Marine boundary layer sulfur cycle

Yvon S. A., Saltzman E. S., Cooper D. J., Bates T. S., and Thompson A. M. (1996) Atmospheric sulfur cycling in the tropical Pacific marine boundary layer (12 degrees S, 135 degrees W) a comparison of field data and model results 1. Dimethylsulfide. J. Geophys. Res. Atmos. 101(D3), 6899-6909. 

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.

Once the importance of DMS to the global sulfur cycle was established, numerous measurements of DMS concentrations in the marine atmosphere have been conducted. The average DMS mixing ratio in the marine boundary layer (MBL) is in the range of 80-1 lOppt but can reach values as high as 1 ppb over entrophic (e.g., coastal, upwelling) waters. DMS mixing ratios fall rapidly with altitude to a few parts per trillion in the free troposphere. After transfer across the air-sea interface into the atmosphere, DMS reacts predominantly with the hydroxyl radical and also with the nitrate (N03) radical. Oxidation of DMS is the exclusive source of methane sulfonic acid (MSA) in the atmosphere, and the dominant source of S02 in the marine atmosphere. We will return to the atmospheric chemistry of DMS in Chapter 6. 

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. 

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