The Global Distribution of Acidifying Wet Deposition - ACS Publications

F-911 91 Gif-sur-Yvette Cedex, France ... acidsoccur in eastern parts of North America, Europe, and ... Europe, and northern China the estimated pH is...
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Environ. Sci. Technol. 2002, 36, 4382-4388

The Global Distribution of Acidifying Wet Deposition HENNING RODHE* Department of Meteorology, Stockholm University, SE-106 91 Stockholm, Sweden FRANK DENTENER

TABLE 1. Typical Composition of Precipitation in (a) an Acidified Region, e.g. Central Europe and in (b) a Dusty Region, e.g. Urban Site in NE India (43)a anions

(a) acidified

(b) dusty

SO4 NO3ClHCO3sum

70 30 15

20 10 15 15 60

cations

(a) acidified

2-

115

EU Joint Research Centre, TP 280, I-21020 Ispra (Va), Italy MICHAEL SCHULZ CNRS, Ce Saclay, Bat 709, F-911 91 Gif-sur-Yvette Cedex, France

The acid-base status of precipitation is a result of a balance between acidifying compoundssmainly oxides of sulfur and nitrogensand alkaline compoundssmainly ammonia and alkaline material in windblown soil dust. We use current models of the global atmospheric distribution of such compounds to estimate the geographical distribution of pH in precipitation and of the rate of deposition of hydrogen ion or bicarbonate ion. The lowest pH valuessmainly due to high concentration of sulfuric acidsoccur in eastern parts of North America, Europe, and China. A comparison with observed pH values shows fair agreement in most parts of the world. However, in some areas, e.g. western North America, southwestern Europe, and northern China the estimated pH is too low, indicating that we have underestimated the deposition flux of alkaline material, probably mainly CaCO3. Our neglect of organic acids may have contributed to an overestimate of pH especially in certain tropical areas. To illustrate the potential effects of acidifying deposition on nitrogen saturated terrestrial ecosystems we also calculate the deposition of “potential acidity” that takes into account the microbial transformation of ammonium to nitrate in such ecosystems, resulting in the release of hydrogen ion. Compared to the deposition of acidity, with its maxima over Europe, eastern North America, and southern China, the deposition of potential acidity exhibits an additional maximum in India and Bangladesh and in several other smaller hot spots where the cycling of ammonia is enhanced by a dense cattle population. To the extent that soils in these areas of high potential acidity deposition actually become nitrogen saturated a depletion of base cations and other changes in soil chemistry and biology should be expected. Potential problem areas for future soil acidification include several regions with sensitive soils in southern, southeastern, and eastern Asia as well as in central parts of South America.

Introduction Acidic deposition is known to have affected sensitive terrestrial and limnic ecosystems in Europe and North America * Corresponding author phone: +46 (0)8 164342; fax: +46 (0)8 159295; e-mail: [email protected]. 4382

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+

NH4 Na+ Ca2+ Mg2+ K+ H+ sum

30 15 12 5 3 50 115 pH ) 4.3

(b) dusty 20 13 20 5 2 60 pH ) 6.2

a Unit: microequivalents per liter (µequiv/L). To get the concentration in molar units (µM/L) the values have to be divided by 2 for all ions with double charge.

(1) and, more recently, also in China (2). In these regions, monitoring of the chemical composition of precipitation has been carried for several years, and the spatial and temporal patterns in the wet deposition of the ionic components that determine the acidity/alkalinity (H+, SO42-, NO3-, NH4+, Ca2+, etc.) have been quite well determined (3, 4). Much less is known about the situation with regard to acidic deposition in the rest of the world (5). A rapid advance in recent years in the development of emission inventories and in the formulation of Global Chemistry/Transport Models (GCTMs) has made it possible to estimate the cycling through the global atmosphere of several key species including sulfur compounds (6-9), oxidized nitrogen compounds (10-12), reduced nitrogen compounds (13), and soil dust (14). We use results from such GCTM simulations, and a simple model of the ionic balance in precipitation, to estimate the global distribution of pH and deposition of acidity/alkalinity. Acidity of Precipitation. The hydrogen ion concentrations and thereby the pHsin precipitation is determined by the balance between acidic and alkaline species. The pH can be directly measured or it can be calculated based on electroneutrality if all other ionic components have been determined. Table 1 shows two examples of typical ionic composition of precipitation, one case from central Europe acidified by industrial emissions of oxides of sulfur and nitrogen and another from northern India moderately affected by acidifying species and, in addition, by some alkaline dust. In both cases the concentration of Cl- is nearly balanced by an equal amount of Na+ implying little influence of these salt components on the H+ concentration. Neglecting the relatively small contributions from Mg2+ and K+, one may conclude that the molar concentration of H+ in precipitation can be approximately estimated from eq 1a

[H+]precip ) 2[SO42-] + [NO3-] - [NH4+] - 2[Ca2+] (1a) where [ ] is the concentration in mol L-1. If the right-hand side is negative, its absolute value can be interpreted as the HCO3- concentration, i.e. HCO3- is the 10.1021/es020057g CCC: $22.00

 2002 American Chemical Society Published on Web 09/17/2002

only negative ion that can make the charge balance in our simple model:

[HCO3-]precip ) - 2[SO42-] - [NO3-] + [NH4+] + 2[Ca2+] (1b) The value of [H+]precip in this case can then be determined from the carbon dioxide/water equilibrium (15, pp 345348). Our simple charge balance model neglects the contribution of organic acids to the H+ concentration. Several studies have shown that formic and acetic acids may also contribute substantially to the acidity of precipitation, especially in tropical climates (16, 17). On the other hand, once deposited such organic ions are normally quickly consumed by microorganisms such that their acidifying influence disappears (18, 19). We will also use a modified H+ model in which we take into account the potentially acidifying influence of NH4+ if this ion is deposited on an ecosystem which is nitrogen saturated. Under such circumstances the NH4+ will be oxidized to NO3- and the NO3- leached out of the ecosystem leaving behind two H+ ions (20). Since one hydrogen ion is consumed in the initial neutralization reaction between NH3 and an acid (e.g. H2SO4), the net result in the overall process is one hydrogen ion. We thus define the potential H+ concentration in precipitation falling onto a nitrogen saturated ecosystem as

[pot.H+]N-sat ) 2[SO42-] + [NO3-] + [NH4+] - 2[Ca2+] (2) The (pot.H+)N-sat concentration multiplied by the precipitation amount is an upper limit of the effective input, from precipitation, of H+ to an ecosystem. If, on the other hand, the ecosystem is nitrogen limited, then both NO3- and NH4+ will be taken up by the plants and not contribute to the acid/base balance of the soil (21). In this case the potential H+ could be defined as

[pot.H+]N-lim ) 2[SO42-] - 2[Ca2+]

(3)

In real situations some of the nitrogen-containing ions are likely to be taken up by the plants, others not. The hydrogen ion contribution to the soil will then be somewhere between the two situations described by eqs 2 and 3. The crucial factor is the amount of NO3- leached out of the ecosystem: The more leaching the higher the acidifying contribution from the deposition of nitrogen, be it NO3- or NH4+ (20). Dry deposition of acidifying and neutralizing compounds will also influence the acidity of the soil. Equations 1-3 can be modified to include this contribution, cf. Erisman et al. (22). The Global Chemistry Transport Model. The transport of the various chemicals, contributing to the acid deposition, is simulated using the TM3 model, which is a developed version of the TM model formulated by Heimann (23). TM3 is an off-line model with a horizontal resolution of 5° long. and 3.75° lat. and with 19 hybrid vertical levels up to 10 hPa. It uses 6-hourly meteorological fields from the European Centre for Medium-Range Weather Forecast (ECMWF). Convective tracer transport is calculated with an updated mass flux scheme that accounts for shallow, midlevel, and deep convection (24). Dry and wet deposition processes are parametrized according to Ganzeveld et al. (25) and Guelle et al. (26), respectively. The transport and scavenging characteristics of the model have been tested using 222Rn (27) and 210Pb (26). The sulfur simulations are based on the chemistry scheme of Jeuken et al. (28), which includes natural emissions of

DMS (24 Tg S a-1) and man-made emissions of SO2 and aerosol sulfate (taken together 80 Tg S a-1). Sulfur in the form of sea salt is not included in the model since it is considered not to influence the acid/base balance of precipitation or of the ecosystems receiving such deposition. Oxidation of DMS (to SO2) and SO2 to sulfate occurs through reaction with OH and nitrate radical. The global turnover time of sulfate in the model is 3.5 days. The annual deposition (wet plus dry) of oxidized sulfur (SOx ) SO2 + SO42-) is shown in Figure 1a. The simulations of oxidized nitrogen compounds (NOy ) NOx + HNO3 + NO3-) are based on the work of Lelieveld and Dentener (29). The chemical scheme includes 47 species, 32 of them explicitly transported, that describe the CH4-CONMHC-NOx-SOx chemistry. Photodissociation rates are taken from Houweling et al. (30), and heterogeneous processes are described according to Dentener and Crutzen (12). Natural emissions of NOx, from soils and lightning, amounts to 9 Tg N a-1 and man-made emissions to 35 Tg N a-1 (28 from industrial sources and 7 from biomass burning). The annual deposition (dry plus wet) of oxidized nitrogen is shown in Figure 1b. The simulations of reduced nitrogen (NHx ) NH3 and NH4+) are similar to those presented by Dentener and Crutzen (13), the main difference being an updated emission inventory from Bouwman et al. (31) and inclusion of ammonium nitrate aerosol (32). The NH3 emissions include 30 Tg N a-1 from man-made sources, mainly cattle and fertilizer, and 15 from natural sources, mainly oceans and soils/vegetation. The chemical processes responsible for the transformation of NH3 to NOx and to aerosol (NH4)2xH2-2xSO4 (0 < x 5 ms-1with a threshold velocity and a regional source strength optimized by comparing modeled dust occurrence with TOMS aerosol index in arid regions (35). We assume that aerosol Ca is completely dissolved in cloud and rainwater. Sources of Ca other than wind blown soil dust, e.g. fly ash and soil dust from agricultural activities, are not considered. This omission implies a negative bias that may be substantial in some areas, cf. below. The annual deposition of Ca is shown in Figure 1d. A systematic comparison between modeled and observed concentrations of Ca remains to be done. However, the modeled Ca values are closely linked to dust simulations, which in turn have compared with measurements with reasonably positive results (34). VOL. 36, NO. 20, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Annual total (wet plus dry) deposition of (a) SOx, (b) NOy, (c) NHx, and (d) Ca. Note the similarity between the patterns of SOx and NOy with deposition maxima in eastern North America, Europe, and China. The NHx deposition has additional maxima in south Asia and in eastern South America associated with high concentrations of cattle. The white color in (c) represents areas where the net flux of NHx is directed upward. The Ca deposition has a completely different pattern with maxima in Sahara and inner parts of Asia. Unit: mmol m-2 a-1.

FIGURE 2. Annual average pH of precipitation calculated from volume weighted averages of H+ according to eq 1. The most acidified regions occur in eastern North America, Europe, and China. The maximum in South America is associated with emissions from a smelter in northern Chile coupled to low amounts of rainfall in that region.

FIGURE 3. Observed values of volume weighted annual average pH of precipitation. Data from precipitation chemistry networks in North America, Europe, China, and parts of South Africa have been included in the form of inserted maps.

Results The deposition of the four major contributors to atmospheric acidity exhibits distinctly different patterns, cf. Figure 1. SOx and NOy show major maxima over the industrialized regions in eastern North America, Europe, and East Asia. NHx, whose emissions are more related to intensive agriculture, has maxima not only in the three regions mentioned above but also in India, parts of central Africa, and southeastern South America. The NHx deposition in northern India is particularly

high reaching levels of more than 200 mmol m-2 a-1. The deposition of soil derived calcium is mainly confined to the arid regions and their surroundings in northern Africa and central and southern Asia. The pH corresponding to an annual average (volume weighted) of the concentration of H+ in precipitation, calculated from eq 1, is shown in Figure 2. This is the pH that would be measured if all precipitation during a year were collected and mixed in a bucket, i.e. a volume weighted VOL. 36, NO. 20, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Annual wet deposition of acidity according to eq 1. Positive values refer to H+, negative to HCO3-. Note the approximate similarity with the pattern shown in Figure 2. Unit: mol m-2 a-1.

FIGURE 5. Annual wet deposition of potential acidity onto nitrogen saturated ecosystems according to eq 2. Positive values refer to H+, negative to HCO3-. The same color code is used as in Figure 4. The difference between the deposition of potentail acidity (Figure 5) and acidity (Figure 4) is most pronounced in regions with high deposition of NHx, e.g. India and parts of South America. Unit: mol m-2 a-1.

average. (The dry deposition to the surface of SOx, NOy, and NHx and of the four ions in aerosol form is not included in the calculated pH values.) Major regions with acid precipita4386

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tion show up in eastern North America, Europe, and East Asia. In addition, secondary minima in pH occur in California, along the west coast of South America, and in Southern Africa.

The distinct minimum in South America is associated with large sulfur emissions from a smelter in northern Chile in an area with very little precipitation. Over the North Atlantic pH values range from about 4.5 along the coasts of the industrialized regions to 5-5.5 in the interior. Over the Southern Hemisphere oceans and much of the Northern Hemisphere Pacific the values are generally in the range 5.2-5.8. Higher values over the equatorial Pacific and Indian Ocean (5.8-6.6) are mainly due to low concentrations of sulfate associated with low DMS emissions. Except for North America and Europe, the data available to test the model calculations are limited. In Figure 3 we have plotted quality controlled pH-data that cover several years so that a reasonably reliable annual average can be estimated. The data have been grouped in four color categories corresponding to the colors in Figure 2. The figure includes data for China from Ding et al. (37); Japan: Hara (38); Europe: EMEP (3); North America: NAPAP (4); Russia: Ryaboshapko (39); several remote locations worldwide: Galloway (40); East Asia: Fujita et al. (41); Korea: Lee et al. (42); India: Norman et al. (43), Pillai et al. (44), Granat et al. (45); Thailand: Granat et al. (46); Indonesia: Gillett et al. (47); Malaysia: Ayers et al. (48); and South Africa: Held et al. (49). Overall, the simulated values (Figure 2) agree quite well with the observations (Figure 3). However, certain discrepancies are obvious. The simulated pH values in western U.S.A. are substantially lower (by about 0.5) than the observations, probably due to an underestimate of the deposition of Ca and possibly also NHx. A similar situation occurs in southwestern Europe (Spain and France) and in northern China. The neglect of fly ash as a source of Ca is likely to have contributed to the underestimated pH, particularly in China. An overestimate of pH in Venezuela might be related to the neglect of organic acids in the model. More observations of the chemical composition of precipitation are clearly needed, especially in tropical and subtropical regions, before the model can be properly assessed. The annual wet deposition of acidity, i.e., H+ whenever the right-hand side of eq 1 is positive and HCO3- when it is negative, is shown in Figure 4. Highest values of the deposition of H+ occur in eastern North America and in East Asia. High rainfall areas, e.g. certain equatorial regions, tend to receive large amounts of H+ even when the pH value is not particularly low. The simulated deposition rates shown in Figure 4 can be compared with observations taken at the same sites as those of pH shown in Figure 3. To judge whether the rate of deposition of H+ could be of ecological concern, one has to take into account the sensitivity of the various soils to acid deposition. Comparing Figure 4 with the global soil sensitivity map published by Kuylenstierna et al. (50), which is based on base saturation and cation exchange capacity for different soil types, the following potential risk areas emerge: eastern North America, parts of central and northern Europe, southern China, parts of Southeast Asia, Zambia, and parts of Central America and Colombia. If, instead, we consider the deposition of potential acidity assuming nitrogen saturation in the receiving ecosystems (eq 2), the picture looks substantially different, cf. Figure 5. Areas with a high deposition of NH4+ now tend to turn more red than in Figure 4. Conspicuous examples include India and central parts of South America. In northeastern India and Bangladesh the estimated annual wet deposition of (potH+)N-sat exceeds 0.1 mol m-2 a-1. We estimate that the total deposition, including dry deposition, will be between 20 and 50% higher. To judge the relevance of the deposition estimates shown in Figure 5, information is required on the extent of ecosystems that are nitrogen saturated or that may become

saturated in the foreseeable future. Unfortunately, we are not aware of such information. Investigations of the ecological effects of high nitrogen deposition in key ecosystems worldwide would be of great interest. It may well be that acidification is not the most important problem associated with high nitrogen deposition such as that estimated to occur e.g. in northern India. Prospects for the Future. The rate of acid deposition during the decades to come will depend mainly on how the deposition rates of the four components (SOx, NOy, NHx, and Ca) develop over time. In North America and Europe the deposition of SOx and NOy is likely to continue to decline. Assuming that the deposition of NHx and Ca at least will not increase, the acid deposition should decrease. In some other parts of the world, including southern and eastern Asia, the deposition of SOx and NOy may well increase well beyond present levels. Although the most recent IPCC scenarios for global sulfur emissions (51) are much reduced compared to the 1992 scenarios (52), some of them still project an increase by more than 50% by the year 2020. An even larger increase is projected for NOx (25-100% by 2030). Trends in NHx and Ca are more difficult to judge. Galloway (53) argues that global NHx emissions are likely to increase because of an increase of world population, larger demand for protein rich food, and intensification of agriculture. The fly ash component of Ca is likely to decrease as better cleaning techniques are applied. Arguments may be put forward both for an increase in soil dust emissions (i.e. more soil degradation) and against it (soil conservation measures). Taken together, these trends do not provide any clear indication of how the pH of precipitation and the deposition of H+ are likely to change in the future. But there is a definite possibility that the deposition of potential acidity onto nitrogen saturated ecosystems will increase substantially in the coming decades, leading to a depletion of base cations and other changes in soil chemistry and biology (54).

Acknowledgments This work has been supported by the Swedish International Development Cooperation Authority (Sida) and Stockholm Environment Institute (SEI). We thank Dr. Lennart Granat for constructive comments.

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Received for review March 6, 2002. Revised manuscript received August 8, 2002. Accepted August 12, 2002. ES020057G