Atmospheric flux, dimethyl sulfide

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DMS Dimethyl sulfide, (CHsjzS a reduced sulfur gas produced by marine microbiota Dry deposition The flux of material from the atmosphere to the surface by processes other than precipitaHon. Examples are the gravitaHonal settling of large parHcles and absorpHon of gases such as CO2 and SO2 at the ocean surface... 

Fig. 10-9. Flux diagram for sulfur in the unperturbed marine atmosphere. Fluxes are given in units of p.gS/m2day. Numbers in boxes indicate column densities in units of p.gS/m2. DMS, Dimethyl sulfide MSA, methane sulfonic acid (associated with the aerosol). The mixing ratio of S02 is 60 ng S/m3, independent of altitude. The mixing ratio of SOis 280 ng S/m3 in the boundary layer and 80 ng S/m3 in the free troposphere. Contrary to the model of Kritz (1982), the fluxes are confined to the boundary layer. There exists no significant net flux into or out of the free troposphere. The dry deposition velocity for S02 is 5mm/s.

Figure 22.1 depicts the major reservoirs in the biogeochemical cycle of sulfur, with estimated quantities [in Tg(S)] in each reservoir. Directions of fluxes between the reservoirs are indicated by arrows. The major pathways of sulfur compounds in the atmosphere are depicted in Figure 22.2. The numbers on each arrow refer to the description of the process given in the caption to the figure (not to fluxes). Note the small amount of sulfur in the atmosphere relative to that in the other reservoirs. Also note the significant amount of sulfur in the marine atmosphere this is the result of dimethyl sulfide (DMS) emissions from the sea.

Dimethyl sulfide (DMS), CH SCH, is the largest natural contributor to the global sulfur flux (see Section 2.2.1). The DMS-OH rate constant, approximately 5 X 10 cm molecule s at 298 K, exceeds that of DMS-NO, by a factor of 4. (In contrast, the reactions of H2S and CH,SH with NO, are, respectively, 6000 and 40 times slower than that with OH.) The DMS lifetime in the marine atmosphere as a result of both OH and NO, reactions is on the order of one to several days, with the majority of the path occurring by OH at low latitudes and by NO, in colder, darker regions. Because of the photochemical source of OH, DMS removal by OH occurs only during daytime, leading to a pronounced diel cycle in DMS concentration. 

Dimethyl sulfide (DMS), through its oxidation to sulfate in the troposphere, acts as a source of cloud condensation nuclei, thus potentially influencing the radiative balance of the atmosphere. DMS is formed in sea water through the microbial decomposition of dimethyl sulfonioproprionate (DMSP), a compound believed to act as an osmolyte in certain species of marine phytoplankton. The flux of DMS to the atmosphere is controlled by its concentration in surface sea waters, which is controlled in turn by the rate of its decomposition. Estimates indicate that 7-40% of the total turnover of DMS in the surface waters of the Pacific Ocean is due to the photosensitized destruction of this compound, illustrating the potential importance of this pathway in controlling the flux of DMS to the atmosphere. 

Summary The flux of dimethyl sulfide through the atmosphere exceeds that of all other reduced sulfur compounds combined. Dimethyl sulfide is emitted from the ocean surfaces and is rapidly oxidized in the marine boundary layer via reactions with OH and NO3. These initial reactions in the oxidation of DMS have received considerable attention from the scientific community and are reasonably well understood. The reactions with OH and NO3 result in the production of the CH3SCH2OO radical. The reaction between DMS and OH also produces a radical adduct, CH3S(OH)CH3, which reacts with O2 to produce dimethyl sulfoxide (DMSO) with a yield of about 50%. The remaining pathway(s) for the adduct -I- O2 reaction are unidentified. 

As for the primary trace chemical species in the remote natural atmosphere where there is no influence of anthropogenic activities, methane (CH4) emitted from lakes and marshes, biogenic volatile organic compounds (BVOCs), nitric oxide (NO) from natural soils and lightning, dimethyl sulfide (DMS) from marine organisms, O3 descended from the stratosphere can be conceived. Among them, the most important chemical species that is subjected to photolysis by the actinic flux (see Sect. 3.5) in the troposphere is O3 and NO2 (see Sects. 4.2.1 and 4.2.2). 

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