Sulfur compounds, tropospheric sources

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). 

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). 

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. 

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... 

Both heterogeneous and homogeneous nucleation appear to play roles in ice formation in the upper troposphere. Liquid droplets several microns in radius can be found at temperatures down to about T —35°C [38] (Fig. 2). These are probably concentrated solutions of sulfuric acid, ammonium sulphate, and possibly other nitrogen compounds formed on deliquescent aerosols. Some of the aerosols on which the ice particles nucleate are formed aloft by gas-to-particle conversion in the clear air surrounding clouds ([39]) some originate from volcanos, and some arise from gas and/or particle anthropogenic sources. 

---