The Atmospheric Sulfur Cycle in the Tropics...the atmospheric sulfur cycle in equatorial regions. The two main biomes in these regions are the forests and the savannas. It is difficult

Acidification in Tropical CountriesEdited by H. Rodhe and R. Herrera@ 1988 SCOPE. Published by John Wiley & Sons Ltd

CHAPTER 2

The Atmospheric Sulfur Cycle in theTropics

ROBERT DELMAS AND JEAN SERVANT

2.1 Introduction. . . . . . . . . . . . - . . . . . . . . . . .2.2 The Atmospheric Sulfur Cycle in Equatorial Regions. . . . . . .

2.2.1 The Tropical Environment. . . . . . . . . . . . . . . .2.3 Production of Atmospheric Sulfur. . . . . . . . . . . . . - .

2.3.1 General Survey of the Transformation of Sulfur. . . . . . .2.3.2 Influence of the Environment on Biogenic Sulfur Sources. . .

2.4 Biogenic Sources. . . . . . . . . . . . . . . . . . . . . .2.4.1 Biomass Burning. . . . . . . . . . . . . . . . . . . .2.4.2 Marine Sources. . . . . . . . . . . . . . . . . . . .2.4.3 Soil-derived Sources. . . . . . . . . . . . . . . . . .2.4.4 Other Sources. . . . . . . . . . . . . . . . . . . . .2.4.5 Conclusions. . . . . . . . . . . . . . . . . . . . . .

2.5 Sulfur Compounds in the Tropical Atmosphere. . . . . . . . . .2.5.1 Concentrations of Sulfur Compounds. . . . . . . . . - .2.5.2 Concentration of Sulfur Compounds in the Equatorial

Atmosphere. . . . . . . . . . . . . . . . . . . . .2.6 Sinks of Sulfur Compounds in Intertropical Regions. . . . . . . .

2.6.1 Sinks. . . . . . . . . . . . . . . . . . . . . . . .2.6.1.1 Dry Deposition. . . . . . . . . . . . . . . . .2.6.1.2 Wet Deposition. . . . . . . . . . . . . . . . .2.6.1.3 Seasonal Variations in Dry and Wet Deposition. . .2.6.1.4 Acid Rainwater and Influence of Sulfur Compounds.2.6.1.5 Transport-Influence of Equatorial Regions on the

Global Atmospheric Sulfur Cycle. . . . . . . . .2.7 Conclusions. . . . . . . . . . . . - . . . . . . . . . . .

References. . . . . . . . . . . . . . . . . . . . . . . . .

2.1 INTRODUCTION

Sulfur is ubiquitous in nature: it exists in soils mainly as organic compounds,but also as sulfate or sulfide (0.5 mg S/g); in sea water as sulfate (2.65 mg

43

434S474848485153555656565757

57606060606161

666667

44 Acidification in Tropical Countries

S/g); in plants as sulfate, sulfide, or organic compounds (0.1 to 2 mg/g ofdry matter); and in the atmosphere in gaseous and solid states (0.3 /Jg to 1/Jg/kg). The exchange of sulfur between these different media generally takesplace in the atmosphere. Sulfur is emitted into the atmosphere in a more orless reduced state as hydrogen sulfide, organic sulfides, or as sulfur dioxide.Much of it is then oxidized into sulfuric acid or its salts and removed mainlyby precipitation.

In the absence of major climatic changes this natural sulfur cycle is sta-ble, disturbed mainly by volcanic eruptions. A recent perturbation is theresult of industrialization, mainly as a consequence of increasing energydemand. Based on evidence from Arctic polar ice samples (Herron, 1982;Murozumi et at., 1969), this perturbation became significant around 1900and is mainly confined to the Northern Hemisphere. Assessments of sulfurfluxes from both natural and anthropogenic sources vary (Kellog et at., 1972;Friend, 1973; Robinson and Robbins, 1975; Granat et at., 1976; Cullis andHirschler, 1980; Sze and Ko, 1980; Muller, 1984a, b). In these studies thenatural emissions were poorly known owing to the small number of reliablemeasurements. Natural processes have become better understood during thepast decade thanks to data obtained by Aneja et at. (1979a, b), Adams etat. (1979, 1980), Delmas et at. (1980a), Delmas and Servant (1983). Thesestudies, carried out in the eastern US and in equatorial Africa, showed thata positive gradient of the S-flux is to be expected with decreasing latitudesand that the maximum occurs in equatorial regions.

The global natural and anthropogenic emissions based on the most recentestimations are presented in Table 2.1. The production of sulfur by sea-spray has been omitted. The natural emissions emanating from land and sea

Table 2.1sources

Global emissions of sulfur compounds from natural and anthropogenic

Source Intensity References

1012g(S)/year

Natural

Marine 40-48 Andreae and Raemdonck, 1983(revised by Savoie, 1984)

Moller, 1984a

Moller, 1984a; Cadle, 1980

Continental

Volcanic

35-50

2-12

Anthropogenic in 1975 69-104 Moller, 1984b; Cullis andHirschler, 1980

The Atmospheric Sulfur Cycle 45

are approximately equal, and man-made emissions are of the same order ofmagnitude.

Sulfur compounds in the atmosphere are rapidly removed or oxidized,except for carbonyl sulfide (CaS). According to Granat el at. (1976), theproduction of submicron particles is considered the most important reasonfor the elaboration of a global sulfur budget. These submicron particlesconsist partly of sulfuric acid and sulfates. Sulfuric acid is probably themain agent of rainwater acidification even in the presence of other strong(e.g. nitric) or weak (e.g. formic and acetic) acids. However, in the ionicstate of sulfate, sulfur is a natural fertilizer in air and in some soils (Tamm,1976).

This chapter describes the sulfur cycle in tropical regions in order toprovide data on the state of acidification of air and rainwater in a tropicalenvironment before further industrialization.

THE ATMOSPHERIC SULFUR CYCLE INEQUATORIAL REGIONS

In order to describe the effects of the atmospheric sulfur cycle on a partic-ular ecosystem, three components should be considered: (i) the external orinternal sources, (ii) the chemical and physical transformations of the dif-ferent compounds emitted to the atmosphere, and (iii) the sinks that includedeposits and transport outside the ecosystem.

Different sources of atmospheric sulfur are found in equatorial regions.The main source is associated with the biological transformations of sulfurin soil and in the biosphere. The reduction of sulfates to sulfides by themineralization of organic matter under special conditions supplies the at-mosphere with gaseous sulfur compounds. These compounds are emittedin reduced forms such as hydrogen sulfide (H2S) and different methyl andcarbon sulfides.

The complexity of the cycle and of the processes concerned are illustratedin Figure 2.1. In the total sulfur cycle, reactions in the soils are bidirectional.In the strongly oxidizing environment of the earth's atmosphere, however,natural conversion from reduced to oxidized forms is the rule; there is noimportant reverse reaction from a higher oxidation state to a lower one.While the direction of the reaction does not vary during the day, reactionrates do. The processes in the soils are continuous, but their intensity ismodulated by the type of soil and vegetation, and by climatic variations.

Biomass burning is the other source linked to the biosphere; it includesbushfires in dry savannas, and fires used in agriculture or for deforestation.This source is mainly of anthropogenic origin and is sporadic, occurring onlyduring the dry season.

2.2

Soils'winderosion

Oceans'-Bubbl ing-Diffusionof methy I

1 suIf ides

1 = natural

2 = anthropogenic

Figure 2.1

CH3S03 H

. Bushfires in

savannaregions

1 Biological transformations of S in soils 2 Biomass burning

Cont inental biosphere

Simplified sulfur cycle in the tropics

Solid orliquidphase-------------Gaseous

phase

-Deforestationdue to

shiftingagriculture

-Savanna andforestburnings

~0'

Volcanoes

S02

Man-madeemissions

2


In the equatorial regions, atmosphere-biosphere exchanges are the mainsources of atmospheric sulfur, although other sources exist. The mainsources are:

(a) The oceans: all the continental zones in the intertropical regions areinfluenced by oceanic air in the lower layers of the atmosphere, eitherpermanently (South America, equatorial Africa, Oceania), or sporadi-cally, according to the latitudinal position of the meteorological equator(West Africa, tropical Asia) (Riehl, 1954).

(b) The soils: important quantities of dust, moved into the air by strongwinds, are a source of particulate sulfur. This phenomenon is particu-larly important in the Asian and African desert zones.

Other sources are industrial and domestic activities and volcanic emissions.

Some compounds introduced into the atmosphere are stable, such as par-ticulate sulfates (oceanic, soil-derived); others are unstable. The unstablecompounds can be reduced gases such as H2S (biosphere), methyl sulfides(ocean, biosphere), or carbonyl sulfides (biosphere, fires); oxidized gasessuch as sulfur dioxide (S02), emitted during combustion or derived fromthe oxidation of reduced gases; and some other species with short lifetimes.

Sulfur compounds are eliminated from the atmospheric reservoir by dryand wet deposition, with different speeds at different steps in the oxida-tion chain. Wet deposition arises from in-cloud scavenging and below-cloudscavenging. Rain acidity is due to the incorporation of acidic compounds(S02, H2SO4)' Vertical and long-range horizontal transports could also beconsidered as sinks, particularly for carbonyl sulfide (COS).

2.2.1 The Tropical Environment

The previous discussion described how the natural environment influencesthe atmospheric sulfur cycle in equatorial regions. The two main biomes inthese regions are the forests and the savannas. It is difficult to estimate thesurface of the equatorial forests in the world, as they are evolving constantlyand decreasing rapidly as a result of deforestation for cultivation. Lanly andClement (1979) give a value of 17.6 x 10 6 km 2 for all intertropical forests,with primary forests representing less than 3 x 10 6 km2. The total biomassof the equatorial forests comprises about 50% of the continental biomassof the planet. The equatorial forested zones have a high rainfall, usuallyestimated at 1,400 mm/year (Richards, 1964), and some regions receive morethan 3,000 mm/year.

Savannas cover about 20 x 106 km2 and receive an annual rainfall of be-tween 120 and 900 mm. The biomass of the savannas decreases with rainfall

amount, but no linear relation between these two parameters exists.


2.3 PRODUCTION OF ATMOSPHERIC SULFUR

2.3.1 General Survey of the Transformation of Sulfur

The natural cycle of sulfur in the soils includes a complex chain of oxi-dation and reduction reactions controlled by microorganisms. In the soils,most sulfur (75% to 99.5%) appears in organic compounds. The propor-tions of organic sulfur depend largely on the nature of the soil; the inorganicfraction remains low. Plants absorb most of the sulfur in the form of sulfateions. Organic sulfur compounds then become mineralized due to the actionof microorganisms such as bacteria, heterotrophic organisms or fungi. Stud-ies have shown that methyl mercaptan (MeSH), dimethyl sulfide (DMS),dimethyl disulfide (DMDS), and carbonyl sulfide (COS) are produced un-der aerobic conditions, and carbon disulfide (CS2), MeSH, DMS, and COSformation prevails under anaerobic conditions (Katoda and Ishida, 1972;Banwart and Bremner, 1975).

Sulfates can be fixed in soil by organisms that use them for cellular synthe-sis, or absorbed by plants and eventually animals. They can also be reducedto sulfides. Sulfate reduction is the most important cause of the emissionsof biogenic sulfur compounds into the atmosphere. One type of sulfate re-duction is the assimilatory pathway, in which organisms reduce sulfate inorder to meet their nutritional requirement for sulfur. The formation ofvolatile sulfur by this process is believed to be limited. A second type ofsulfate reduction is the dissimilatory pathway. Sulfates serve as a terminalelectron acceptor in anaerobic respiration for bacteria of the genera Desul-fovibrio and Desulfotomaculum. As a consequence, large amounts of sulfideaccumulate in some soils, the main product being H2S.

Numerous authors have studied the sulfur cycle in soils: Asami andTakai (1963), Postgate (1969), Trudinger (1969), Katoda and Ishida (1972),Schlegel (1974), Banwart and Bremner (1975, 1976) are some of them.Other works present some global analyses (Dommergues and Mangenot,1970; Alexander, 1977; Adams et at., 1980).

2.3.2 Influence of the Environment on Biogenic Sulfur Sources

Many factors have a direct influence on the emission of gaseous sulfur com-pounds from the soils: the S-content and composition of the organic matter;the distribution of bacteria; the type and hydrology of the soils, which de-termine their pH and Eh; the temperature; and the atmospheric proximityof the production zone of S-gases.

The composition of organic matter influences the nature and quantity ofsulfur gases emitted. Emission rates were linked positively to soil organicmatter by Farwell et at. (1979). However, there seems to be a threshold of


about 2% organic matter below which sulfur volatilization does not occur.Two conditions are necessary for sulfate reduction: the source of sulfatesmust be unlimited and the content of electron donors (organic acids orhydrogen) must be sufficient. Decomposing litter could be an important sitefor volatilization (Haines, 1983).

The hydrous state of the soils determines the oxygen content and theemission of sulfur compounds (Table 2.2). Water-saturated soils promotesulfate reduction through dissimilation. A laboratory study on different typesof soils carried out by Farwell el al. (1979) showed that emissions of H2Sand CS2 greatly increase with water content in swamp soils.

The most important sulfur species emitted to the atmosphere is H2S. Anejaet al. (1980) found that about 50% to 70% of the total measured sulfurflux in the eastern United States consists of H2S. Saline marshes are strongemitters compared with other types of soils.

Important variations of H2S emissions with different soils have been ob-

served by Delmas el al. (1980a) and Jaeschke et al. (1978). The intensityof the emission was much more important under anaerobic conditions, thusconfirming the dominance of dissimilatory sulfate reduction as a biogenicsource of sulfur.

Soil temperature influences the rate of microbial transformation of sulfurin soils. Enzyme activity is multiplied by a factor 2 to 3 for every 10 °Cup to 50°C. Above this threshold, enzymes in solution are rapidly inac-tivated (Katoda and Ishida, 1972). This dependence has been observed byseveral authors (Hill et al" 1978;Jaes~hket;Cal,! 1978),A linearrelationshipwith the mean ambient temperature appears, with a correlation coefficient of

Table 2.2 Sulfur fluxes of different soils in North Carolina, USA (Adams et ai.,1979

Soil order AverageS flux (glm2/year)n H2S OCS CH3SH DMS CS2 DMDS L:S

Alfisol 9 0.010 0.002 - 0.001 0.002 0.002 0.017

Inceptisol 19 0.0028 0.0022 - 0.0002 0.0012 0.0014 0.008

Histosol 11 0.0178 - - 0.0007 0.0001 0.001 0.019

Saline swamp 1 0.0194 0.0016 - 0.0069 - - 0.028

Saline marsh 7 139.50 6.36 6.56 - - - 152.40

Alluvial clay 11 0.0004 0.004 - 0.0001 - - 0.0002


250

200~0Q)>-

.......II)c:0+-

100

/,'.-/'..-.-/'

/'

150

"U~01

>-.J:Ix::3-~::3-::3

(/) 00 5 15

T °C

Figure 2.2 Regression of mean ambient temperature by East-West Sure Grid tiersversus sulfur fluxes (from Adams et at., 1980). Copyright 1980, Electric PowerResearch Institute (EPRI) report EA-1516, Biogenic Sulfur Emissions in the SURERegion. Reprinted with permission

0.85, from 158 S-flux measurements performed in the eastern United States(Adams et ai., 1980) (Figure 2.2).

Gaseous sulfur compounds are emitted into the atmosphere if the pro-duction zone is near the surface. In an oxygen zone, sulfides can be oxidizedinto sulfates by bacteria such as Beggiatioa and Thiorix. In this environment,gaseous sulfides cannot reach the atmosphere, which is why hydromorphicsoils such as flooded forest soils, swamps, and paddy fields are importantsources. This is not the case for lakes and lagoons except near the shoresor under eutrophication conditions. The sulfides produced in the sedimentlayer are oxidized before reaching the surface, as exhibited in a lake of theAmazon basin by Brinkmann and Santos (1974) (Figure 2.3). These resultswere confirmed recently by Staubes et at. (1986).

It has been believed for years that H2S formed in anaerobic sites in culti-vated soils could not escape oxidation by FeH ions, but this assertion wasdisproved recently (Delmas et at., 1980a). From a general point of view,any type of soil can be a source of reduced sulfur compounds either duringmineralization of organic matter under aerobic or anaerobic conditions, orduring sulfate reduction. This last source is probably the most important,especially under anaerobic conditions.


m9

8

Lake surface

2

(504 ~" .-

" -I /I '1/

y'.',/" ,_/ ,, ,III!.."

, \\

6

4

00 .2 .3 .4 :5 .11

mg/liter

Figure 2.3 Distribution of oxygen (02), sulfates (SO~-) and hydrogen sulfide(H2S) in an Amazonian flood plain lake (from Brinkmann and Santos, 1974). Copy-right 1974, Tellus. Reprinted with permission

2.4 BIOGENIC SOURCES

Despite its probable importance, this source is not well known. Detailedstudies, such as those carried out by Adams el al. (1980) and Aneja el al.(1979a, b) in midlatitude zones in the USA, have not been undertaken inthe tropics. A study of H2S emissions from the humid tropical forests ofthe Ivory Coast was conducted by Delmas el at. (1980a) and Delmas andServant (1983). The flux was measured using two methods. The first was byaccumulating H2S at night under a forest canopy. The second method in-volved H2S vertical profile measurements using aircraft at different distancesfrom the coast, in the direction of the horizontal advection of air masses inthe monsoon layer « 2,000 meters alt.). The first method yielded a 12.5 pgS/m2/hour flux for dry soil and 300 pg S/m2/hour for partly flooded soil.The second method yielded a similar value: 40 pg S/m2/hour above swampsand flooded forests (Delmas el aI., 1980a).

Soil emissions from forested zones were also calculated using an atmo-spheric circulation model in which the planetary boundary layer (the mon-soon layer in that case) was considered to be a box at equilibrium. Theconcentration of sulfur in particulate matter (SO~-), S02 and H2S weremeasured over a one-year period at the entry and exit of the box. The re-sults obtained for the exchanges of sulfur compounds allowed the calculationof soil emissions (Figure 2.4).

Sulfur emissions showed pronounced seasonal changes. Values were lowduring the dry season, 55 pg S/m2/hour; and high during the wet season,270 pg S/m2/hour. The annual mean value was 170 pg S/m2/hour or 1.5 g


300

... 400::>0~......'"E

......CJI

:t

200,Annual mean

value

100

0J F M A M A SON D

Figure 2.4 Monthly variations of the sulfur flux emitted by the soils in a forestedzone in the Ivory Coast (from Delmas and Servant, 1983). Copyright 1983, Tellus.Reprinted with permission

S/m2/year. These values have the same order of magnitude as those deductedfrom measurements and are in accordance with the relationship between S-flux and latitude, increasingtoward the equator. Adams et ai. (1980) predictan annual average H2S flux ranging from 0.5 to 1.0 g S/m2/year near theequator; the total suifur flux might be as high as 2.3 g S/m2/year at theequator.

The above approach to estimating fluxes accounts for emissions from dif-ferent types of soils and shows seasonal variations in intensity of the source.However, this determination is not precise due to uncertainties regardingchemical and meteorological parameters, and all S-compounds were nottaken into account. The flux of organic sulfides is probably low: during thedry season, (CH3h SH + C2Hs SH emissions reach only 1 to 70 X 1O-4gS/m2/year in the La Selva rainforest in Costa Rica (Haines et ai., 1984),about 20 x 10-4 the average value calculated for the Ivory Coast.

In a forest at equilibrium, external inputs and outputs of sulfur must


be equal. Outputs are the sum of emissions into the atmosphere from lit-ter, soils, and river run-off. In the tropical rainforests of Costa Rica andVenezuela, net exchanges of sulfur are 1.17 and 0.4 g S/m2/year respectively(Haines, 1983). If river run-off is taken as negligible, these values representmaximum emissions of gaseous sulfur to the atmosphere. They are compa-rable to the mean H2S fluxes measured in the Ivory Coast, and correspondto a global emission for the tropical forests of 21 and 7 Tg S/year comparedto 27 Tg S/year from the data obtained in the Ivory Coast study. Clearly,any extrapolation from available data presents large uncertainties. Furtherresearch is needed in order to evaluate this source accurately. Such studiespresent difficulties because source intensities vary with the nature of the soilsand with soil water status. For example, data obtained for water-saturatedsoils are an order of magnitude greater than those for dry soils.

A detailed study of all sulfur compounds emitted is needed. Studyingfluxes of COS and CS2 in relation to the global S-cycle would be interestingeven though these gases are less important on a regional scale due to theirlonger residence time in the atmosphere.

2.4.1 Biomass Burning

Biomass burning is an important source of aerosols and gases in tropical ar-eas. Primarily anthropogenic, these burnings occur for three reasons: defor-estation for cultivation; clearing associated with shifting agriculture; and an-nual bushfires, which are traditional in the dry savannas. This phenomenonis particularly important in Africa where 40% to 60% of the savannas lo-cated between 5° and 10° N are burned each year (Deschler, 1974). Seilerand Crutzen (1980) (see Table 2.3) estimate the spreading of these fires andthe biomass consumed. In their approach the total amount of the biomassburned annually (M) in a biome is:

M = A x B x Q X f3 (gram of dry matter per year) = g dm/year

where:

A = total land area burned annually (m2/year)B = the average of organic matter per unit area in the individual biome

(g dm/m2)Q = fraction of the average above-ground biomass relative to the total

biomass B

f3 = the burning efficiency of above-ground biomass.

In order to infer thequantitiesemittedinto theatmospherefrom bioma~~burning, the sulfur content of the biomass and the amount of sulfur


volatilized during the fires must be determined. Such an estimation is diffi-cult because the S-contents of various plant species in forests and savannasvary, ranging from 0.2 to 2 mg/g (0.02-0.2%). However, the fraction ofvolatilized sulfur depends on the type of plant and on temperature of thecombustion. Two examples are given for a savanna in the Ivory Coast (Table2.4). The fraction of volatilized sulfur is 67% for a Loudetia savanna and35% for an Andropogon savanna. Clearly, it is difficult to evaluate accu-rately the intensity of this source; more data are needed. An estimation ofthe magnitude of the source can be made by considering the mean biomassburned annually, 3.6 Tg dm/year (Seiler and Crutzen, 1980), with a maxi-

Table 2.3 Amounts of biomass burned annually in tropical and subtropical areas(Seiler and Crutzen, 1980) (unit: 100 Tg dm/year)

Table 2.4 Sulfur emissions during bushfires in two types of savanna in the IvoryCoast (Delmas, 1982)

Total above-ground biomass beforethe fire (T(dm)/ha)

S content in the dry matter (mglg)

S amount in above-ground biomassbefore the fire (kglha)

Average percentages of sulfur volatilized

S fluxes (kglha)

Activity Burned area Biomass exposed Annually burned(million hectare) to fire biomass

Burning due to 21-62 24-72 9-25shifting agriculture (42) (48) (17)

Deforestation due to 8.5-15.1 16-25 5.5-8.8population increase (12.0) (20.5) (7.2)and colonization

Burning of savanna 12.2-23.8 4.8-19and bush lands (600) (18) (11.9)

Loudetia Andropogonsavanna savanna

7 9

0.29 0.33

2.03 3.0

67 35

1.40 1.05


mum S-content of 0.2% (Bowen, 1969; Shriner and Henderson, 1978) andan S-volatilization efficiency of 50%. This yields a maximum emission of3.6 Tg S/year. In the intertropical zone as a whole, this source is weakerthan the biological source, but it may be significant in some regions. Ourestimate is lower than that of Andreae (1985), which is about 7 Tg S/year,corresponding to an annual biomass burning of 6,800 Tg dry matter. Ourcalculation concerns only the tropics and does not include the use of woodas fuel, which represents about 1,000 Tg dry matter/year.

Compounds formed during combustion have not been analyzed. Measure-ments in the Ivory Coast showed that S02 was dominant in the smoke plumeand that H2S was also formed (Delmas, 1982). Very hot air displaced by thewind before the flames causes the vegetation to parch and become partiallypyrolized before burning. This is likely the cause of the emission of reducedgases such as H2S. Emissions of COS have also been observed by Crutzenet ai. (1979). The ratio of COS/C02 concentrations = 6 x 10-6 obtainedfrom aircraft measurements corresponds to a global source equal to 4 x1010 g (COS-S)/year, or one-tenth of the total emission of COS into theatmosphere (Crutzen et ai., 1985).

2.4.2 Marine Sources

Oceans are the major natural source of sulfur in the troposphere, with SO~-a ubiquitous trace species in the oceanic atmosphere. A large fraction of thisSO~- is derived from sea-spray and is not discussed further here. An ex-tensive survey on sulfate concentrations from nonmarine origins over 25regions of the world ocean concluded that the average S-concentration isabout 0.7 j.tg/m3.This concentration differs across regions, e.g. by a factorof 30 to 50 for polluted regions of the Mediterranean Sea and the pristineareas of the southern oceans (Savoie, 1984). In tropical areas, the high-est concentrations were recorded to the west of the continents: 1.6 j.tg/m3in the Gulf of Guinea, 5 j.tg/m3 west of South America, 1.1 j.tg/m3 in theNorth Atlantic Ocean, 2 j.tg/m3 in the Gulf of Mexico, and 1.5 j.tg/m3 inthe Bay of Bengal. In the intertropical oceans, values of 0.3 to 0.6 j.tg/m3were observed. Higher concentrations are often linked to terrigenous duststransported from the deserts of Arabia, India, Pakistan, and Sahara. Nearthe west coast of South America a high sulfate concentration results fromindustrial activity in Chile. Near the northwest coast of South America thehigh sulfate concentration can be attributed to either pollution or a naturalsource.

The S02 concentration in air is not necessarily linearly correlated withoceanic productivity (Nguyen et ai., 1978; Bonsang, 1980; Andreae et at.,1983; Cline and Bates, 1983). However, such a relation between the finalproducts of S-compounds and the emission of DMS has been demonstrated.


2.4.3 Soil-derived Sources

Sulfur derived from soils is associated with wind erosion. At high windspeeds, dusts are suspended in the atmosphere, and the finest aerosol parti-cles are carried by air masses over thousands of kilometers during dry hazeepisodes (Prospero and Bonatti, 1969; Bertrand et al., 1974; Rahn, 1981;Duce et al., 1980; Bravo et aI., 1981; Darzi and Winchester, 1982; Ue-matsu et aI., 1983; Parrington et aI., 1983; Ryaboshapko, 1983). In Africa,this source represents 15% of the atmospheric SO~- in the equatorial zone(Delmas et aI., 1978; Delmas and Servant, 1983). This value was estimatedfrom chemical criteria and explained by the presence of gypsum in the zoneswhere the dry haze originates.

2.4.4 Other Sources

Volcanic emissions and man-made pollution are other potential sources ofsulfur compounds in intertropical zones. Active volcanoes in tropical zonesare numerous, but most of the sulfur emitted during eruptions, about 2 TgS/year globally (Muller, 1984a), probably enters the stratosphere directly.Estimations from Naughton et al. (1975) and Cadle (1980) are higher, at23.5 and 20-30 Tg S/year. If we consider the amount of sulfur in the oceans,sedimentary rocks, and evaporites, these values are too high on a long-termbasis. Volcanic emissions into the troposphere during noneruptive periodsappear to dominate and a global release rate of 12 Tg S/year is probablya higher limit for the volcanic flux. This source may be important on theregional scale during volcanic episodes.

The present anthropogenic sulfur emissions over the world are estimatedas 104 Tg S/year (Cullis and Hirschler, 1980) and 75 Tg S/year (Muller,1984b). The difference between estimates is due to the higher emission fac-tors for coal and lignite used in Cullis and Hirschler's work. Global S02emissions are expected to increase between 1975 and 2000, mainly due toincreased coal combustion. In 1979, man-made sulfur emissions are esti-mated to have been 1.1, 2.1, and 0.35 Tg S/year for Africa, Central andSouth America, and Oceania, respectively. The total emissions of these re-gions represent 8% of the world emissions (Varhelyi, 1985).

2.4.5 Conclusions

The major component of the atmospheric sulfur cycle in the temperate lat-itudes is anthropogenic. In inter-tropical regions, however, the main terres-trial source is natural biomass decomposition. Recent studies indicate that


most soils and litter decomposition can be major sources of sulfur com-pounds for the atmosphere, with the greatest emissions occurring underanaerobic conditions. In humid equatorial regions where rains are abundantand anaerobic sites are common, the high temperature and organic contentof the soils present favorable conditions for greater S-emissions. Floodedsoils, probably the greatest source, depend on the annual rainfall distri-bution. The emission of sulfur compounds in savanna soils has not beenmeasured, although one might assume that its value is lower than in humidareas because anaerobic conditions are less common. One important sourcecould be biomass burning, but this is a difficult source to evaluate as it issporadic and occurs during the dry season.

In some tropical countries, particularly those with high population den-sities in tropical Asia, anthropogenic emissions might exceed natural ones.In China, very high S02 concentrations are recorded south of the YangtzeRiver as a result of coal consumption (Zhao and Sun, 1986).

2.5 SULFUR COMPOUNDS IN THE TROPICAL ATMOSPHERE

2.5.1 Concentrations of Sulfur Compounds

The data on concentrations of sulfur compounds are limited. Most measure-ments concern total S or SO~- concentrations in particulate matter (Table2.5). No measurements of carbon sulfide concentrations are available for thetropical zone.

2.5.2 Concentration of Sulfur Compounds in the Equatorial Atmosphere

Several authors have measured the size spectrum of particulate SO~-. Theresults obtained in the Amazon basin by Orsini et at. (1982) are shown inTable 2.6. The SO~- particles were collected with a cascade impactor six-stage Battelle Model); the analysis of sulfur was carried out by Pixe (particleinduced X-ray emission).

The spectrum obtained differs from that of oceanic and continental atmo-spheric particles. The maximum mass of SO~- corresponds to the submicronparticles. Similar results were obtained in the Ivory Coast (Delmas, 1980)and in the Congo (Delmas et at., 1986). The cascade impactor used in thelatter study was a six-stage high-volume sampler (Weather Measure model),and the analysis was carried out by atomic absorption. About 70% of theSO~- mass corresponded to the submicronic fraction. On the other hand,chemical criteria for marine- and soil-derived origins have shown that 70%of the massof SO~- in parti~ulate matter correspondsto airborne sulfatesin the Ivory Coast (Delmas el at., 1978).

V100

Table 2.5 Atmospheric concentrations of sulfur compounds in the intertropical zone (average values in parentheses)

ReferencesDMS

Location Concentration (nglm3)

SOz SO~- HzSS particulate

Lawson andWinchester, 1979

Orsini el ai., 1982

Lodge el ai., 1974

Penkett el ai., 1979

Delmas el ai.,1978 andDelmas andServant, 1983

Bolivia, mountainManaus, BrazilSalvador, Brazil

51131260

Amazon Basin 215-342

Panama,equatorial forest

850-3,150

Sudan 500-9,000

500-3,8001,700)

50-3,500(600)

2.7-7.1Ivory Coast,equatorial forest

500-2,000(650)

n- - - -

This particle-size distribution, mainly in the submicronic range, impliesthat SO~- is formed primarily from homogenous gas-phase reactions. If theformation had been the absorption and subsequent reaction of vapor-phaseH2SO4 on seasalt or soil-derived particles, the SO~- mass distribution wouldhave been similar to the surface distribution of these particles. In that case,SO~- would have shown a mean mass diameter of 4 J.l.m.

No in situ study on the mechanism of S-oxidation in equatorial regionshas been reported, but results were obtained using a budget method (Delmasand Servant, 1983). Above a forested zone in the Ivory Coast, the calculatedmonthly mean values of the oxidation rate show a variation from 5.3% to12.9%/hour with an annual arithmetic mean of 8.4%/hour. This value is anorder of magnitude higher than those observed at midlatitude rural zones. Itwas obtained through an indirect budget method, so precision is likely to below. However, it seems to account for the concentrations of sulfates observedon the continent. A direct measurement of this oxidation rate in Australia

(Ayers, 1986) yielded a lower value of O.25%/hour, but the photochemicalcharacteristics of the atmosphere in Australia and equatorial Africa mayhave been different.

Rapid gas-phase oxidation suggests that the equatorial atmospherepresents favorable features, in particular a relatively high concentration ofOH-radicals. Between 15 x Nand 15 x S, the photochemical model ofLogan €l at. (1981) givesan OH-concentration of about 2 x 106 atoms/em3at ground level. Crutzen and Gidel (1983) obtained similar results.


Table 2.6 Granulometric mass-spectrum of sulfur in atmospheric aerosol in theAmawnian basin (Orsini et al., 1982) (units: ng(S)/m3)

Sample IC stage 0 I 2 3 4 5 (S)Size range (jlm) <0.25 0.25-0.5 0.5-1 1-2 2-4 >5

IC AM 4A 27 160 92 23 14 26 342

IC AM 4C 19 157 132 23 24 20 375

IC AM 3A - 136 57 12 7.8 2.0 215

IC AM 3C 5.8 96 106 - 8.5 3.0 219

IC AM 2A 3.1 138 48 5.7 12 - 235

IC AM 2C - 127 74 9.3 12 3.0 226


2.6 SINKS OF SULFUR COMPOUNDSININTERTROPICAL REGIONS

2.6.1 Sinks

Sulfur compounds leave the atmosphere by dry and wet deposition. Theelimination of sulfur in an ecosystem also occurs via horizontal advectionand flux of matter outside the limits of the ecosystem. Deposition processesdepend on the chemical and physical characteristics of the compounds, suchas size spectrum for aerosols and solubility for gases. The residence time ofa compound in the atmosphere depends on all these parameters.

2.6.1.1 Dry Deposition

Gases or particles are deposited by mechanisms that depend on near-surfaceconcentrations and on the deposition velocities of the elements. During theday, the near-surface concentration is close to the mixing layer value due toatmospheric turbulence. At night, under stable atmospheric conditions, thenear-surface concentration decreases. Deposition velocity is independent ofconcentration but depends on aerodynamic conditions, the size distributionof the particles, and the solubility (or chemical reactivity) of the gases. Forthe S02 example, the Henry's coefficient is 1.3 M/atm, but the effective H-coefficient varies from pH 2 to pH 6 (Schwartz, 1984). Deposition of solublegases on vegetation depends on whether the stomata are open or closed.

Because of pollution, S02 is the most extensively studied gas in the in-dustrialized countries (Belot et at., 1974; Garland, 1974; Petit et ai., 1976;Garland and Branson, 1977; Fowler, 1978; Shreffler, 1978), where depositionspeeds have been measured between 0.3 and 3.2 em/see, averaging around 1em/sec. No similar study has been carried out in the intertropical regions.

The deposition velocity of SO~- particles is low « 0.1 em/see) and doesnot depend on gravity due to the particles' submicronic diameter. In thecontinental zones, Cawse (1974) and Muller and Schumann (1970) indicateda minimum in the 0.1-1 Jim range, and measured a deposition velocity of0.01 em/sec. Based on past estimates of dry deposition, Galloway (1985)gives values for dry deposition rates of 0.02 and 0.02 g S/m2/year for S02and excess SO~- in remote marine areas, and 0.07 and 0.03 g S/m2/year inremote continental areas.

2.6.1.2 Wet Deposition

Wet deposition is proportional to the total annual rainfall and concentrationof sulfur compounds in the rainwater. The compounds are mainly sulfatesand sulfites, which could result from the dissolution and oxidation of S02


in the liquid phase or from rainout and washout of particulate SO~-.In most experiments, the collectors are not closed between rain events,

thus the dry and wet deposits are analyzed together. The data obtained byauthors at different sites are displayed in Table 2.7. Concentrations varyby a factor 5, the lowest corresponding to the highest rainfall intensity.Delmas, Mathieu, Galloway et al. and Lacaux et al. obtained data under goodconditions for collection and analysis. In particular, Galloway et at. (1982)used collectors that opened and closed before and after the rain, to precludedry deposition. The analysis was done by an ionic separation followed byconductivity measurement (Dionex apparatus) giving good precision andreproducibility in the sub-ppm concentration range. Lacaux et at. (1986)used an automatic sampler and a Dionex apparatus for SO~- analysis. Theyfound an SO~- washout ratio (the ratio of concentration of an element orion in one liter of rainwater to its concentration in one cubic meter of air)of 103. This is compared to 3 x 102, the geometric mean of the ratios atSamoa, Miami, and Bermuda, which have been used to calculate the SO~-deposition flux over the world oceans (Andreae and Raemdonck, 1983).

2.6.1.3 Seasonal Variations in Dry and Wet Deposition

The study undertaken in the Ivory Coast by Delmas (1980) on particulateSO~- and on the main gaseous S-species H2S and S02 affords the oppor-tunity to compare the relative importance of dry and wet depositions in atropical atmosphere. Studied over a one-year period, the seasonal variationsare apparent (Figure 2.5).

Dry depositions are calculated from average monthly concentrations ofS02, H2S, and SO~- measured in a forested zone, with deposition veloci-ties of 1 em/see for H2S and S02, and 0.1 em/see for SO~-. Wet depositionestimates are based on the weighted means of data obtained at 11 stationsin this zone. The wet sulfur deposition, representing about two-thirds ofthe total, is the highest. This coincides with the assessment of sulfur deposi-tion in remote continental areas given by Galloway (1985). Wet depositionmodulates seasonal variations, and total deposition is highest during the wetseason due to an increased source of biogenic sulfur in the atmosphere, anddue to washout.

2.6.1.4 Acid Rainwater and Influence of Sulfur Compounds

In pristine areas without anthropogenic sources of sulfur, the pH of rainwa-ter is thought to be controlled by the carbonate system buffer CO2/H2C03and has a value of 5.6. The natural parts of the atmospheric cycles of sulfur,nitrogen, and carbon introduce strong acids (H2SO4, HN03) and weak acids(HCOOH, CH3COOH) sufficient to lower the pH of natural rainwater by

0\N

Table 2.7 Concentrations of SO;- in rainwater in the equatorial zone (average values in parentheses)

References Location Remarks Concentration Rainfall Deposits(mg(SO;-S)/liter) (mm/year) (g(S)/m2/year)

Visser, 1961 Kam pala (0.6) (0.8)Uganda

Reponed by Granat et at., 1976Hesse, 1957 Kiyuhu 0.1

Rodhe et at., 1981 Kenya, Av. 9 locations, 3 yearsTanzania Monthly samplesUganda 0.21

Eriksson, 1966 Zaire 0.28

Bromfield, 1974 Nigeria N. Av. 11 locations, 2 years 0.13 877 0.11

Mathieu, 1972 Ivory Coast Av. 1 location, 2 years (0.26)

Delmas, 1980 Ivory Coast Av. 11 locations, 1 year (0.43) 1,604 (0.69)

Lacaux et at., (unpublished) Lamto, Av. 1 location, 1 year 687 (0.35)Ivory Coast

Reponed by Haines, 1983Johnson et at. 1979 La Selva, Av. 1 location, 1 year (0.35) 3,400 1.25

Costa Rica

Jordan and Murphy, 1982 San Carlos, Av. 1 location, 1 year (0.42) 3,500 1.52Venezuela

Galloway et at., 1982 San Carlos. Av. 1 location, 6 months (0.17) 3,9100Venezuela


N

E.....c>E

200

...~::::c>E

EE

400

.5

300150

100 200

50

0M A M A 5 0 N

0D

Figure 2.5 Monthly variations of sulfur deposition in a forested zone in the IvoryCoast (from Delmas, 1980). (1) Wet deposition (mg(S)/m2/month); (2) dry deposi-tion (mg(S)/m2/month); (3) concentrations of SO~- -S in rainwater (mg/liter); (4)monthly rainfall (mm)

about one unit from that value. The impact of sulfur compounds alone mightcause pH values to range from 4.5 to 5.6 (Charlson and Rodhe, 1982). At themost central locations of the Antarctic plateau, before industrial pollutionand far from the marine influence, ancient ice cores up to 30,000 years oldwere found to be acidic at pH 5.2 to 5.5 (Legrand, 1980; Delmas el ai.,1980b, 1982). Sources of base-line acidity are probably the biogenic activityof the Antarctic Ocean and atmospheric HN03. The volcanic contributionto the total H2SO4 fallout on the Antarctic plateau is estimated to be 30%for the last 100 years (Delmas el ai., 1982).

Over the oceans, where pollution is low (the estimated total deposition ofSO~- of nonmarine origin equals the estimated rate of DMS emission), thepH values are 5.4 over the temperate southern Atlantic zones, 5.0 over thenorthern Atlantic tradewind zones and 5.3 over the north Pacific (Savoie,1984). Over the continents, pH values may decrease partly due to the S-emission. However,low valuesareobservedovertheequatorialbelt despitehigh ammonia (NH3) emissions and in contrast with the predictions by Daw-


son (1977)basedon theacidityof tropicalsoils.These NH3 emissionshav@been inferred from the NH4+ concentration in air and rainwater in the IvoryCoast (Servant et al., 1984). As NH3 is a base, it neutralizes the acidity ofH2SO4.

Examples of acidification of natural rainwater are given by the data ob-tained in the Global Precipitation Chemistry Project (GPCP) (Galloway etal., 1982). Precipitation from five remote areas of the world is analyzed.Two areas are located in the intertropical zone; Katherine, Australia, and StCarlos de Rio Negro in the Venezuelan tropical rainforest (Table 2.8). Theaverage pH values at the different stations were near 4.8. At times the pHwas quite acidic, below 4.5, which is the value Charlson and Rodhe (1982)quote as the lowest expected natural value. One interesting new result ofthis study is the evidence of a fairly constant background of strong mineralacidity with most of the pH-variability associated with organic acids. Thisfinding was suggested by a preliminary analysis of weak organic acids, butrequires confirmation at other sites to ensure its generality. This study showsthat air and rainwater acidities are linked to the cycle of major elements inthe atmosphere of which the sulfur cycle is one. Such a result seems to beconfirmed by our recent measurements in the Congo where we have de-termined the chemical composition of aerosols in a tropical rainforest site(Dimonika, 400 km west of Brazzaville). Organic carbon represents about70% to 80% of the total mass of particles (Table 2.9). The size spectrum ofparticulate organic carbon (POC) corresponds to submicron particles (di-ameter ::; 1 J.lm),suggesting a gas-particle conversion from gaseous organiccompounds and probably the presence of an acidic component.

In the Ivory Coast, at a site at the border of the savanna forest 200 km fromthe coast, rainwater was collected for the BAPMON Project. The samplerwas opened during rain only, and the water was frozen immediately and sent

Table 2.8 pH and contribution of acids to free acidity (%) in rainwater (Gallowayet al., 1982)

St. Georges, Poker Flat, Amsterdam Katherine, St. Carlos,Bermuda Alaska Island Australia Venezuela

pH 4.8 4.9 4.7 4.7 4.8(4.1-6.0) (4.7-5.4) (4.0-5.5) (4.0-5.4) (4.0-5.4)

H2SO4 111 65 73 33 18

HN03 .::;35 17 '::;14 26 17

HX* >0 >18 >13 >41 >65

HX* could be HCl, organic acids or H3P04; the authors believe it was an organic acid.


Table 2.9 Chemical composition of the atmospheric aerosol in a tropical forestsite, Dimonika in the Congo (Delmas et al., 1986)

to French laboratories. The pH, conductivity, and concentrations of SO~-,NO.3, PO~-, Cl-, NH: (by ion chromatography) and of Ca2+, Mg2+, K+,Na +, FeH (by atomic absorption) were measured.

From May 1983 to May 1984, 97 events were recorded with pH valuesbetween 4.3 and 6.2 (Figure 2.6). About 19% of the rain events have a pH< 5.0, 46% a pH between ~ 5.0 and < 5.6, and 35% a pH ~ 5.6. Themore acidic events represent 10% of the total rainfall. In the ionic balance,sulfates and nitrates account for about 70% of the free acidity of rainwater.The remaining 30% might come from organic acids, such as formic acid,the result of isoprene oxidation by OH radicals leading to the formation offormaldehyde. The pH shows slight seasonal variations: values are higherduring the dry season when soil-derived particles bring alkaline materialfrom sahelian zones (Lacaux el ai., 1986).

5.6 5.9 6.2 pH

Figure 2.6 Histogramof the pH of precipitationcollectedat the Bapmon site ofLamto, Ivory Coast (from Lacaux et al., 1986)

4.4 4.7 5.0 5.3

Dimonika, Concentration (nglm3)

Congo POC SO- NH: NO) K+ Na+

End of rainy season 6,850 1,709 422 322 212 155

End of dry season 7,000 207 385 18 180 247

30

25

en 20-c:Q)>Q) 15-0

10Q).DE::J 5z

0


Transport-InfluenceofEquatorialRegionsontheGlobalAtmospheric Sulfur Cycle

The horizontal transport of trace compounds by air masses depends on theiratmospheric residence time. In the continental zones with high precipitationand humidity, the oxidation of S-gases, with the exceptions of CS2 and COS,is rapid and their residence time is probably less than one day. In these zones,the major source of sulfur is biomass decomposition and most airborne sul-fates stay in the planetary boundary layer. This layer presents very efficientrainout with rainfall between 1,500 mm/year and 2,000 mm/year.

In the Ivory Coast, the residence time of aerosols in the atmosphere isestimated at several hours in the rainy season and about two days in the dryseason. Under such conditions, transport of the sulfur compounds will belimited mainly to a few hundred kilometers, again with the exceptions ofCS2 and COSo Most of the sulfur emitted is recycled locally or in nearbyecosystems, such as the savanna. On the other hand, bushfires occur duringthe dry season, and particles formed in the plumes by gas-particle conversionare mostly in the submicronic range. Their residence time in the atmospherelikely approaches two or three weeks, the value usually given for the freetroposphere. These particles can be carried away by air masses and dispersedover regions far away from the generating zones. They may, for example,supply the oceanic atmosphere with SO~- from nonmarine sources.

In the equatorial regions, a vertical transport of sulfur compounds intothe stratosphere is conceivable. These regions are located at latitudes wherethe tropospheric air enters the stratosphere through the ascending branchesof the Hadley cells. Most of the sulfates of the stratospheric layer appear tooriginate in the COS photolysis over a 20 km altitude during periods withlow volcanic activity (Crutzen and Schmaitzl, 1983; Servant, 1986).

2.6.1.5

2.7 CONCLUSIONS

Attempting to estimate the sulfur budget over the intertropical regions isdifficult due to limited and often unreliable atmospheric chemistry data forthese regions. According to current knowledge, the sulfur cycle in the trop-ics still appears to be basically natural. This assertion does not mean thatanthropogenic pollution in tropical regions is always negligible, but ratherthat the background of sulfur in the tropical atmosphere is natural. In theindustrialized countries of the Northern Hemisphere, the background ofsulfur is anthropogenic. However, man-made emissions may already exceednatural emissions in tropical countries with high population densities andthese are expected to increase in the future.

The natural part of the sulfur cycle in the tropics presents some typi-cal features: reaction speeds in the atmosphere and pedosphere are very


fast; the main source of sulfur compounds is the biodegradation of organicmatter in humid zones; and the turnover of the cycle is dependent on nat-ural and climatic conditions on a regional scale. Rapid degradation of litterdue to luxuriant micro-fauna, fungal, and bacterial activity favored by con-stant high temperatures and soil humidity, and the presence of numeroushydromorphic zones favorable to S-emissions in relation with high rainfallaffect source intensity. Features of the tropical sulfur cycle also influence theatmospheric oxidation of sulfur compounds and the recycling of these com-pounds through wet deposition associated with high rainfall. With a nearlyconstant year-round air temperature, the atmospheric fraction of the sulfurcycle is modulated by the annual rainfall distribution for both emissions anddepositions.

One difficulty in studying the sulfur cycle over tropical regions is theregional variation in climatic conditions. For example, during monsoons theocean could have a great impact in some regions. In addition, the sulfurcycle should not be studied alone: atmospheric oxidation rates for sulfurare influenced by the nitrogen, carbon cycles through their impact on theconcentrations of the hydroxyl radical. The acidification of air and rainwateris caused by sulfur compounds. In the tropics, the pedosphere seems to bea natural source of ammonia, which partly counterbalances the acidifyinginfluence of sulfur compounds. This effect needs further study. Nevertheless,with low anthropogenic emissions, the pH of rainwater can be reduced tobelow 4.5. One reason could be the presence of organic acids, as is probablein the forested zone, with the atmospheric oxidation of the gaseous organiccompounds emitted by the biosphere.

Despite the great intensity of sulfur sources in intertropical regions, theyexert only a small influence on the global sulfur cycle. Most of the com-pounds emitted are recycled locally and contribute to the stability of theprimary ecosystems. One exception is carbonyl sulfide (photolized in thestratosphere), which could be emitted in the forested zones where condi-tions are favorable. This compound might be the source of the sulfate layerin the stratosphere during volcanic 'quiescent' periods.

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Documents

The Atmospheric Sulfur Cycle in the Tropics...the atmospheric sulfur cycle in equatorial regions. The two main biomes in these regions are the forests and the savannas. It is difficult