Reports

Estimate of the Contribution of Biologically ProducedDimethyl Sulfide to the Global Sulfur Cycle

Abstract. Atmospheric dimethyl sulfide (DMS) measurements were made on theAtlantic Coast of the United States at Wallops Island and Cape Henry, Virginia,during June 1975. The very low concentrations, typically less than 30 parts per tril-lion observed at the Cape Henry site, were thought to result from the smog chem-istry associated with the Norfolk metropolitan area. Atmospheric DMS concentra-tions at the Wallops Island site were much higher, having a geometric mean of 58parts per trillion and a geometric standard deviation of 2.1. At this site the DMSsource strength was estimated to be 6 milligrams ofsulfur per square meter per year.

Because of wind conditions during this experiment, the DMS source strength isthought to be representative of the DMS source strength of the ocean in the WallopsIsland area and is much less than the 130 milligrams of sulfur per square meter peryear needed to balance the ocean-atmosphere portion ofthe global sulfur budget.

Biologically produced sulfur com-

pounds are thought to account for one

half of the total sulfur entering the atmo-sphere (1, 2). Despite the potentiallygreat effect that these materials may

exert on our climate and environmentthrough their aerosol-forming properties(3, 4), our knowledge of these materialsis very limited. Early investigatorsthought that H2S was the primary sulfurcarrier because this compound is pro-duced in the decay processes of animaland plant tissue and by anaerobic micro-bial action in the sediments of lakes,marshes, and oceans. However, recentmeasurements of H2S concentrationsover land (5) indicated a background H2Sconcentration of approximately 40 partsper trillion (ppt) by volume which, if weassume a mixing depth of 1 km and a

residence time of I day (4), leads to an

estimated source strength for land-basedbiologically produced H2S of 2 x 1012 g

year-' or about 4 percent of the 58 x

1012 g of sulfur per year expected forland-based sources (1). Ostlund and Al-exander's studies (6) of the oxidation ofH2S in seawater indicated that, relativeto its rate of diffusion from the sedimentswhere it is thought to be generated, H2Sis quickly oxidized by the oxygenatedsurface layers of the ocean. This resultimplies that the biologically produced at-mospheric H2S arises only from land andshore areas and thus accounts for only2 x 1012 g year-' of the estimated 106 x

6 MAY 1977

1012 g year-' of the sulfur emitted intothe atmosphere by biological sources (1).The identity of the sulfur carriers thus

remains to be determined. Lovelock et

al. (7) have suggested that dimethyl sul-fide (DMS) might be an important carriersince it is produced by some species ofmarine algae (8) and it has the requiredproperties of volatility, insolubility inwater, and stability toward oxidation byaqueous oxygen. They found that water

Fig. 1. Map showing the sites monitored inthis study: site 1, Cape Henry; site 2, WallopsIsland.

in the North Atlantic is saturated withDMS and that some soils produce thiscompound. Their attempts to detectDMS in the atmosphere failed; however,they pointed out that oxidants trappedwith DMS in their cryogenic trappingprocedure could have destroyed theDMS before the analysis could be car-ried out.We have analyzed for DMS at two

sites on the east coast of the UnitedStates (Fig. 1). Site 1 was located atCape Henry, Virginia, near the mouth ofChesapeake Bay. Site 2 was located atthe National Aeronautics and Space Ad-ministration launch facility at WallopsIsland, Virginia. Wallops Island is a bar-rier island on the Virginia Eastern Shorenear the Maryland border. Hourly mea-surements were made for 1 week at eachsite during June 1975. About 30 minuteswas required to obtain each sample.The DMS measurements were made

with a gas chromatograph equipped witha flame photometric detector. The liquidphase in the column used to separate thesulfur compounds consisted of poly-phenyl ether mixed with 0.1 percentH3P04; Teflon beads (40/60-mesh) wereused as support. We concentrated thesample by a factor of 330 before injectingit into the gas chromatograph by pullingabout 1000 cm3 of ambient air through asample loop (3 cm3) cooled to 87 K withliquid argon. The sample loop was a Tef-lon tube (1.8 m long, 0.20 cm in outsidediameter) whose center portion waspacked with 40/60-mesh glass beads. Af-ter a short cycle in which hot water wasused to release the compounds from theglass beads, the carrier gas was divertedthrough the sample trap; in this way thesample was swept from the trap, throughthe column, and into the flame photomet-ric detector. In calibrating the system,we used Teflon permeation tubes havingknown permeation rates. We carried outcalibrations in the field environment byplacing the permeation tubes directly inthe sample manifold. The minimum con-centration of DMS detectable by ourmethod was approximately 10 ppt.The concentrations of DMS at Cape

Henry were typically less than 30 ppt.Many zero values were obtained, an in-dication that the instrument sensitivitywas not great enough to characterize thedistribution function for DMS at thissite. However, measurable DMS con-centrations were found most often whenthe wind was coming off the ChesapeakeBay. An unknown sulfur compound(compound U) having a retention timevery close to that of CS2 was observedconcurrently with the DMS. A diurnal

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pattern was not observed for either DMSof compound U.The concentrations of DMS were

much higher at the Wallops Island site.The distribution of the DMS data for thissite was clearly log-normal, and themeasurements had a geometric mean of58 ppt and a geometric standard devia-tion of 2.1. A few values exceeding 500ppt were observed during early morninghours; these we attributed to the forma-tion of strong low-level radiation in-versions.As expected, there was an approxi-

mately inverse relation between theDMS concentration at the Wallops Is-land site and the wind speed. A subgroupof DMS measurements made at windspeeds of less than 13 km hour-' had ageometric mean of 70 ppt and a geomet-ric standard deviation of 1.5; a similargrouping for wind speeds exceeding 20km hour-' had a geometric mean of 32ppt and a geometric standard deviationof 1.7.Although the nature of the sources ex-

pected to contribute to the DMS loadingat the Wallops Island site had a high di-rectional dependence, only a weak cor-relation between the DMS concentrationand wind direction was observed. TableI summarizes the results of a statisticaltreatment of the DMS data subgroupedaccording to the wind direction at thetime of measurement. Whereas openocean should be the only important areasource for subgroups with wind direc-tions between 900 and 1800 and between1800 and 2200, marshes, shallow bays,land, and open ocean can all be impor-tant area sources for the subgroup withwind direction between 00 and 900. Inview of the weak correlation betweenDMS concentration and wind direction,we conclude that for this site marshes,shallow bays, and land are not dramati-cally stronger sources of DMS than openocean.The DMS data for Wallops Island

clearly exhibited a diurnal pattern (Fig.2). The DMS concentration increasedfrom a minimum of 40 ppt near sundown

100 -

80- A A AAAAAA

00 4 8 12 16 20 24

Hour of day (E.S.T.)

Fig. 2. Average diurnal variation in the DMSconcentration at the Wallops Island site. Fivedata points were averaged to obtain eachpoint shown on the graph.

to a maximum of 110 ppt near sunrise;after daybreak the DMS concentrationdecreased rapidly to about 40 ppt andthen maintained this concentration forthe remainder of the day. Lapse ratesmeasured at 7 a.m. and 7 p.m. indicatedthat the diurnal pattern was determinedprimarily by low-level radiation in-versions. Compound U was also ob-served at the Wallops Island site. It ap-peared concurrently with and at aboutthe same concentration as the DMS.The large differences in the DMS con-

centrations at the Cape Henry and Wal-lops Island sites are difficult to reconcile.Our Wallops Island data suggest that theocean is an important source of DMS.The same ocean area that contributes tothe DMS loading at the Wallops Islandsite with southwest winds can also con-tribute to the DMS loading at the CapeHenry site with northeast winds. The in-fluence of the smog chemistry associatedwith the nearby Norfolk metropolitanarea, however, might in part explain thelow DMS concentrations found at CapeHenry. This thesis is supported by Coxand Sandalls's (4) observations that theDMS reactivity in smog systems is verygreat, being comparable to the reactivityof simple olefins such as propylene.We have estimated the emission rate

of DMS per unit area, using a method de-scribed by Shaffer (9) for determining therate of emission of radon from soil. Byanalogy with Shaffer's treatment of theradon problem, we assumed that theDMS emission rate was constant over awide area, that the DMS flux at 300 mduring the nighttime was negligible, and

Table 1. Results of a statistical treatment of the DMS data for the Wallops Island site sub-grouped according to wind direction.

GeometricWind Number mean Geometric Area sourceof concen- standard .c .direction samples tration deviation description

(ppt)00 to 900 80 44 2.7 Marsh, shallow bays, land, ocean900 to 1800 42 53 2.2 Open ocean1800 to 2200 47 62 1.7 Opean ocean2200 to 3600 0 Marshes, shallow bays, land

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that the DMS concentration at 300 m wasconstant at the daytime average DMSconcentration. Under these conditionsthe DMS emission rate R is given by theexpression (9):

d ( 300

dt J (1)

where z is the vertical coordinate and c isthe DMS concentration. By assumingthat the DMS concentration decreasedexponentially with height during the eve-ning hours, we have evaluated the in-tegral in Eq. 1 on an hourly basis be-tween 6 p.m. and 6 a.m. A plot of the neg-ative value of this integral as a functionof time was a straight line whose slopeyielded an emission rate for DMS of 6mg of sulfur per square meter per year.

This emission rate for DMS is prob-ably representative of the emission rateof this compound from the ocean nearWallops Island and is comparable to the10 mg of sulfur per square meter per yearobtained by Liss and Slater (10), usingthe DMS data of Lovelock et al. (7) and atwo-layer model for gaseous transportacross the air-sea interface. Both ofthese estimates are much smaller thanthe average emission rate for biologicalsources of 130 mg of sulfur per squaremeter per year (1) required to balancethe ocean-atmosphere portion of theglobal sulfur cycle. Therefore, unless theemission rate of DMS is much larger forother areas of the ocean, it would appearthat DMS from ocean sources makes avery small fractional contribution to theglobal sulfur cycle.

PETER J. MAROULISALAN R. BANDY

Department ofChemistry,Drexel University,Philadelphia, Pennsylvania 19104

References1. J. P. Friend, in Chemistry of the Lower Atmo-

sphere, S. I. Rasool, Ed. (Plenum, New York,1973), chap. 4.

2. C. E. Junge, Air Chemistry and Radioactivity(Academic Press, New York, 1963); E. Robin-son and R. C. Robbins, ''Sources, abundancesand fate of gaseous atmospheric pollutants"(Project Report PR-6755, Stanford Research In-stitute, Menlo Park, Calif., 1968); W. W. Kel-logg, R. D. Cadle, E. R. Allen, A. L. Lazrus, E.A. Martell, Science 175, 587 (1972).

3. R. D. Cadle, in Chemistry of the Lower Atmo-sphere, S. I. Rasool, Ed. (Plenum, New York,1973), chap. 2.

4. R. A. Cox and F. J. Sandalls, Atmos. Environ.8, 1269(1974).

5. D. F. S. Natusch, H. B. Klonis, H. D. Axelrod,R. J. Teck, J. P. Lodge, Jr., Anal. Chem. 44,2067 (1972).

6. H. G. Ostlund and J. Alexander, J. Geophys.Res. 68, 3995 (1963).

7. J. E. Lovelock, R. J. Maggs, R. A. Rasmussen,Nature (London) 237, 452 (1972).

8. F. Challenger, Adv. Enzymol. 12, 429 (195 1).9. W. A. Shaffer, thesis, Drexel University (1973).10. P. S. Liss and P. G. Slater, Nature (London)

247, 181 (1974).23 March 1976; revised 15 November 1976

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