 1966 (Oden, 1968; Engstrom 1972). Similar conclusions were soon reached for the northeastern parts of the north American continent (e.g. by Likens and Bormann, 1974). The conclusions reached in these early works were that growing anthropogenic emissions of sulfur dioxide and oxides of nitrogen in these heavily industrialised regions were the cause of the observed secular trend in acidity, produced by chemical conversion of these gases in the atmosphere to the respective strong acids (sulfuric and nitric). It was further concluded that the levels of acidity had reached the point where they posed a serious threat to sensitive environments. Over the years there has been some questioning of these and related conclusions (e.g. by Stensland and Semonin, 1982; Kallend et al., 1983), however it is now generally seen as indisputable that there have been very large perturbations to the natural atmospheric cycles of sulfur and nitrogen in heavily industrialised regions, and that resultant increases in atmospheric acidity have a number of quite adverse environmental consequences including deleterious effects upon sensitive soils, plants, aquatic ecosystems, and building materials. Historical perspectives on the acidic deposition phenomenon have been given by Cowling (1982) and Gorham (1989), while recent reviews by Mohnen (1988) and Schwartz (1989) summarise the current understanding from an atmospheric chemistry perspective. * Plenary speaker. Environmental Monitoring and Assessment 19: 225-250, 1991. 9 1991 Kluwer Academic Publishers. Printed in the Netherlands.

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The situation outside the populated regions of Europe and the US/Canada has received very little scientific scrutiny in comparison with the resources that have been allocated to acidic deposition research and evaluation in these two regions. Yet based on the European and North American experience the message for other parts of the globe seems clear: where anthropogenic activities add significant quantities of SOR and NOx to the atmosphere, acidification of the environment may follow, and adverse environmental consequences cannot be ruled out. The situation in Asia is a case in point, and is the focus of the discussion in this paper. The large population of Asia, significant rate of population growth in the region, and current emphasis on agricultural and economic development in many of the countries of the region virtually ensures that large increases in SO2 and NOr emissions will occur in the decades to come. In light of experiences in the northern mid-latitudes it seems very pertinent now to ask the questions: what is the current state of atmospheric acidification in the Asian region, and what is the potential for future acidification and acidification-related environmental problems?

2. Background Before moving into a discussion of the current state of knowledge about the Asian region it is worth setting the scene with some background information. Note that the 'Asian Region' discussed here is defined as the countries from Pakistan in the west, through to China and Japan in the east, south to Australia and New Zealand. In other words the area considered covers the tropical and sub-tropical parts of central/eastern Asia and Oceania, and specifically excludes the Soviet Union. 2.1. PROCESSES 2.1.1. Emissions

The prime anthropogenic emissions leading to atmospheric acidification are the gaseous species SO2 and NOx (----NO + NOR) which are released in large quantities by controlled combustion processes (NOx mostly as NO) - thermal power stations fired with fossil fuels, internal combustion engines (petrol and diesel), industrial/mineral refining and smelting processes, etc. (Mohnen 1988; Schwartz 1989; Galloway 1989). Emission rates from such sources can be estimated from (measured) sulfur contents, measured source-specific NOx emission factors (e.g. Logan, 1983), and a knowledge of the rates of fuel consumption. Emission inventories for SO2 and NOx at scales from regional up to global, and times back into last century, have been based on such information (e.g. Dignon and Hameed, 1989; Galloway, 1989). The fact that these combustion sources tend to be concentrated in populous urban/industrial regions ensures that it is these regions in which acidic deposition has been most evident, prime examples being the industrialised regions of central Europe and the northeastern part of north America (Mohnen 1988; Schwartz 1989; Galloway 1989). The perturbations to the atmospheric sulfur and nitrogen cycles in such regions can be very large. For example Galloway et al. (1984) compared precipitation composition data

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from the five sites in eastern North America with data from five remote, unpolluted, continental sites elsewhere on the globe, and found that the rainwater nitrate and sulfate levels were enhanced an order of magnitude or more in the North American rain. Several-fold increases in rainwater free acidity, [H+], accompanied the elevated sulfate and nitrate concentrations. Another perspective showing the extent of the perturbations to the atmospheric S and N cycles is given by Schwartz (1989): he suggests that evidence of the effects upon aquatic ecosystems of acidic sulfate and nitrate deposition point to the need to limit these depositions (a 'deposition standard'?) to 20-40 mmole/m2/y for sulfate and double this for nitrate, whereas in eastern North America regional average emissions currently run at 130 and 120 mmole/m2/y, respectively. The imbalance is clear. It is pertinent to note that sources of reactive nitrogen and sulfur other than urban/industrial sources should not be ignored when discussing the question of acidic deposition. For example emissions of nitrogen and sulfur species from biological/agricultural sources and biomass burning sources may not be negligible, particularly in some tropical regions (e.g. estimates by Ayers and Giltett, 1988a). Emissions of alkaline species such as ammonia, and calcareous aerosols also need to be considered - these may have natural as well as anthropogenic sources. Finally emissions of reactive species other than acids, bases or their precursors can be important to atmospheric acidity where these emissions affect the oxidising capacity of the atmosphere. A prime example is emission of reactive hydrocarbons (HCs), which may have a multiplicity of anthropogenic and natural sources (fugitive emissions from fuels, solvents and a variety of industrial processes, automotive exhausts, biological emissions, biomass burning emissions, etc.). Among other things, reactive HCs participate strongly in the atmospheric oxidant cycle (Atkinson, 1990), which has a major role in the conversion of SO2 and NOx into the respective strong acids.

2.1.2. Chemical Transformations The primary emissions of SO2 and NO are neither strongly acidic, nor particularly soluble in water; indeed NO is relatively insoluble. The acidification of cloud and rainwater results from conversion of these 'acid precursors' in the atmosphere into sulfuric and nitric acids, both strong, highly soluble mineral acids. There is a variety of oxidation pathways, both gas phase and aqueous phase by which the oxidation may occur (see the volume edited by Calvert, 1984). The pathways thought to be most important in the northern mid-latitude regions currently impacted by acidic rain are listed in Table I. The gas phase oxidation of SO2 and NOx by- OH radical provides a major sink for both species (and source of the respective strong acids) under cloud-free, daytime conditions, with effective second order rate constants of about 1 X 10-~2and 10 )< 10-12cm 3molec -1 s-~, respectively (Calvert and Stockwell, 1984). The sufuric acid produced is relatively involatile and exists in the atmosphere primarily in the form of a sub-micrometer aerosol. On the other hand nitric acid can exist in considerable quantities in the gas phase (Stelson and Seinfeld, 1982, and references therein). Theoretical estimates by Calvert and Stockwell (1984) of oxidation rates under mid-latitude conditions of high (mid-day, mid-summer)-OH concentrations (of order 107 molec cm -3) gave oxidation rates of 4 +__

G . P. A Y E R S

228

TABLE I Some important chemical pathways for a t m o s p h e r i c S O 2 a n d N O x. Gas phase 9O H + S O 2 + 0 2 + H : O ~ H2SO 44- H O 2. NO + 03 ~ NO 2 + 02 N O + H O 2" - - N O 2 4- 9O H N O 2 + 9O H ~ H N O 3 NO 24-O 3~No 34-O 2 N O 3 4- N O 2 r N205 Aqueous phase H S O 3- 4- H 2 0 2 ~ SO42- 4- H + 4- H 2 0 SO32 + 0 3 ~ SO42- 4- 0 2 N205 4- H 2 0 ~ 2 H + 4- 2 N O 3 H 2 S O 4 ~ 2 H + 4- SO42HNO 3 ~ H § + NO 3

2% h -l for SO2 and 34 _ 17% h -l for NOR. For. O H concentrations a factor of 4 lower the SO2 oxidation rate dropped to 1 _ 0.5% h -l while for NOR the rate dropped to 18 _ 9% h -1. By contrast the aqueous phase oxidation pathways can be orders of magnitude faster, with the oxidation of dissolved SO2 by H 2 0 2 in clouds and rain potentially occurring at rates measured in percent per minute (Martin, 1984). Homogeneous gas phase oxidation by. O H is a maximum in mid-day when insolation is maximum and thus 9O H production is greatest, and falls to zero at night when 9O H concentration goes to zero. Seasonal variations in. O H concentration occur as well at mid to high latitudes where insolation varies significantly with season. Crutzen (1982) shows theoretical estimates of- OH concentration virtually independent of month, at about 2 X 106 cm -3 (daytime average), in the latitude band 0 ~ - 10~ but varying from about 1.5 X 106 cm -3 in summer down to 0.5 X 106 cm -3 in winter at latitude 40 ~ Another reaction subject to strong periodicities is the oxidation of NO2 to NO3 by 03, followed by reaction with NO2 to yield Y 2 0 5 . This reaction is important only at night, as N205 is readily photolysed during the sunlit hours. Thus overall lifetimes of SO2 and NOx against oxidation to the respective acids can be complex functions of time of day, latitude and season, and oxidising species. Additional complexities can arise if other oxidation pathways not mentioned here take on importance (Hoffman and Jacob, 1984), one example being the aqueous phase oxidation of dissolved SO2 by 03, which is a pH-limited reaction pathway, but can be extremely rapid if cloud or rainwater p H is much above 5 (Penkett et aL, 1979).

2.1.3. Transport With lifetimes against oxidation of order hours to days depending on latitude, time of year and cloudiness, it is not surprising that acidic deposition is not confined to regions immediately adjacent to strong sources of SO2 and NOx (Schwartz, 1989). The overall atmospheric lifetimes of NOx and SO2 are often estimated to be about 1 and 3 days, respectively (e.g. Rodhe et al., 1981; Schwartz, 1989). Thus with typical horizontal wind

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speeds of order a few m s-~ in many regions of the globe, transport of significant quantities of SO2 and NOx over distances of some hundreds to a thousand km or more can be anticipated before oxidation to the strong acids and subsequent deposition. Clearly the effects of acidic deposition may be exported well away from the source region. A notable example is the case of the northern European and Scandinavian countries, in which acidic deposition is dominated not by local emissions but transport of SO2 and NOx from the strong source region ofindustrialised central Europe. For example B o w et al. (1989) show figures of 77% of S deposition in the Netherlands originating abroad, and 67% of N deposition originating abroad. Another notable example is the case of Whiteface Mountain, NY, which has no local sources but is significantly impacted by acidic emissions from the southwest industrial/urban source regions (Mohnen, 1989). It has become apparent only recently that in addition to long-range transport via horizontal advection, vertical transport to the upper troposphere of pollutants emitted near the surface may be very rapid in strongly convective cloud systems. (Gidel, 1983; Chatfield and Crutzen, 1984). Chatfield and Crutzen (1984) drew the conclusions that this process is likely to be of importance in "rainy tropical jungles and mid-latitude industrial regions, since both regions have large sulfur emissions and frequently active cumulonimbus convection". They also concluded that "once in the upper troposphere, substantial long-distance transport beyond the synoptic scale is possible". Z 1.4. Deposition

Three pathways have been demonstrated as important for the deposition of atmospheric acids - wet deposition (rain/snow), dry deposition, and 'occult precipitation' (impaction of acidic cloud/fog droplets directly at the earth's surface). Soluble aerosols such as acidic sulfate aerosol are readily incorporated into cloud at the cloud droplet nucleation stage, where such soluble particles are very effective as the cloud condensation nuclei upon which the cloud droplets form. This is a very efficient mechanism for incorporating aerosol mass into clouds, typically incorporating all the aerosol material larger than about 0.1 ~m (Jensen and Charlson, 1984). Scavenging of highly soluble gases and very small particles in-cloud is also very efficient, as Brownian diffusion is a rapid transport path. This can be seen from a simple calculation based on Maxwellian diffusion (Twomey, 1977), which would have n = no exp (--4rcRmNDT)

(1)

where n is the atmospheric concentration of gas molecules (or tiny particles) having diffusion coefficient D,R,, is the mean radius of the cloud droplets to which the molecules are diffusing, Nis the cloud droplet concentration (number per unit volume) and t is time. For typical cloud conditions just above the condensation level (Pruppacher and Klett, 1978), R,, is of order 5 - 10/~m, N of order 100 cm -3, D of order 0.1 cm 2 s-a, so this simple calculation indicates that diffusional scavenging of highly soluble gases, such as nitric acid, would be complete in just a few tens of seconds. More rigourous numerical calculations confirm this conclusion (e.g. Ayers and Larson, 1990). The advent of precipitation from the cloud then leads to wet deposition of the aerosol

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and gas material scavenged by the cloud droplets. Additional material is incorporated into raindrops falling through the sub-cloud layer, which may be heavily polluted. Soluble gases again find their way into raindrops via diffusional transport, while aerosol particle larger than about 1/am are efficiently scavenged by impaction (Slinn, 1982). As mentioned earlier the aqueous phase in the atmosphere provides a medium which can promote chemical reactions at rates far in excess of similar, gas phase reactions. The example given above was for oxidation of SO2 to sulfuric acid, which proceeds at rates of percent per hour in the gas phase by the- OH oxidation pathway, but can occur at percent per minute in cloud or rain droplets via the 02 and H202oxidation pathways (Calvert and Stockwell, 1984). Quite recent airborne field work by Hegg and Hobbs (1988) has also suggested the possibility fo a hitherto unsuspected aqueous phase pathway for nitrate production. Thus the final chemical composition of wet deposition comprises material from soluble gases and aerosols scavenged in cloud, plus that scavenged below cloud, as modified by aqueous phase reactions. While there is still uncertainty about some of the fundamental physics of the scavenging processes (Slinn, 1982), numerical models incorporating most of the air/cloud processes involved in wet deposition has occurred to the point where models currently available can offer very useful interpretive and predictive capabilities (see comments by Schwartz, 1989). Dry deposition has been studied less and is less well understood at a quantitative level than wet deposition, though considerable advances have occurred in recent times. The reason is that dry deposition is inherently more difficult to measure than wet deposition (e.g. Nicholson, 1988), the latter in concept at least simply involves placing a clean collector out in the field to collect rain for subsequent chemical analysis. The case for dry deposition is far more complicated, as fluxes of material to the earth's surface depend upon the turbulence properties of the atmospheric boundary layer, surface roughness properties, particle size (density and shape) for aerosols, chemical properties of the surface and the depositing molecule for gases, and environmental conditions such as temperature and humidity if the deposition surface is biologically mediated (for example deposition to plants can depend for soluble gases on whether the stomates are open to allow exchange with leaf water; for some insights see Hicks et al., 1986). Nevertheless, dry deposition is an important pathway in acidic deposition, and must be accounted for if regional scale budgets of acidic species are to be constructed (Whelpdale et al., 1988). In many cases the difficulty of experimentally assessing dry deposition fluxes is avoided by use of the 'deposition velocity', V, of a species of interest, where Vi is a semi-empirical constant relating [0, the concentration of gas or aerosol species i, with its rate of dry deposition in given environment (Seinfeld, 1986): surface flux of i = Vi" [0.

(2).

Of course the problem with this very simple approach, widely used as it is, is that Vi must vary with all the factors listed above that influence dry deposition, so is very hard to specify with confidence. Finally the deposition pathway of 'occult precipitation' must be acknowledged

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whereby in places where cloud and fog are common more water may be deposited by impaction of cloud or fog droplets onto plants, buildings etc. than by the normal forms of precipitation. Thus this form of deposition can be of particular importance in mountainous regions subject commonly to cap clouds or fogs, especially if the mountain is located downwind of a strong source ofanthropogenic emissions (e.g. Mohnen, 1989. In other instances even where cloud and fog interception at the surface are less common, this deposition pathway may still be of great importance because of the extremely high concentrations of dissolved species that often occur in the cloud or fog droplets. These concentrations arise because non-precipitating clous and fogs have lower liquid water contents than precipitating systems: typical values might be of order 0.1 g m -3 liquid water for fogs, perhaps a few times this for non-precipitating clouds, compared with figures of 1-3 g m -3 for precipitating clouds (Pruppacher and Klett, 1978). An additional factor is the fact that surface level fogs are located right in the atmospheric boundary layer where source emissions are highest, and usually form in calm conditions that imply minimum ventilation and dilution of polluted boundary layer air. The volume edited by Unsworth and Fowler (1988) contains many recent papers of relevance to this topic. 2.2. EFFECTS Since the emphasis here is on atmospheric aspects of the acidic deposition question, rather than on the adverse effects, what follows is merely a brief indication of some of the areas involved. Four areas that have been discussed in the literature are: effects upon aquatic ecosystems, effects upon soils/plants, effects upon building materials, and effects upon humans and animals. The effects of acidic deposition upon aquatic ecosystems are those that have been most convincingly demonstrated. These involve reductions in the diversity and population size offish, other fauna and aqutaic plants in lakes and rivers as a function of pH. At pH much above 6 the ecosystems are unaffected, but as pH declines the effects become evident, so that pH values much below 4 most fish cannot survive, and few species of plants and invertebrates survive (Schofield, 1976). A major contributor to fish morbidity and mortality in aquatic ecosystems and a major suspect as a cause of injury to plants is soluble aluminium, which becomes mobilised in soil waters and lake sediments as pH falls below about 5. High levels of soluble aluminium (and manganese) affect gill function in fish, and fine root function in trees. Likewise it is mobilisation of toxic substances into groundwaters followed by human or animal drinking of these waters that is a likely source of'direct' effects of acidic deposition upon humans and animals. Another effect in highly polluted regions is than on lung function caused by inhalation of acidic fine aerosols and acidic gases (for a summary document see SMA, 1982). There has been considerable discussion in the literature about the role played by acidic deposition in the forest dieback that has been observed in Europe and the northeastern parts of the US. The idea prevalent a decade ago that acidic deposition was the prime cause has now been modified, with the current consensus being that a combination of factors is most probably the cause of forest dieback, with air pollution being one of the major ones. In this context acidic deposition is one of several air pollutants probably implicated, others being ozone, oxides of nitrogen, and toxic substances including heavy

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metals and complex organics (see McDowell, 1988; Galloway, 1988; Cowling 1989; Likens, 1989; Falkengreg-Grerup, 1989 and Stone and Seip, 1989). It should be noted in this context that pH alone may not be a good measure of the possible effects of wet deposition on the environment in cases where acidity may be largely neutralised by high levels of atmospheric ammonia. Nihlgard (1985) has argued that ammonium ion itself in high concentrations contributes substantially to forest decline. Corrosive effects of acidic deposition upon building materials seem very plausible, though here again the separation of effects into those due to rainwater, gases, aerosols and oxidants is difficult. Suffice it to say that atmospheric acidity in some form has been identified as a cause of corrosion in polluted areas of Europe, the US and China (e.g. Zhao and Xiong 1988; Kucera, 1988). 3. Asia / Oceania within the Global Context

3.1. IMPERATIVES The question can now be asked - what is the situation in Asia/Oceania with respect to atmospheric acidity and acidic deposition? Before directly addressing this question, it is worth spending a little time to summarise some imperatives that impinge upon this question.

3.1.1. State of Knowledge The SCOPE Project entitled Acidification in Tropical Countries, which culminated in a workshop in Caracas in 1986 (Rodhe and Herrera, 1988) was the first attempt at summarising on a global scale the available knowledge on atmospheric acidity for a region outside the northern mid-latitudes. The tropical regions include a large part of Asia/Oceania, and as part of the SCOPE Project case studies were initiated for four countries in the region, India, Bangladesh, China and Australia. Of these only the latter two came to fruition (Rodhe and Herrera, 1988). It was clear from this exercise that for the tropics generally, including the Asian region, there is insufficient current knowledge to make a proper assessment of the state of acidification now, and potential for the future, except in very few places. To quote from the Preface to the SCOPE 36 report (Rodhe and Herrera, 1988): "The book should therefore not be regarded as a final assessment. It is rather a beginning that needs to be completed by much additional data". For the Asian region the SCOPE Project concluded that "areas of probable sensitivity to soil acidification include southern China, and other areas of southeast Asia, and southwestern India", but that the state of knowledge of factors such as emission source strengths for acidic and alkaline substances, and sensitivities of plants, soils and groundwater systems to acidic deposition throughout the region is insufficient for an adequate assessment to be made.

3.1.2. Population The global perspective is that total population at present stands at 5 billion, with annual

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growth rate near 1.7% (PCGlobe, 1989). Continuation of this growth rate for just 40 years would see global population at 10 billion. The implications for our atmospheric environment of such a change cannot be predicted with any certainty, but it is hard to envisage that current atmospheric concerns such as the greenhouse effect, secular changes in both tropospheric and stratospheric ozone concentration, and atmospheric acidification (indeed the 'global change' issue in general) will not be exacerbated as population, and emissions of pollutants, dramatically increase. There can be no doubt within this context that population pressures in the Asia/Oceania region must be acknowledged as a prime imperative for asking what effect human activities in the region are having now, and will have in the future, on the regional chemical climate. The combined population of the most populous ten countries in the region alone exceeds 50% of the global population (PCGlobe, 1989), as can be seen from Figure 1. Current population growth rates in the majority of these already populous countries likewise are reported to be above the global average of around 1.7% per annum (PCGlobe, 1989), and are depicted in Figure 2. If the countries in the region are to avoid the regional pollution (and acidification) problems that have occurred in the northern mid-latitudes in the latter half of this century it seems clear that the current lack of knowledge about the chemical climate of the region should be addressed now, since it seems most probable that atmospheric emissions of such anthropogenic effluents as SO2 and NOx must increase with population. POPOLflTIOH 1989 (in thousands) CHIHA

1.33,z4

INDIA INDONESIA JAPAN

1

123,1Se

BflI~LADESH

1

112,823

PAKISTAN

1le,369

UIETNflN

I66,8e2

PHILIPPINES

~64,81~

THAILHND

~55,517

,SOUTH KOP,]~

~43,342

flUSl~flLIfl

Fig. 1.

16,458

Estimated population figures for the ten most populous countries in Asia/Oceania, plus Australia, for 1989 (PCGlobe, 1989).

234

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AVEBAGE I~I4NLII~LPOPULATII]N GBOUTH BATE ( i n percent) I~KISTt~N 154 I L I I ' P I N E S BCt~d~DESN

q IETN~ INDIA

~

INDOflESIA

2

. . . . .

.

0

- .

.

1

.

.

1[ . 9 6

I LAND +...:,..:o.: ~." (v2~/'+-.:~....:~-.:+.2o.:r - : , ' 1[ . 3 3 $00114 ](OBEA ";.~~,... .: ;.~2, ,.: : ,.~ ,,.~ : ,.~? :,~2,,,-~2, :.~?:,,~ 'i,~ :~~v :'i 'i,', ~,,' '.] CHINA IqdJSTBAL I A

JAPAN

F i g . 2.

~ 8 . 4 6

Estimated

population

growth

rates for the countries

l i s t e d in F i g u r e

1 (PCGlobe,

1989).

3.1.3. Economic Growth

An increase in emissions resulting from population increases in Asia/Oceania can only be compounded by increased economic (industrial and agricultural) development in the region, since such development towards the western model is energy intensive, and most energy produced in the region currently is from combustion of fossil fuels. Economic development on a large scale seems quite plausible: the region's share of world gross domestic product (GDP) increased from 14.5% in 1970 to 21.2% in 1988 (PCGlobe, 1989). Certainly there is plenty of incentive, and plenty of room for economic development, given the relatively low per capita GNP that exists in many of the ocuntries in the region. Figure 3 gives a comparison for the 11 countries included in the population comparison of Figure 1. Japan and Australia are typical of countries at the mid to upper end of the per capita GNP range for industrialised countries - the comparison in Figure 3 makes very obvious the disparity in wealth that must be a potent driving force for development in the remaining countries listed in the figure. Another perspective on the pressures likely to be faced by the regional atmosphere as a result of economic development can be obtained by seeking comparisons directly in terms of per capita SO2 and NO xemissions. Reliable figures for countries in the Asian region are scarce, but the figures in Table II which include the global averages again show a clear disparity between the western, industrial economies and the rest of the world, including especially the countries of Asia presented in Table II. The conclusion from these figures is

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GHP PER CRPITR (in ~US)

ll6,zE

JaP~ RUS'IBALIfl

lll,ze Z,915

SOI.II'H ROllER THRILRND

~861

PHILIPPINES

B573

ININI"IESIR

[~qZ8

PRKISI~

|351

INDIfl

307

CHIIIR

299

IJIEll~N

t98

BAN6LRI)ESIt

161

Fig. 3.

Per capita Gross National Product (GNP) for the countries listed in Figures 1 and 2, US dollars.

quite clear: changes in the economies of Asian countries towards those of the industrialised, wealthy countries can be expected to involve large per capita increases in SO2 and NOx emissions on top of any increases related to population growth. 3.1.4. Regional Factors There are some factors specific to the Asia/Oceania region that should be taken into account when comparing the atmospheric acidity situation in the Asian region with that at northern mid-latitudes. These will not be discussed in any detail, but the existence of 'local' factors needs to be acknowledged: this list is not exhaustive. First, it must be acknowledged that many countries in Asia/Oceania are located in the tropics/sub-tropics, rather than at mid/high latitudes. Organic acids, particularly formic and acetic acids, have recently been shown to be important components of precipitation, which had previously gone unnoticed (Likens et al., 1987, and references therein). These organic acids may be relatively more important in tropical regions where temperatures are high year-round leading to year-round emissions of acids from biological sources, though at this stage the sources of atmospheric formate and acetate are not understood (Keene and Galloway, 1988). Another consequence of the high temperatures in tropical regions is the likelihood that chemical processes will proceed at rates different from those at the cooler, higher latitudes. Ayers and Gillett (1988a) give some examples, one example of which is the solubility of SO2, which was estimated to be a factor of 3 lower in tropical clouds than high latitude

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TABLE II Per capita emissions of SO 2and NO2, as kg per annum S and N, respectively. Sources: Hidy et al. (1984), Moiler (1984), Gschwandtner et al. (1986), Dovland (1987), Galbally and Gillett (1988), AEC (1989) and Dignon and Hameed (1989). Region

Year

pop. (millions)

SO2 (Tg S/y)

kg S/y/person

world world India TVM China USA UK Australia

1970 1980 1980 1980 1980 1980 1984/85 1984/85

3,800 4,400 680 115 970 226 56 15.7

62 62.8

16.3 14.3

18 10.6 1.8 0.76

18.6 46.9 32.0 48.4

NOx (Tg N/y)

kg N/y/person

22.1 0.77 0.15 5.0 3.5 0.49 0.24

5.0 1.1 1.3 5.2 15.5 8.8 15.3

clouds, with obvious implications for scavenging and wet deposition of sulfur. Some tropical regions are also subject to more persistent meteorology than locations at higher latitudes - the directional persistence of winds for large parts of the year in some regions could have very important implications for both dry and wet deposition patterns. Other implications exist for the relative importance of wet and dry deposition in countries located in the wet-dry tropics, where it can rain almost every day for months on end, then change to months on end of zero rain. Strong vertical transport in convective cloud systems is also a feature of tropical meteorology, with the consequence that pollutants emitted near the surface and rapidly transported to the upper troposphere would there have extended lifetimes enabling lengthy transport and deposition far afield, especially poleward. Other features that should be noted include the prevalence of biomass burning, with consequent strong emissions of reactive hydrocarbons and NOx, with smaller amounts of sulfur, to the regional atmosphere. There is good evidence that these emissions strongly influence the tropospheric oxidant cycle, as revealed by perturbations to ozone concentrations (Crutzen et al., 1985). The Asian region is reported to contain 40% of the world's acid sulfate soils - fish kills are a well known phenomenon when rainwater leaches such soils (Singh, 1982). How would such soils respond if there were widespread, secular increases in atmospheric acidic deposition? Active vulcanism exists in some regions, for example the Indonesian archipelago - what role do emissions from these strong natural sources of acidic gases have on the local and regional atmospheric acidity? Finally we can note that in warm climates especially, emissions of reactive gases from soils and vegetation are important to the atmospheric oxidant cycle and atmospheric acidity. For example isoprene, an extremely reactive C5 hydrocarbon (lifetime < 1 h at mid-day in the tropics) is emitted in huge quantities by tropical trees (Ayers and Gillett, 1988b). Removal of tropical forests among other agricultural developments in the Asian region may have considerable implications for the atmospheric chemistry of the region. One such development could involve fertilizer usage. Emissions to the atmosphere of

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reactive N from fertilizer applied to the soil can provide a substantial contribution to the reactive nitrogen in the atmosphere. Again a comparison between Asian countries and more developed countries can illustrate the potential for change in Asia. Galbally and Gillett (1988) calculated that in tropical Australia fertilizer use as N in per capita terms was 59 kg/y, compared with 5.7 kg/y in India and 4.4 kg/y averaged over Thailand, Vietnam and Malaysia. 3.2. EXISTING OBSERVATIONALDATA The question posed at the outset may be now be asked: what is the current state of atmospheric acidification in the Asian region, and what is the potential for future acidification and acidification-related environmental problems? The answer to the initial part of this question lies in observational data, the first line of which in areas with significant rainfall is rainwater composition data. What follows is a summary of data for the Asia/Oceania region known to this author. A certain amount of this comes from the so-called 'grey literature' - it is likely that additional material of this type existed, buried in government or institute reports that are not widely available. The quality of such material is, of course, not always easy to determine. By far the largest set of Asia/Oceania rainwater composition data available from a single source is the World Meteorological Organisation's BAPMoN (Baseline Air Pollution Monitoring Network) database, which in the region under consideration has data from 23 sites in I 1 countries, some of which extend back to the early 70s (WMO, 1990). The data records contained in this database are not all up to date, not all complete in terms of rainwater species determined, and require careful scrutiny before use, but when approached with these limitations in mind do provide a substantial primary source of information. 3.2.1. India

There has been a long history of rainwater composition studies in India, almost certainly the longest in the Asian region, continuing with a high level of interest during the last decade. A selected bibliography for the period since 1982 is indicative of this activity: Maske and Nand (1982); Handa et al. (1982); Nand (1984); Khemani et al. (1985a, b, c, d); Mukherjee et al. (1986); Khemani et al. (1987a, b, c, d); Naik et al. (1988); Das (1988); Varma (1989a, ab); and Khemani et al. (1989a, b). While the majority of Indian work has been on rainwater composition, these studies include work on aerosol composition and some work on acid precursor gases, while the very recent paper of Krishnayya and Bedi (1989) addresses the question of the effects of SO 2 on three tropical tree species. The consensus from these Indian studies is that Indian rainwater (and cloud and fogwater) is alkaline, relative to the standards of the northern mid-latitudes (Mohnen, 1988; Schwartz, 1989) where precipitation pH values below 5 are very common, pH values of 4 are common, and fog/cloudwater values in the range 2-3 are observed (Unsworth and Fowler, 1988). Indian rain and fog typically has hydrogen ion concentrations 2 or more orders of magnitude lower than acidified regions of Europe and the US, at mean pH values between 6 and 7. These relatively high pH values in Indian rain result from alkaline

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soil aerosols, originating especially in the arid regions of the northwest (Khemani et al., 1985c, d; Khemani et al., 1987c, d, Naik et al., 1988, Varma, 1989a, b). Thus there appears to be no immediate concern over acidification over most of India. However the long-term effects of increasing emissions of anthropogenic acids have been noted via increases in rainwater acidity and changes in composition over the last 2-3 decades (Varma, 1989a; Khemani et al., 1989a, b), and it has been suggested that the southeast corner of the country where pH is currently in the range 5-6 may be considered to be a sensitive zone (Varma, 1989a). The indication is that continued increases in emissions with continued economic development (according to Khemani et al. 1989b, SO2 emissions have already tripled since the early 60s) will certainly lead to increased acidification of Indian rain: it is impossible with present data to predict to what extent. 3.2.2. China Given the substantial combustion-derived emissions of NOx and SO2 in China (5 and 18 Tg/y, respectively, Galloway, 1989), derived primarily from coal combustion in low-level sources, the question of acidification has recently been accorded some attention in China (Zhao and Sun, 1986). The current understanding, along with precipitation and aerosol composition data, is summarised in the works of Zhao and Sun (1986), Galloway et al. (1987), Zhao et al. (1988), Huebert et al. (1988) and Zhao and Xiong (1988). The situation in China is characterised by rainwater and aerosol nitrate loadings comparable with those in the acidified mid-high latitudes of the northern hemisphere, and sulfate loadings several times higher, indeed these may be the highest routinely observed anywhere in the world. Yet the pH of rain in northern China averages about 6.5, giving no obvious hint of the elevated sulfate and nitrate concentrations. Like the Indian situation, in the north of China atmospheric acids are neutralised by high levels of alkaline soil material (primarily calcium carbonate), but unlike India there is additional neutralisation provided by very high levels of atmospheric ammonia, presumably produced by decomposition of animal waste/volatilisation from the alkaline northern soils. However in southwestern China where the soils are not alkaline, indeed are acidsensitive (Zhao and Xiong, 1988) precipitation in the vicinity of some cities shows mean pH values near 4. Tables III and IV, taken from Zhao and Xiong (1988) illustrate some chemical properties of the rain in this region. Despite the high levels of calcium in the rain from the southwestern sites, relatively low ammonia concentrations and greatly elevated sulfate concentrations compared with northern rain are clearly the cause of the high levels of rainwater acidity observed (Galloway et al., 1987; Zhao et al., 1988; and Zhao and Xiong, 1988). 3.2.3. Taiwan

The only recent publication mentioning rainwater composition studies in Taiwan known to this author is the paper by Jeng and Tu (1989). This paper mentions nine sampling stations spread across the island, and that for the three year period to February 1988 300 rain samples were collected, of which 56% had pH < 5.6. Rainwater data for hydrogen ion, sulfate, chloride and nitrate were used by Jeng and Tu (1989) in multiple regression

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TABLE III Precipitation pH data (monthly means) from four locations in Guizhou province, China, in 1984(data from Zhao and Xiong, 1988).

month

Guiyang (urban)

Liizhang (rural)

Keyang (rural)

Shisun (rural)

Jan. Feb. March April May June July Aug. Sep. Oct. Nov. Dec.

3.9 4.0 3.8 4.1 4.0 4.5 4.5 4.1 3.7 3.8 3.7 3.4

4.3 4.4 4.4 4.2 4.5 4.9 4.9 4.6 4.8 4.3 5.4 5.4

4.3 4.1 4.2 4.6 4.6 5.4 5.1 4.5 4.5 4.6 4.5 4.3

5.9 4.3 3.9 4.7 4.3 5.1 4.7 4.5 4.4 4.5 4.7 4.7

TABLE IV Mean annual precipitationpH and ionic concentrations0zeq/L) in 1984for the Chinesesiteslistedin Table III (data from Zhao and Xiong, 1988). site

pH

SO42-

NO 3-

Ca2+

NH4+

Guiyang Luizhang Keyang Shisun

3.95 4.68 4.56 4.65

444 143 163 97

10.3 9.1 20.8 15.9

256 165 132 73

57 30 87 38

models of corrosion rates, b u t were not directly presented, so despite the large dataset gathered this information does not appear to be the public d o m a i n at this time (neither is information on the methods of sampling, analysis etc.).

3. 2. 4. South Korea As in the case of Taiwain, only one contemporary publication o n rainwater composition in South Korea is k n o w n to this author, the paper of Shin et al. (1989). This paper reports the results of sampling with wet-only event samplers at 10 sites in the Seoul area between August a n d November 1985. The volume-weighted m e a n p H raged from 4.39 to 4.64 across these 10 sites, with sulfate the d o m i n a n t a n i o n at 31-70 # e q / L . Nitrate was low, less than 6 # e q / L as volume-weighted m e a n at all sites. Calcium was almost as i m p o r t a n t as hydrogen ion o n a n equivalents basis, indicating as in the cases of India a n d C h i n a that alkaline aerosol material is i m p o r t a n t to the acid base balance. However absolute levels of calcium in Korea were much less than in China, a n d overal this small dataset suggests that acidification of rain in Seoul by sufuric acid is a widespread p h e n o m e n o n , a n d at a level not greatly below that of the acid-impacted regions of Europe a n d north America.

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AYERS

3.2.5. Japan

Cloud and fogwater analyses were reported by Okita (1968) for samples taken on Mt Norikura, Mt Tsukuba and Shiobara. in 1963. The most notable feature of these samples was that several of the samples from Mt Norikura had pH values near and below 4, with high levels of sulfate, indicating significant acidification of the cloudwater by sulfuric acid, presumably of anthropogenic origin. Okita and Ohta (1979) and Ohta et al. (1981 ) discussed cloudwater chemistry of clouds sampled from Mt Tsukuba between 1975 and 1978: very high acidities were found with pH values in the range 4 down to less than 3, with sulfate in the hundreds of micromolar to millimolar range, and nitrate somewhat lower. A study on rainwater pH in Tokyo (995 samples, 1973-1980) and Tsukuba (736 samples, 1980-1984) was reported by Kanazawa et al. (1984). They reported overall mean pH values of 4.5 for both series of measurements, with 26% of samples in Tokyo having pH < 4, while 10% of those at Tsukuba had pH < 4. Without complete chemical analyses of these samples the acids involved cannot be identified, however these statistics are sufficient alone to point to possibility of a significant anthropgenic acidification of rain at Japanese sites. Finally the Japanese BAPMoN station at Ryori (39~ 142~ in northern Japan is somewhat removed from the heavily populated region around Tokyo, and has reported data to the WMO databank since 1976 (WMO, 1990). The long-term volume-weighted mean pH at Ryori is 4.8, with nss-sulfate at 20.1/zeq/L and nitrate at 9.3 ~eq/L. These values are 2-3 times higher than the nss-sulfate and nitrate levels considered by Galloway et al. (1984) to be representative of continental areas unaffected by anthropogenic emissions: a significant anthropogenic influence on sulfate and nitrate deposition is indicated. This conclusion is consistent with the fact that the long-term mean pH is significantly less than 5, though neither this value of 4.8 nor the non-sea-salt (nss) sulfate and nitrate levels at Ryori are presently at the levels causing concern in Europe and north America. 3.2.6. Hong Kong

The only rainwater composition data available from Hong Kong is that generated by the Royal Observatory, from the BAPMoN station ofYuen Ng Fan located at Sia Kung. The data record dates from early 1988, and was obtained for calendar year 1988 directly from the Royal Observatory (W.L. Chang, 1989, personal communication). The data are for weekly, wet-only samples, and have a volume-weighted mean composition not unlike that from the Japanese site at Ryori: ph 4.68, nss-sulfate 26.6 #eq/L and NO2- 10.1/~eq/L. Thus a similar conclusion ensues, that the data from Hong Kong show a significant influence by anthropogenic emissions on rainwater sulfate and nitrate concentrations, but levels currently are less than those causing concern in Europe and the US Galloway et al., 1984; Schwartz, 1989).

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3.2.7. Thailand A small dataset of just 12 wet-only rainwater samples, each of one month's duration taken at intervals between November 1983 and July 1985, is available from the BAPMoN site at the island site of Ko Sichang (WMO, 1990). This small dataset has a volume-weighted mean pH of 6.54, low nitrate ( 5.6, with less than 10% of samples at all sites having pH < 5. The lowest pH value observed was 4.5. 3.6.8. Philippines

This author is aware of a very small dataset from the BAPMoN site at Mt St Thomas, covering the period September 1988 to January 1989. The data are incomplete, comprising just a few results for pH, magnesium, calcium and chloride, and cannot be used to draw meaningful conclusions. 3. 6. 9. Malaysia

Long data records are available from the BAPMoN station at the mountain site of Tanah Rata (1540 m altitude) in the Cameron Highlands. Monthly precipitation composition data cover the period December 1975 to March 1987, while weekly data are available from July 1974 (WMO, 1990). The volume-weighted mean pH vales for both sets of data are within the range 5.0-5.5, suggesting that at this mountain site somewhat removed from urban/industrial activities the levels of acidification are low. Indeed with total cation and anion sums < 50 # e q / L as volume-weighted means the average total dissolved ionic loading in precipitation from Tanah Rata is consistent with this site being relatively unpolluted. A more extensive picture of rainwater acidity comes from the work of Leong et al. (1988), which presents data from a very extensive network (17 sites) of precipitation composition stations operated by the Malaysian Meteorological service (Tanah Rata is one of the 17 sites). The stations at Petaling Jaya, Senai and Perai had the highest mean sulfate concentrations (in the range 25-50/~eq/L) and lowest pH values (in the range 5.05-4.40) of the network, these observations leading Leong et al. (1988) to suggest that this was related to the presence of industrial and motor vehicles in the vicinity of these sites. Overall for calendar year 1987 the network yielded 8 of the 17 volume-weighted mean pH values below 5, with 6 of these below pH 4.7. Significant acidification in some regions of Malaysia is suggested by these results.

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3.6.10. Indonesia

Datasets from the Jakarta-BMG BAPMoN station are available from the BAPMoN data archive (WMO, 1990) covering the period January 1981 - July 1984 for monthly data, and December 1984 -October 1987 for weekly data. The datasets lack values for sodium and potassium, and have rather different volume-weighted mean pHs, at 4.79 for the monthly data and 5.33 for the weekly data. There are also some other aspects of the datasets which are notable (unusually high nitrates, on occasion), so in the absence of a proper analysis of the data no conclusions are warranted. Nevertheless, these lengthy datasets are available for external scrutiny, and the other precipitation sampling sites are operated by the Meteorological and Geophysical Institute at Puncak, Bandung, Kenten and Sampali, though no data seems available via WMO at this time. 3.6.11. Australia

Bridgman (1988) provides an excellent summary of work on rainwater composition that had been carried out in Australia prior to 1988: he referenced 16 separate datasets covering clean marine, clean continental, and polluted continental regions. Several other works have been published since the review of Bridgman (1988); see Ayers and Gillett (1988c, d), Ayers and Ramsdale (1988), Gillett and Ayers (1988), Bridgman et al. (1988), Ayers and Ivey (1988), Noller et al. (1990) and Gillett et al. (1990). The situation in Australia so far as it can be determined from these datasets (the continent is so large that the studies cited cannot be considered representative of all the continental environments, including all regions of industrial activity) has been summed up by Bridgman (1988) as follows: "Non-tropical rural pHs average from about 5.0 to 5.7. In Sydney, urban pHs average about 4.4, with the acidity created mainly by local sources. In tropical Australia pHs average about 4.5, with the extra acidity caused mainly by organic acids, most likely derived from emissions of volatile organics from vegetation." The large land area, small population (about 17 million), large proportion of inhospitable, virtually unpopulted landscape, and the concentration of the population in 6 cities between 1 and 3 million ensures that there are good opportunities in Australia for studying the composition of precipitation in an almost 'natural' environment, far from anthropogenic sources. Likens et al. (1987), for example, chose central north Australia as the site for a remote, continental rainwater sampling site in their Global Precipitation Chemistry Project. Results from this site, and the BAPMoN baseline station at Cape Grim, Tasmania (Ayers and Ivey, 1988) show convincingly that nss-sulfate and nitrate concentrations of < 10 # e q / L are to be expected in 'unpolluted' regions. In contrast the studies of Ayers and Gillet (1981) and Ayers et al. (1986) in the urban area of Sydney (pop. 3 million), and Avery (1984) and Rothwell et al. (1987) in the industrial Hunter Valley region show nss-sulfate levels of 20-30 ~eq/L, and nitrate levels of order 10/~eq/L. Thus while overall levels of pH and sulfate and nitrate concentrations in these regions fall well short of those causing concern in the US and Europe, even in Australia the processes of atmospheric acidification by sulfuric and nitric acids can be discerned clearly. One remarkeble outcome of the GTCP (Likens et al., 1987) has been the identification

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of the importance of organic acids, especially formic and acetic acids, to rainwater acidity. We will return to this point later: suffice it to say here that pH values down to and below 4, and cloud water pH values mostly less than 4 (Ayers and Gillett, 1988) have been observed in tropical Australia. This 'acid rain' is not caused primarily by sulfuric and nitric acids, for as noted above the concentrations of sulfate and nitrate are low in these samples. This seems to be a natural phenomenon, presumably one which is not a problem for the local environment, since organic acids are readily consumed by natural biological, processes (Herlihy et al., 1987) so should not contribute to soil/groundwater acidification. Finally, it is worth noting that two additional rainwater composition datasets spanning more than 5 years each of weekly wet-only samples have been produced by the BAPMoN regional stations at the two mid-latitude sites of Wagga Wagga and Coifs Harbour in the east of the continent. (WMO, 1990). The results from these sites accord with the quote from Bridgman (1988), given above: long-term mean pH values at Coffs Harbour and Wagga Wagga respectively are 5.08 and 5.60, while nss-sulfate/nitrate levels respectively are 8.0/4.0 and 12.4/9.8 #eq/L. 3. 6.12. New Zealand

As for Australia Bridgman (1988) summarises the situation for New Zealand prior to 1988. He cites just 4 datasets, to which can be added the discussion by Harvey and Clarkson (1988) of extensive datasets from Kelburn and Lauder (the latter a BAPMoN site). All these datasets refer to rural areas, removed from strong, local, anthropogenic sources. Each of the datasets had mean pH values between 5 and 5.6, with nss-sulfate and nitrate levels of just a few ~teq/L, reflecting little anthropogenic influence at the sites studied. Discussion

When the precipitation composition dataset cited above are compared with those available over the last 3 decades from the US and Europe (e.g. those underpinning the review by Schwartz, 1989), it is clear that the Asia/Oceania data are extremely limited in both scope and quality. For some regions, such as India, China and Australia there is a useful amount of precipitation composition data available, but even in these cases there are very large gaps in the whole infrastructure of basic atmospheric observations necessary for understanding of processes regulating the acid-base balance in the atmosphere over the countries involved. As a consequence of this paucity of data there has been no attempt in the Asian/Oceania region to model the atmospheric processes involved in generation of atmospheric acidity at the quantitative level now commonplace in the European and US situations (e.g. see Schwartz, 1989). Yet the imperatives seem clear: pressures on the atmospheric environment from population growth, changing agricultural practices, and economic development will almost certainly in the coming decades lead to increases in anthropogenic contributions to the atmospheric budgets of reactive sulfur and nitrogen oxides (SO2 and NOx) in Asia/Oceania. The reality of these imperatives is supported by the limited data that are

2~

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available: southwestern China does have an identified acidification problem (Zhao and Xiong, 1988); a secular trend towards in increasing acidity has been identified in Indian precipitation (Varma, 1989a; Khemani et al., 1989a, b); precipitation in Korea (Shin et al., 1989) and Japan (WMO, 1990), shows evidence of significant acidification; in Malaysia (Leong et al., 1988) and even in a major city in Australia (Ayers and Gillett, 1985; Ayers et al., 1987) the processes of atmospheric acidification from anthropogenic emissions o f S O 2 and NOx have been identified. Thus the need to quickly improve our knowledge in the Asia/Oceania region seems unquestionable, if the problems presently faced in acidified areas of Europe and the US are to be properly considered as possiblities that perhaps might be avoided in Asia/Oceania. At a minimum, four requirements for improvement in our current understanding seem necessary: 1. a co-ordinated, high quality, dataset on precipitation composition covering the complete range of regional environments is a basic necessity; 2. a necessary adjunct for understanding the processes controlling the chemistry of precipitation are complementary measurements of aerosol and gaseous species relevant to rainwater composition, including atmospheric oxidants (such as 03, and H202 etc.), including where possible direct measurements of dry deposition fluxes of acidic/alkaline species; 3. high quality emissions inventories, listing source strengths and seasonal characteristics of all significant sources of atmospheric S and N in all countries of the region, including for example large natural sources such as the volcanos in the Indonesian archipelago, and extensive biomass burning in some regions; and 4. ecosystem sensitivity studies, in which the potential for adverse effects of acidification on soil/groundwater/plant/animal systems is assessed. Particular scientific questions not so far addressed in Asian studies will need to be identified and addressed in the process of filling in the gaps in our knowledge of the Asia/Oceania atmospheric environment. Two examples are the question of the role played by natural sulfur emissions in the production of atmospheric sulfuric acid, especially in coastal regions (according to Andreae, 1990, the global oceans emit on the order of 40 Mt S per year), and the role played by organic acids, particularly formic and acetic acids (Keene and Galloway, 1988), in regional atmospheric acidity. The paper of Herlihy et al. (1987) confirms that all past Asian rainwater studies (except those in northern Australia) would not have registered the presence of these organic acids, since these acids are rapidly destroyed by bacterial action in the rain sample if a biocide is not added to the sample. Thus the true pH and organic acid content of Asian rains remain to be measured. Figure 4 shows northern Australian rainwater composition data which emphasises the absolute necessity to account for organic acids, which at this location are the dominant anions. Finally, the issue of data quality for all types of measurements will need to be addressed. Consider just the available precipitation composition datasets cited in the previous

245

ATMOSPHERIC A C I D I F I C A T I O N IN THE ASIAN REGION

180 fowic

160

/

140

acetic

120 03 (D

E ~o 0

E

nitrote

100 80

sulfate

60 40 20 0 1

5 2

5 4

8

7

11 15 16 22 9 12 14 17 25 somple number

Fig. 4. Stacked bar graph depicting rainwater composition at Jabiru (12~ in the Northern Territory of Australia during the wet season of 1984/85. Daily rainwater samples were taken at approximately half-weekly intervals in the 30 months period from December 1984.

section: there are great disparities in sampling protocols (bulk vs wet-only; manual vs automatic; wide variations in sampler design), sample handling protocols (some sites filter samples, some refrigerate, some add biocides - others do some or none of these things), and analytical methods (classical wet-chemistry, often with marginal detection limits vs more modern sensitive, automated/instrumental methods). The message again seems clear: proper scientific conclusions to questions of acidification in the Asia/Oceania region will be difficult to achieve at all without the achievement of uniform, high quality standards for all measurement programs implemented in the region. 6. Conclusions

Anthropogenic perturbations to the global, atmospheric sulfur and nitrogen cycles have produced levels of acidic deposition in some highly populated and industrial regions of the northern hemisphere that have contributed significantly to deterioration of terrestrial ecosystems. Similar effects cannot be ruled out for some parts of the Asia/Oceania region, given the large and growing population in this region and the global pressures for economic development, the latter usually being based on energy-intensive activities which cause large emissions of SO2 and NOx to the atmosphere. The brief summary given above of information currently available from precipitation composition studies in Asia/Oceania shows that the data are extremely limited in amount, quality, and geographical coverage. Most countries in the region appear to have

246

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essentially no available data. This paucity of precipitation composition data, coupled with no generally available emissions inventory data for major sources of atmospheric S and N throughout the region, and the lack of any systematic documentation of the susceptibility of regional ecosystems to acidification, ensure that informed conclusions regarding acidification now or in the future cannot be made for Asia/Oceania. Co-ordinated, region-wide, multidisciplinary studies are needed now to address this large gap in our understanding of the global atmospheric chemical climate.

Ackowledgement Robert Gillett rendered considerable assistance to the author via discussions, comments and preliminary scrutiny of several datasets.

References AEC: 1989, AcidRain in Australia: a NationalAssessment, Australia Environment Council Report No. 25, Aust. Gov. Publishing Service, Canberra. Andreae, M.O.: 1990, 'The global biogeochemical sulfur cycle: a review', in: Trace Gases and the Biosphere, Schimel, D. S. and Moore, B. (Eds.), University of Arizona Press. Atkinson, R.: 1990, 'Gas-phase tropospheric chemistry of organic compounds: a review', A tmos. Environ., 24A, 1-41. Avery, R.: 1984, 'A preliminary study of rainwater acidity around Newcastle, NSW', Clean Air (Aust.), 18, 94-101. Ayers, G. P. and Gillett, R.W.: 1985, 'Some observations on the acidity and composition of rainwater in Sydney, Australia during the summer of 1980-81', J. Atmos. Chem., 2, 25-46. Ayers, G. P., Gillett, R. W., and Cernot, U.: 1987, 'Rainwater acidity in Sydney, an addendum', Clean Air (~lust.), 21, 68-69. Ayers, G. P. and Gillet, R. W.: 1988a, 'Acidification in Australia', in: Acidification in Tropical Countries, Rodhe H. and Hererra R. (Eds.) SCOPE Report 36, J. Wiley and Sons, Chichester, England, pp. 347--402. Ayers, G. P. and Gillett, R. W.: 1988b, 'Isoprene emissions from vegetation and hydrocarbon emissions from bushfires in tropical Australia', J. Atmos. Chem., 7, 177-190. Ayers, G. P. and Gillett, R. W., 1988c, 'First observations of cloudwater acidity in tropical Australia', Clean Air (Aust.), 22, 53-57. Ayers,G. P. and Ivey, J. P.: 1988, 'Precipitation composition at Cape Grim, 1977-1985', Tellus, 40B, 297-307. Ayers, G. P. and Ramsdale, S. R., 1988, 'Wet deposition of excess sulfate at Macquarie Island, 50~ ', J. Atmos. Chem., 7, 317-323. Ayers, G. P. and Ivey, J. P.: 1989, 'Methanesulfonate in rainwater at Cape Grim, Tasmania', Tellus B, in press. Ayers, G. P. and Larson, T.V.: 1990, 'Numerical study of droplet chemistry in oceanic, wintertime stratus cloud at southern mid-latitudes', J. A tmos. Chem., 11, 143-167. Bovy, M. W. L., Mieras, M., Posma, G. H. M. M. J., Stallen, P. J. M. and Wieringa, K.: 1989, 'Acid rain policy in The Netherlands - application of mediation techniques', Ambio, 18, 416-422. Bridgman, H.A., Rothwell, R., Pann Way, C., Peng Hing, T., Carras and Smith, M.Y.: 1988, 'Rainwater acidity and composition in the Hunter region, New South Wales', Clean Air (~lust.), 45-52. Bridgman, H. A.: 1988, 'Acid rain studies in Australia and New Zealand', Arch. Environ. Contain. Toxicol., 18, 137-146. Calvert, J.G. (Ed.): 1984, 'SO 2, NO and NO z oxidation mechanisms: atmospheric considerations', Acid Precipitation Series, Volume 3, Butterworth Publishers, 254 pp. Calvert, J. G. and Stockwell, W. R.: 1984, 'Mechanism and rates of gas-phase oxidation of sulfur dioxide and nitrogen oxides in the atmosphere', in: S O ~, NO and NO e Oxidation Mechanisms: Atmospheric Considerations, Acid Precipitation Series, Volume 3, Calvert J. G. (ed.), Butterworth Publishers, pp. 1-62. Chatfield, R. B. and Crutzen, P.J.: 1984, 'Sulfur dioxide in remote oceanic air: cloud transport of reactive

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Atmospheric acidification in the Asian region.

Atmospheric acidification in the Asian region is discussed from the perspectives of currently available regional measurements, and the knowledge now a...
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