Sulfur compounds troposphere

Varhelyi, G., 1977 Wet removal of tropospheric sulfur compounds. Idojaras 81, 85-93. 

TABLE 2.3 Average Lifetimes and Observed Mixing Ratios of Tropospheric Sulfur Compounds... 

As discussed in Chapter 2 and in more detail in Chapter 11, a variety of organic sulfur compounds in addition to inorganics such as H2S and COS are emitted by biological sources. In the troposphere, they may ultimately be oxidized to S02 and H2S04. However, the chemistry of these compounds tends to be complex, and a variety of partially oxidized sulfur compounds is formed first. 

Table 8.17 summarizes the rate constants and estimated tropospheric lifetimes of some of these sulfur compounds with respect to reaction with OH or NO-,. The assumed concentrations of these oxidants chosen for the calculations are those characteristic of more remote regions, which are major sources of reduced sulfur compounds such as dimethyl sulfide (DMS). It is seen that OH is expected to be the most important sink for these compounds and that NO, may also be important, for example, for DMS oxidation (see also Chapter 6.J). 

Barnes, I., K. H. Becker, and R. D. Overath, Oxidation of Organic Sulfur Compounds, in The Tropospheric Chemistry of Ozone in the Polar Regions (H. Niki and K. H. Becker, Eds.), NATO ASI Series, Vol. 17, pp. 371-383, Springer-Verlag, Berlin, 1993. 

Photoreactions are often complex reactions that not only control the fate of many chemicals in air and water, but often produce products with chemical, physical, and biological properties quite different from those of their parent compounds more water soluble, less volatile, and less likely to be taken up by biota. Photooxidation removes many potentially harmful chemicals from the environment, although occasionally more toxic products form in oil slicks and from pesticides (Larson et al., 1977). Biogeochemical cycling of organic sulfur compounds in marine systems involves photooxidation on a grand scale in surface waters, as well as in the troposphere (Brimblecombe and Shooter, 1986). 

Current research on the atmospheric cycling of sulfur compounds involves the experimental determination of reaction rates and pathways (see Plane review, this volume) and the field measurement of ambient concentrations of oceanic emissions and their oxidation products. Photochemical models of tropospheric chemistry can predict the lifetime of DMS and H2S in marine air however there is considerable uncertainty in both the concentrations and perhaps in the identity of the oxidants involved. The ability of such models to simulate observed variations in ambient concentrations of sulfur gases is thus a valuable test of our assumptions regarding the rates and mechanisms of sulfur cycling through the marine atmosphere. 

All sulfur compounds showed relatively constant profiles throughout the lower fright levels which is consistent with the neutral conditions in the mixed layer. As compared to the data obtained during the ship cruise (Tables I and II) the concentrations in the lowest flight level (30 m) were about 2-3 times lower for DMS, nearly the same for SO2, and about 3 times higher for MSA and nss-SC>42 The fraction of nss-SC>42 to total sulfate at this level was 18%, similar to the results from the ship cruise. The relatively higher concentrations observed for MSA and nss-SC>42 indicate a significant accumulation of aerosol particles in the mixed layer due to the postfrontal inversion between the mixed layer and the free troposphere. 

Biogenic Sulfur Emissions from the Ocean. The ocean is a source of many reduced sulfur compounds to the atmosphere. These include dimethylsulfide (DMS) (2.4.51. carbon disulfide (CS2) (28). hydrogen sulfide (H2S) (291. carbonyl sulfide (OCS) (30.311. and methyl mercaptan (CH3SH) ( ). The oxidation of DMS leads to sulfate formation. CS2 and OCS are relatively unreactive in the troposphere and are transported to the stratosphere where they undergo photochemical oxidation (22). Marine H2S and CH3SH probably contribute to sulfate formation over the remote oceans, yet the sea-air transfer of these compounds is only a few percent that of DMS (2). 

Dissolved metals affect the concentration of atmospheric trace gases, such as ozone, organics and sulfur compounds. Ozone is formed in the troposphere through a complex series of homogeneous reactions as shown schematically in Fig. 4. The chain represented in the figure by thick arrows involves the cooperative oxidation of organic molecules and NO with the intermediacy of HO, formed by subsequent photolysis of NO2 and O3, and HO2 radicals. 

The tropospheric sulfur chemistry is different. Unlike the nitrogen and carbon chemistry, where combustion is an insignificant source, the combustion source of SO2 appears to be very important. While OH reactions can be shown to convert sulfides to SO2, it is not clear that normal atmospheric chemistry is important in the next step—the conversion of S02 to H2SO, which is then removed from the atmosphere by rainout. It has also been suggested that a large amount of SO2 is removed directly by rainout. Unfortunately we have the fewest data, both kinetic and atmospheric, on sulfur compounds. Most of the kinetic data we do have are at high temperatures, and most of the atmospheric data are for polluted environments. 

Cadle R. D. (1975) Volcanic emissions of halides and sulfur compounds to the troposphere and stratosphere. J. Geophys. Res. 80, 1650-1652. 

Rodhe H. and Isaksen I. (1980) Global distribution of sulfur compounds in treh troposphere estimated in a height/latitude transport model. J. Geopkys. Res. 85, 7401 -7409. 

Sulfur compounds are very important atmospheric constituents, since in clean tropospheric air as well as in the stratosphere the majority of aerosol particles are composed of ammonium sulfate or sulfuric acid (see Chapter 4). This finding is particularly interesting since with the exception of sea salt sulfur, a predominant portion of sulfur emission is in gaseous form. 

On the basis of the foregoing discussion it is concluded that tropospheric background particles consist mainly of sulfur compounds. Sulfate particles contain hydrogen or ammonium ions as a function of the ammonia gas available. These mostly Aitken size particles are (externally or internally) mixed with some organic material at least near the Earth s surface.16 More research is needed to know in more detail the chemistry of this aerosol. 

As an illustration of the concept of mean lifetime, consider all sulfur-containing compounds in the troposphere. If the average mixing ratio of these compounds is 1 part per billion by mass (ppbm) and a steady state is assumed to exist, then with the mass of the troposphere about 4 x 1021 g, the total mass of sulfur-containing compounds in the troposphere is Q = 4 x 1012 g. If natural and anthropogenic sources of sulfur contribute to give a total P of about 200 x 1012gyr, the mean lifetime of sulfur compounds in the troposphere is estimated to be... 

Carbonyl sulfide is the most abundant sulfur gas in the global background atmosphere because of its low reactivity in the troposphere and its correspondingly long residence time. It is the only sulfur compound that survives to enter the stratosphere. (An exception is the direct injection of S02 into the stratosphere in volcanic eruptions.) In fact, the input of OCS into the stratosphere is considered to be responsible for the maintenance of the normal stratospheric sulfate aerosol layer. 

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