Contribution of anthropogenic and natural sources to atmospheric sulfur in parts of the United States

Atmospheric En&wnem, Vol. 15, pp. l-9. 0 Pergamon Press Ltd. 1981. Printed in Great Britain.

CONTRIBUTION OF ANTHROPOGENIC AND NATURAL SOURCES TO ATMOSPHERIC SULFUR IN PARTS OF THE UNITED STATES* HARBERT

RIC‘E

Estimation Research Assoc., Inc., Acton, MA, U.S.A. D. H. NO~HUMSON Harvard University, Cambridge, MA, U.S.A. and G. M. HIDY Environmental Research and Technology, Inc., Westlake Village, CA, U.S.A. (First received 27 April 1979 and in jinalform

11 February

1980)

Abstract - This paper presents an estimate of the contributions to atmospheric sulfur of natural vs anthropogenic processes in areas of the United States. The areas were selected on the basis of population density, industriali~tion and potential for different kinds of geographically unique natural emissions. The sulfur emissions were estimated in part from land use practice and from geochemical arguments relating sulfur to biological carbon cycling. The natural or quasi-natural processes considered include sulfur gas production in freshwater sediments and intertidal mudflats, soil processes and vegetation. Agricultural activities and acid mine drainage were also taken into account as a perturbation to the available natural sulfur resources. The emissions appear to be heavily influenced by contributions from sulfate reduction in freshwater sediments and intertidal mudflats, and acid mine drainage. The anthropogenic emissions were calculated from the U.S. Environmental Protection Agency’s inventories in the late 1960s. The natural vs man-derived sulfur were compared for 2” longitude by 2” latitude sectors in New England, the mid-Atlantic States, the Atlantic Coastal South, the Midwest, and the arid Southwest. In the sample regions where the anthropogenic emissions exceed 50-100x lo3 tonne S y-l over a 2 x 2” sector, or Z 1530kg(S) ha-’ y-‘, they tend to dominate the biogenic emissions. This appears to be thecase for ind~triaIiz~ Ohio, Iliinois, and New England. If 10% of the available biogenic sulfur is released to the atmosphere, natural or quasi-natural emissions may be a significant contributor in air over Minnesota and Wisconsin, Florida, and perhaps the rural areas of Virginia and remote parts of Arizona and Utah.

INTRODUCTION

As a result of systematic air monitoring over the past decade, a picture of the geographi~l distribution of airborne sulfate has been obtained (e.g., USEPA, 1975). The data are based on the chemical analysis of water soluble extracts of suspended particulate matter collected on a glass fiber filter substrate. Even though the historical observations are uncertain in quality, they have shown that high 24-h average sulfate concentrations exceeding 7-10 pg me3 are widespread over most of the eastern United States, especially the greater Northeast. The distribution of high sulfate concentrations generally coincides with the zone of high emissions of sulfur dioxide associated with the consumption of fossil fuels. There is evidence that the oxidation of sulfurous gases in the atmosphere can account for much of the sulfate observed. This conclusion comes from the fact that primary particulate

* Presented in part at the Ninth international Conference for Atmospheric Aerosols, Condensation and Ice Nuclei, October 1977, Galway, Ireland.

are insufficient to account for global airborne sulfate levels and the applicable oxidation chemistry can be identified (e.g., Friend, 1973). It has been recognized for some time that natural processes of material exchange between the atmosphere, hydrosphere and lithosphere can influence sulfur in the atmosphere. The processes which have been identified include the injection of sulfate by sea spray or wind blown dust and sulfur compounds by volcanic eruptions, and by biological activity in soils or bodies of water. Quasi-natural sources of sulfur are those related to agricultural practices such as soil treatment and animal husbandry, eutrophication in polluted waters, and acid mine drainage. Global sulfur budgets have provided evidence that a significant fraction and perhaps the major amount of sulfur in the earth’s atmosphere comes from such sources. These sources probably emit hydrogen sulfide and organic sulfides such as dimethyl sulfide. Hitchcock (1976) has speculated that a signi~cant fraction of atmospheric sulfate in the northeastern United States may come from oxidation of natural gaseous emissions. In parts of the United States, it is conceivable that natural or quasi-natural sulfur emissions may be comparable or sulfur sources

even larger than anthropogenic

sources. Direct evidence for large biogenic emissions of sulfurous eases is limited. Although organic sulfide gases have been measured in the atmosphere, the dominant natural gaseous sulfur is likely to be H,S. The principal mechanism for the production of atmospheric H,S is thought to be mediated by sulfatereducing bacteria which inhabit muds and waterlogged soils in anoxic environments. Anoxic muds can be found both in freshwater and salt marshes, estuaries, and certain lakes and rivers. Evidence for H,S release readily can be found by odor detection in marshes, streams and polluted river sections. Simple odor detection, however. does not lend itself to quantification of emission rates. Additional support for the conclusion that H,S is emitted in large quantities from coastal waters is provided by measurements of sulfur isotope distribution and abundance for gaseous and particulate matter. Agricultural activities and acid mine drainage also may be a factor in release of sulfur into the atmosphere. These may involve the distribution of large quantities of sulfate which add to the natural reservoir or sulfur and may be reduced eventually to H,S in surface waters. Once H,S is emitted into the atmosphere, it may be reabsorbed at the surface or may oxidize to form sulfur oxides. Since H,S is relatively insoluble, it is unlikely that oxidation in water droplets would be significant. However, Cox and Sandalis (1974) have reported that experiments show H,S is readily oxidized by photochemically initiated reactions involving hydroxyl radicals. Recently, Graedel(1977) has proposed a plausible mechanism for the oxidation of H,S to sulfate via the production of sulfur dioxide. Although the steps of such gaseous free radical reactions remain uncertain, Cox and Sandalls’ (1974) laboratory measurements suggest H2S can be oxidized rapidly at atmospheric conditions with a lifetime of about one day. Graedel’s (1977) estimate for photochemical oxidation appears to be longer. These are comparable with estimated atmospheric lifetimes of SO, of one to three days. Assuming the ground absorptive rates of H2S and SO, are similar, one can take the ratio of natural or quasinatural H,S emissions and SO2 emissions in a given region as a useful comparison for the contributions of natural and anthropogenic sources of sulfurous gases to the atmospheric sulfate burden. Because of their potential importance as sources of atmospheric sulfur oxides, workers have attempted to estimate quantitatively the natural and quasi-natural emissions of sulfur. The approaches have been: (a) indirect deduction either considering global scale sulfur balances (Friend, 1973), or statistical evidence (e.g., Hitchcock, 1976), (b) indirect or direct measurement by microscale studies of the air-- water boundary processes (e.g., Jaeschke et al., 1978; Hansen ef ul., 1978). In this paper, an attempt is made to deduce the magnitude of annual averaged natural and quasinatural emissions on a subglobal or regional scale of

roughly 1O’km’ for selected regions of the United States. These calculations essentially update earlier estimates reported in Hidy ef (11.(1977) and assume: 1. The sulfur emissions can be disaggregated by contributions, and the upper limit of sulfur mobilized annually in the biosphere can be estimated from the carbon to sulfur ratio in vegetation or biological components, as well as from ~~gri~ulturai practice and acid mine drainage. 2. The sulfur mobilized is emitted from the lithosphere and hydrosphere without consideration of the return flow from the atmosphere. 3. Coarse land use information by region provides a meaningful estimate of the active area for biogenic sulfur emissions. 4. Upper limit sulfur emissions by anaerobic reduction of sulfate in fresh water and intertidal areas is estimated extrapolating recent global budget values and land area assessment. The regions chosen for analysis were 2” longitude by 2” latitude squares of approx. 200 km on a side (about 4 x 10’ha area). They include locations which are influenced by : (a) industrialization, (b) areas of agricultural production and soil treatment, (c) forested areas, (d) wetlands and coastal areas, and (e) arid, nonindustrial areas. Because of the high degree of uncertainty in such calculations, a check was needed for verification. To do this, the direct estimates have been compared with a regional airborne sulfur material balance, taking into account anthropogenic sources of sulfur dioxide and atmosphere processes (Hidy et ul., 1977). The approach for estimating biogenic emissions used in this paper has two major limitations. First, there is little clear direct evidence accounting for major biogenic emissions on which to base the release rates for “active” biogenic areas. The natural factors, which are thought to be significant in influencing atmospheric variations, cannot be quantified readily. Direct quantification is restricted by current literature sources to estimates of H,S release from stagnant wetlands near lakes, ponds, marshes, and estuaries, which may range over orders of magnitude. A second major constraint is the lack of consistent region-wide data on airborne sulfur which would allow the cnnstruction of regional sulfur budgets. Normally. biogenie emissions are a major uncertainty in any budget. This uncertainty would increase as the geographical resolution is increased, and inventories are disaggregated. Given these limitations, the objectives of this emission’s study is to compile empirical estimates for biogenic emissions for a restricted number of variables where some quanti~catjon is feasible, and to assess the possible magnitude of regional biogenic sources. The work is not intended to be a detailed, authoritative treatment of the mechanisms for release of natural sulfur flow in the environment. It is a proposed extrapolation of global sulfur balances to a regional scale based on available mucroscopic con-

Contribution of anthropogenic and natural sources to atmospheric sulfur siderations. The consistency with microscopic proespecially the dissimilatory reduction process is discussed separately (Hanry and Hidy, 1980).

cesses,

ESTIMATE

OF NATURAL

SULFUROUS

EMISSIONS

Our main interest is to compare for selected regions the amount of sulfur released as gases by biological activity to the amount of sulfur released by man-made combustion sources. We do this by considering the magnitude of the sulfur pool available from natural biological activity, and from mobilization by man’s agricultural and mining industry. Then an estimate is made of the fraction, a, that may be released to the atmosphere as gaseous sulfur, which is assumed to be chiefly hydrogen sulfide, (H$). The remainder, l-a, represents the long-lived sulfur remaining in the hydrosphere and lithosphere. “Long-lived” is defined in terms of a time scale much larger than a half-life deduced from the sulfur mobilization rate in soils or natural waters. Sources of biogenic su~ur

Sulfur is present in soils as sulfate in soil water, and as elemental sulfur and organic sulfur. Plants normally use sulfates as their source of sulfur. Most higher animals require organic sources of sulfur. The major biological transfers of sulfur are mediated by microorganisms which can assimilate sulfur as sulfide, sulfate, thiosulfate, elemental sulfur, or other forms. When sulfur is assimilated by micro- or other organisms, it is reduced to an -SH group which is incorporated into essential amino acids and into protein tissue. The reverse of assimilation is mineralization or decomposition, in which proteins are broken down. These processes result in sulfate formation if carried through to completion. Assimilatory metabolism of sulfur when calculated as organic sulfur provides an upper bound for sulfur emissions available from biological cycling of sulfur. Relatively little of the total organic sulfur is expected to be exchanged with the atmosphere. Non-assimilatory transformations of sulfur involve oxidation and reduction of sulfur by bacteria. Sulfate reduction occurs in anaerobic environments where it is carried out by bacteria of the genus, Desulfouibrio. Reduction takes place in waterlogged soils and in freshwater and marine environments were oxygen levels are reduced to zero, and anoxic conditions may persist. Sulfate reduction continues until the organic matter supply or the sulfate supply is exhausted, or until anoxic conditions are disrupted. Bacteria also are capable of oxidizing reduced sulfur compounds. Most sulfur oxidizing bacteria require oxygen, but strains of Thiobacillus are known which can use nitrogen compound interaction to oxidize H,S. H,S is also oxidized by purple and green photosynthetic bacteria. An additional oxidation reaction of importance is the oxidation of pyrite (FeS*) to sulfuric acid (Ivanov, 1971). Sulfuric acid in mine

3

runoff has been attributed to the action of iron and sulfur bacteria, Ferrobacillus and Thiobacillus. These bacteria are ~liev~ to catalyze the a~ologi~l oxidation of pyrite by ferric iron, by reoxygenation of the ferrous iron produced (Singer and Stumm, 1970). The mobilization of sulfur from surface mining activites, chiefly as sulfuric acid, also may be a factor as a large source of sulfur added to the natural sulfur reservoir. Global budgets and regional emissions

Ideally, the calculations of biogenic sulfur emissions could be made by extrapolating direct observations. Virtually no such data are available, but some limited observations have been reported for certain kinds of soils in the eastern United States (Adams et al., 1978), and for tidal flats (e.g., Jaeschke et al., 1978; Hansen et al., 1978). In the absence of observations, we must use very gross budget c~culations. One approach uses the extrapolation of global estimates. Previous investigations of the contribution from the biosphere have been global in nature. They have involved estimates by differences between total sulfur in the atmospheric reservoir and that accounted for from man’s activities and other natural sources such as sea spray and volcanic eruptions. In principle, these calculations could be extended to regional behavior by scaling these to the active surface for emissions. Another approach to calculating regional sulfur release would be to relate the available sulfur for different surface conditions to probable emissions. If the inventory of processes for sulfur mobilization were known regionally, a direct calculation of sulfur emissions could be done. As a first step in this approach, one can make estimates for organic sulfur available for emissions from mass ratios of sulfur to carbon and nitrogen for different biological classes. This is based on the amount of carbon which is fixed by plants, and which is available for decomposition each year. From information in the literature, the carbon-sulfur ratios for different biological systems can be estimated (see, for example, Hidy et al., 1977). These mass ratios range from 100: 1 for some wetlands (e.g., Nriagu and Coker, 1976) to 1000:1 for forests and grasslands. Data on carbon productivity estimates for various components of the terrestrial and oceanic systems is readily available (e.g., Whittaker and Likens, 1973). Taking a ratio of 1000: 1 for C : S, the amount of sulfur cycled annually on the land is about 100x IO6 tonne (S). On a unit area basis, individual ecosystem types differ in carbon productivity. Thus, using the same C : S ratio, they differ in estimated sulfur cycling with ranges from 4-12 kg(S) ha-’ y-l for grasslands; 6-14 kg(S) ha-’ y-’ for forests; 6-24 kg(S) ha-’ y-i for croplands; and 5-50 kg(S) ha-’ y-l for marshland and estuary production. In addition to the sulfur available from natural organic processes, the inventory should account for quasi-natural sources involving agricultural activities and acidic mine drainage. Sulfur may be mobilizcil from agricultural activities in a variety of ways. The

4

HARBERT

RICE,

D. H. NOCHUMS~N and G. M. HIIX

average sulfur content of humid soils is O&49:,(Buckman and Brady, 1969). Sulfur may be added to agricultural soils in association with phosphorus and potassium in fertilizers. Using the relative abundance of sulfur in fertilizers, and the average application rate calculated by the Department of Agriculture for the U.S. in 1972,4 kg(S) ha- ’ y - ’ is calculated to be added to soils through agricultural activity. Independent estimates have been made of sulfur losses for agricultural crops. Data from Buckman and Brady (1969) show standard estimates of sulfur loss by crop removal of approx. 15 kg(S) ha- ’ y’- ‘. These compare to the average values calculated from Whittaker and Likens, (1973) data on biomass productivity. The regional level of acidic mine drainage is difficult to estimate, Hitchcock and Wechsler (1972) have placed this at 3 x 106 tons S in the United States based on surface mining of coal. This level is given by Geraghty er al. (1976) as 1.3 x lo6 ton y-‘. Using an area of 4 x lo6 acres disturbed by surface mining based on 1972 estimates of the U.S. Dept. of Agriculture, the annual sulfur mobilization from acid mine drainage is placed in the range of 900-2000 kg(S) ha-’ y -‘. Table 1 summarizes the estimated rates for potential biogenic emissions and rural mobilizations of sulfur. The estimates range from < 1 kg(S) ha- ’ y- ’ to 2000 kg(S) ha- ’ y-i. Thus, the land area which is active in sulfur emissions evidently is a key parameter for disaggregation of the natural inventory. Although the marshland and estuary production is large, the consideration of sulfur release by ratio of available (organically) bound sulfur to carbon may be low. Workers have recognized, for example, that the release of sulfur by anaerobic sulfate reduction potentially can be much larger than the estimate considering only bound sulfur. Two recent experimental studies, for example, have reported widely different sulfur fluxes. The first provided an estimate of the flux of H,S from tidal flats near the Island of Sylt in the North Sea (Jaeschke et al., 1978). H,S profiles were measured; calculation of the flux from these yielded a value of about 0.5 kg(S) ha- ’ y-l. In a second study by Hansen et al. (1978), H,S emissions were measured in a ventilated box covering coastal sediments under a thin layer of water. For two locations in Denmark chosen for high sulfur mobilization potential, values of 180 and 440 kg(S) ha-’ y-l were reported. These two results give very different H,S fluxes for similar ambient air levels of H,S (between 0.4 and 3Opg m-“). The reason for this difference is not apparent, though Hansen er al. (1978) note the biological nature of the marsh bacteria represent extreme conditions for sulfur mobilization and have a crucial influence on the sulfur emissions. The two results point to the enormous potential differences to be encountered for biogenic emissions depending on the local microecology of the wetlands. Without knowledge of the activity of the wetlands, it is not possible to extrapolate these individual measurements to regional or subglobal scale emissions. However, an estimate can be made using

macroscopic arguments by assuming as an extreme that biogenic S emissions are dominated by anaerobic sulfate reduction and considering global emissions scaled to active surface areas. Taking the most recent global budget of Granat and Rodhe (19761, the natural S emissions are I? x IO6 tonne (S) y -” for terrestrial land production, and 17 x lo6 tonne (S) y- ’ for the oceans. If marshlands or wetlands are the sole source of production, for example, then S-emission values would be 85 kg(S) ha--’ y-i using 2 x 1O’ha for the marshland area based on Whittaker and Likens (1973). in the same manner, if oceanic sources are assumed to come from estuaries along ( - 2 x 10’ ha), S-emissions could be as high as 8.5kg(S) ha- ’ y _ ‘. If oceanic emissions are area averaged over the continental shelf (27 x 10’ km*), then emissions would be about 6.3 kg(S) ha ’ y-‘. Thus, these global estimates place the wetlands and tideland emissions roughly between 6.3 and 85 kg(S) ha.- ’ y‘ ‘, which is lower than the Hansen et ul. (1978) levels, but significantly larger than the Jaeschke er 01. (1978) observation. The list of sulfur available for injection into the atmosphere given in Table 1 represents a maximum or upper bound expected from mobilization. Thus, for comparisons, the global budget estimates above must be scaled by the fraction of “available” sulfur mobilized (a). There is virtually no information available in the literature from which one can derive an estimate of this fraction. Granat and Rodhe (1976) have argued that a should be a few percent from global budgeting of sulfur. Rice and Nochumson (see Hidy et ul., 1977) speculated that Goldhaber and Kaplan’s (1975) reduction rates between lo- ’ and 10e4 mole SO; y - 1in marine sediments would support a value of D~0.1. Nriagu and Coker (1976) have suggested ~~0.01 based on sulfur mobilization estimates for Lake Ontario sediments. Comparison of the Jaeschke er ul. (1978) observations with those from Table 1 for marshlands and estuaries suggests a 2 0.4. Hansen et ai. (1978) suggest that a z 0.2 to 0.9, bearing in mind the very high emission rates they observed for a saltwater marsh and a lagoon. If the release of mobilized sulfur from terrestrial sources is analogous to the marine sediments, a “probable” regionally applicable value of a can be taken as 0.1 for purposes of this discussion, Thus, the (extreme) upper bound for emissions anaerobic sulfate reduction based on the global argument is 63-850 kg(S) ha- ’ y-l, as listed with other estimates in Table 1. The range of urban and anthropogenic production rates are lo--50 kg(S) ha- ’ y- 1 based on emissions of from 40 to 200 x lo3 tonne (S) y - ’ in 2” latitude by 2” longitude regions in the United States. For biogenic sources to operate on the same order of magnitude, production rates of 5,000 kg(S) ha- ’ y - ’ would be required for 1% of the regional land area or of 500 kg(S) ha-’ y-i for land areas on the order of 10%. Most of the upper bound values for biogenic emissions calculated in Table 1 fall in the range l-25 kg(S) ha- ’

Contribution of anthropogenic and natural sources to atmospheric sulfur

5

Table 1. Estimated upper bound rates for potential biogenic emissions and sulfur mobilization where g = 1. Processes listed are estimated to be the most significant sulfur

sources on a subg~oba~scale Rate kg(S) ha-’ y-l

Source

Description

4-12

Grassland net organic S production

6-14

Temperate forest net organic S production

C :S ratios 500 : 1 and 250:1, and W-L* data C:S ratios of 1OOO:l SoO:i, and W-L* data

Desert scrub

C:S ratios of 1ooO:l 500:1, and and W-L* data

6-24

Cropland net organic S production

5-50

Marshland and estuaries net organic S production

C:S ratios of 1OOO:l 250:1, and W-L* data C :S ratios of 1000 : 1 lOO:l, and and W-L* data

0.3-0.6

63-850

Marshland and estuaries, estimated anaerobic sulfate reduction

Global budget scaled to active surface area, and assuming CL= 0.1

4

Average fertilizer application rate

Estimates in Hidy et al. (1977)

900-2000

Average acid mine drainage mobilization in U.S.

Hitchcock and Wechsler (1973); Geraghty et at. (19761.

* W-L based on Whittaker and Likens (1973) carbon production data.

Table 2. Regions selected for detailed analysis of anthropogenie and natural S sources Region* 44” x 74” New England

38” x 80 Mid-Atiantic, Virginia 28” x 82 Florida

82” x 84” Midwest Ohio

42” x 92” Midwest Illinois 46” x 92 Midwest Wisconsin 38” x 114” Western Arizona

Description New England, including parts of Mass., New Hampshire, Vermont and New York ; high density population ; urban and forest. Southern Atlantic climate, Virginia farming, forest, and some mining. Florida, tropical climate plus

area, some farming and mining Midwest, Michigan and Ohio includes Lake Erie shoreline; isolated heavy industry; farming plus surface mining. Midwest, Illinois, Iowa and Missouri; corn, soybeans, cattle and hog region. Midwest, Minnesota and Wisconsin ; daily farming. Western Utah, Arizona; nonurban, farming and desert vegetation.

* Refers to the upper left corner of a 2” x 2” box.

y-’ and are not likely to provide production rates on the order of urban areas unless operating over lOOo/,of the region, which is clearly not the case. However, the processes of bacterial sediment sulfate reduction and acid mine drainage suggest rates which could be comparable to regional anthropogenic emissions; these are in the range of 50 kg(S) ha-’ y-’ to 2OOOkg(S)ha-’ y-l. Analysis by spec$c region

For regional scale calculations, seven 2” longitude by 2” latitude grid cells were selected, mostly from the eastern United States. The seven regions selected for specific analysis are listed in Table 2 and are shown by location in Fig. 1. The land use for each selected source was obtained from the National Atlas (1970), and was apportioned to the latitude-longitude grid cells. The natural or quasi-natural biogenically related emissions were calculated for each region using the rates shown in Table 1. These are summarized with land use areas in Table 3. For comparison, estimated ant~opogeni~ emissions also are given in Table 3. The anthropogenic emissions were derived from EPA (1974) data by Air Quality Control Region (AQCR). The EPA inventories predate major power plant developments in northern Arizona. As suggested in Table 3, the total upper bound biogenic emissions from sulfur mobilization are in

Fig. 1. AIbers equal area projection map showing 2’ latitude by 2’ longitude study areas.

approximately the same range as the estimated anthropogenic emissions in all cases selected except for the desert Arizona-Utah area, where they exceed the latter. In industrialized New England and Illinois, the highest estimate of biogenic emissions is less than the anthropogenic emissions. However, in Ohio, Florida, Wisconsin and Virginia, the highest estimate exceeds the anthro~genic emissions. In the case of New England, Wisconsin, Virginia and Arizona, forest lands represent a significant fraction of potential natural sulfur emissions. Mine drainage may be a large fraction of exchangeable sulfur in all regions. Mine drainage appears particularly important in the Midwest-Ohio, Virginia, and Florida regions. Agricultural activities including crop production, soil fertilization and animal husbandry generally represent minor potential cont~butors by region. Where large bodies of freshwater exist with shorelines containing marshlands and exposed sediments, the sulfur emissions from peripheral muds are significant. Such sourcescould be important in the Florida or Midwest-Ohio boxes. However, one may assume that surfaces of deep freshwater bodies like Lake Erie are not in themselves significant sources of H& Large parts of Lake Erie may be a source of sulfurous gas emissions during spring and fall when the surface layers are disrupted by large scale turnover forcing water of low oxygen content to the surface. No measurements have been made to investigate this possibility. If the deep water areas are soumes of H,S, then this contribution could significantly increase the potential biogenic emissions given for the Florida and Midwest-Ohio regions. The range of emissions for possible biogenic sources is very large, and could be a factor in several of the areas considered. In an attempt to cross-cheek this significance, the range of total S emissions is calculated for the seven regions within the United States

using a regional box model. The regional box model. the parameters values used and the evaluation results are discussed in detail by Nochumson and Rice in Hidy et al. (1977). The model is based upon an annual atmospheric sulfur budget in a box (Nochumson, 1976). The box area used in the calculations is 2 longitude by 2” latitude and has a height equal to the height of the average mixing layer. The model accounts for the processes of advective transport of sulfur dioxide and sulfates into and out of the box, transformation of sulfur dioxide to sulfate within the box, removal of sulfur dioxide and sulfate by wet and dry deposition, and addition of sulfur dioxide into the box by sources at the ground. The calculations were done for seven regionai areas, and the rest&s are described in Hidy et at. (1977). The anthropogeni~ emission data were based on tabuIations of the U.S. EPA (1974). These reported emissions by Air Quality Control Region (AQCR) were mapped into emissions by area. The material balance or budget model was used to estimate the sulfurous emissions required to account for SOi- In the regional boxes. The difference between the estimated emissions and the anthropogenic emissions by regional box was identified with a residual natural emission r~uirements. After accounting for the emissions from anthropogenic sources, backcalculated residual emissions for the regions were compared with the potential natural nonurban emissions. In general, there was a reasonable agreement between man-made AQCR emissions and the mean values of the back-cahuiated emissions. When the full range of the back-calcufated emissions were examined based on values, it appears that the bank-maculated values tended to overestimate emission sources. The range of uncertainty of the residuals was large so that the upper limit vaiues or the probable biogenic emissions could be a factor in the sulfur balance. However, comparison with the meun estimutes and the

Area* kg(S)? Area* kg@ft Area* kg(S)* Area* kg(S)? Are&* kg@)t Area* kg(S)? No headf kg(S)? No. head: kg(Si)t No. head2 kg@)+ kgfS)t Biogenic {a = 1) Anthropogenic

* Area expressed in lo3 ha. t Emission expressed in 10b kg(S) y-t. $ Thousands of head.

Fertihzers Totals

Beef cattle

Hogs

Dairy animals

Fresh Ha0 sediment

Desert

Mine drainage

Crops

Grasslands

Forest

36-91 97.4

4.7

1163.6 6.7-28 to.3 9.3-21

2654.5 15-37

12 57-270 236

148.6 0.74-130

18-71 28.6 26-57

2964.3

107.1 0.65-1.5

5.5 42-260 35.4

5.0

51-870 84.9

201.7 l-170 317 1.2 918 1.8 288 1.4

1378.2 8.3-33 8.78 7.9--18

2420.2 15-34

892.3 4.5-760

1261.5 7.6-30 29.4 26-59

1230.8 7.4-17

7.3 43-110 41.5

12.9 12-26

1827.6 1l-44

2103.4 13-29

17-39 0.3

0.3

1586.2 OS-l.0

2.05 1.8-4.1

69.0 0.4-1.7

172.4 0.7-2.1

2172.4 13-30

Table 3. Comparison between the upper bound estimates of biogenic emissions and anthropogenic emissions for selected 2” longitude by 2” latitude -. Ohio Virginia Wisconsin Arizona New England Midwest Fforida Midwest Mid-Atlantic west source Variable 44” X 74” 42” x 84” 28” x 82” 46” x 92” 38” x 114” 38” x 80”

83.4 0.3 2973 5.9 610 3.0 16 58- 142 153

11.5 ICI-23

23-94

3925.9

42” x 92”

Illinois Midwest

regions

AQCR emissions suggests that, within it factor of two, the AQCR emissions could account for the atmospberic s&fur without requiring any biogenic emissions. ‘Phus, the rough calculations did not WplpOFi major contributions to regional atmospheric sutfur from biopnic sources. The relation between estimated biogenic emissions and a~~~~~~~~~~~ emissions is shown ~~a~~~~a~~~ in Fig. 2. Both the upper bound leveis, II= 1.0, and the more Iikely levels assuming cc=O.t. are shown for comparison. Except for the arid Southwest and Florida, the upper limit biogenic emissions are similar, with a XBean Ievel ~~g~g from 40 x 103 to 260 X f@ tonne S y-‘, where wetlands are prreyalent in areas like Florida, the (extreme) upper timit of biogenic sulfur emissions is very large. The probable biogenic emissions estimated for the seven regions do not represent a s~~~~ant fraction of overall sulfur emissions unless the anthropogenic contribution is below roughly 50- 100 x 103 tonne S y ._1 in a regional celi. For a 2” longitude by 2” latitude area, this is approx. 15-30 kg(S) ha-’ y-‘~ This focuses inierest for biogenic activity in the middle and south Atlantic states, and the rural are&s ofthe Midwest, and West. It should be noted, of course, that the levels of upper limit biogenic S emissions are large enough to impact the

total sulfur budget in the atmosphere in all the areas examined.

Global sulfur budgets have suggested that a major fraction of the atmospheric sulfur burden may be ~de~ti~~d with natural emissions of sulfurous gases. The most prevalent natufai sulfur contai#~ng gas is beiieved to be hydrogen sulfide. Sulfurous gases, including H,S and SO,, are removed from the air by oxidation to form particulate sulfate, which is a s~g~~~~t fra&on of the totat ~art~~u~ate burden. Because af the paucity of measurements and knowledge of the mechanisms of H,S and other natural. sulfurous gas emissions, it is difficult to estimate directly their contribution to the sulfur oxide conce~~ratjons in the atmosphere. We propose a calcuIati~~~~~~~ upper bound sulfur emissions as related to the mobiIization of suIf& in the biosphere, combined with animal husbandry, fertilization, crop losses and acid mine drainage. This method confirms suggestions ofother workers that )izS and sulfur production from b~o~~~~~~ processes in sragnant water and anoxic muds may be especially important in an atmospheric sulfur balance. The analysis also suggests that acid

Contribution of anthropogenic and natural sources to atmospheric sulfur mine drainage appears to be a potentially major source of mobilized sulfur which may reach the atmosphere in reduced form. The estimates of sulfur from biogenic origins were compared with anthropogenic emissions of sulfur from sulfur dioxide in seven regions in the United States. The regions were identified in terms of 2” latitude by 2” longitude geographical sectors. The regions were selected to range from highly industrialized, to highly agriculturally utilized, to rural semi-desert. Comparison in the seven sample regions suggests that the natural or quasi-natural emissions do not appreciably influence the total atmospheric sulfur burden unless anthropogenic emissions are less than 50-100x 10” tonne (S) y- l, or less than 15-30 kg(S) ha- ’ y- ’ over regions 200 km2 and larger. This restriction is expected to focus on the rural, non-industrial areas of the United States, or to areas where wetlands are prevalent. In the eastern United States, where high levels of airborne sulfate are observed, our calculations suggest a minor, probable contribution from natural sulfur gases assuming that 10% of the available biogenic sulfur pool is released to the atmosphere. There is no observational basis at present for this deduction, though some recent work is consistent with this constraint for the sulfur fraction released. The proposed method for estimation of biogenic sulfur emissions is highly uncertain based on lack of knowledge of the fraction (a) of mobilized sulfur released to the atmosphere and the assumptions about the “active” area for emissions, especially for wetlands. The maximum emission level assuming tzapproaching unity appears unlikely. The estimation of emission of sulfurous gases from biogenic and natural processes is very tenuous without measurements. Recent reports of direct attempts to measure the H,S flux from tidal flats and salt water marshes give widely different results, ranging from less than 1 kg(S) ha-’ y- ’ to greater than 4000 kg(S) ha-’ y- ‘. Assuming that the measurement methods are valid, there is little known about the processes which contribute to such variability. To improve such estimates, careful atmosphere-water or soil investigations continue to be required to measure the sulfur gas transport rates under different natural conditions. Studies of surface water, anoxic mud-air transport and acid mine drainage chemistry are needed to establish directly the flux sulfur emissions into the air, and to give improved estimates ofthe parameter CI,and the ecological nature of the surface area important to sulfurous emissions. Acknowledgements ~ We are indebted to the Air Quafity Committee of the American Petroleum Institute (API) for sponsorship of this work. We are grateful to the members of the API Sulfate Task Force, who assisted us with helpful comments and criticisms of this study. The comments and criticisms in checking the calculations ofDrs. R. C. Henry and D. Hitchcock are gratefully appreciated.

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