Atmospheric sulfur cycle remote marine atmosphere

Figure 13-5 is the box model of the remote marine sulfur cycle that results from these assumptions. Many different data sets are displayed (and compared) as follows. Each box shows a measured concentration and an estimated residence time for a particular species. Fluxes adjoining a box are calculated from these two pieces of information using the simple formula, S-M/x. The flux of DMS out of the ocean surface and of nss-SOl back to the ocean surface are also quantities estimated from measurements. These are converted from surface to volume fluxes (i.e., from /ig S/(m h) to ng S/(m h)) by assuming the effective scale height of the atmosphere is 2.5 km (which corresponds to a reasonable thickness of the marine planetary boundary layer, within which most precipitation and sulfur cycling should take place). Finally, other data are used to estimate the factors for partitioning oxidized DMS between the MSA and SO2 boxes, for SO2 between dry deposition and oxidation to sulfate, and for nss-SO4 between wet and dry deposition. 

Once DMS is emitted into the atmosphere it will eventually be oxidized by OH or NO3 radicals to sulfur dioxide (SO2), methanesulfonic acid (MSA), and, via SO oxidation, to non-sea-salt sulfate (nss-S042 ) as major reaction products (e.g. 10.111. The Southern Ocean represents a relatively unpolluted marine environment. It offers a unique possibility to study the natural sulfur cycle in an atmosphere far remote from man-inhabited continents. 

In the early 1970s there was some concern that the sulfur cycle did not balance and that there needed to be additional global sources. When James Lovelock discovered DMS in the atmosphere of remote Ireland, this seemed a fine candidate to balance the sulfur cycle. The production of DMS in the oceans and its subsequent oxidation have been extensively investigated, particularly because of its role in producing sulfuric acid droplets that can act as an important cloud condensation nuclei in the remote marine atmosphere. 

Fig. 13-5 The sulfur cycle in the remote marine boundary layer. Within the 2500 m boundary layer, burden units are ng S/m and flux units are ng S/m h. Fluxes within the atmospheric layer are calculated from the burden and the residence time. Dots indicate that calculations based on independent measurements are being compared. The measured wet deposition of nss-SO " (not shown) is 13 7 //g S/m /h Inputs and outputs roughly balance, suggesting that a consistent model of the remote marine sulfur cycle within the planetary boundary layer can be constructed based on biogenic DMS inputs alone. Data (1) Andreae (1986) (2) Galloway (1985) (3) Saltzman et al. (1983) (4) sulfate aerosol lifetime calculated earlier in this chapter based on marine rainwater pH the same lifetime is applied to MSA aerosol. Modified from Crutzen et al. (1983) with the permission of Kluwer Academic Publishers.

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