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NIRE Annual Report
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1996

Oxidation Processes of Dimethylsulfide in the Remote Marine Atmosphere

Inter-Spheric Environment Division
Environment Assessment Department
Objectives
Dimethylsulfide(DMS,CH3SCH3), synthesized by phytoplankton, is a primary source of sulfur emitted from the ocean to the atmosphere. Atmospheric DMS is mainly oxidized by hydroxyl radical(OH) and nitrate radical(NO3), and finally converted to sulfuric acid(H2SO4). Sulfuric acid is a principal component of cloud condensation nuclei(CCN) in the marine atmosphere. The results obtained by fixed point observations reveal that the methanesulfonic acid(MSA) to non-sea-salt SO42-(nss-SO42-) molar ratio varies seasonally with a maximum during summer and a minimum in winter. A photochemical box model is used to simulate seasonal variations in concentrations of sulfur compounds at latitude 40oS. The purpose here is to explore the oxidation processes of DMS in the marine atmosphere, and to investigate reasons why the MSA/nss-SO42- molar ratio varies seasonally.
Results
The concentrations of DMS and SO2, the molar ratios of SO2/DMS, MSA/nss-SO42-, and DMS/MSA calculated by the present model were found to be in good agreement with the observed values at Amsterdam Island and Cape Grim. In a number of recent papers, the geographical distribution and diurnal variation of atmospheric DMS, and the relationships among DMS, SO2, MSA, and nss-SO42- were explained only by the reaction of DMS with OH. The results obtained in the present model, however, suggest that, at middle and high latitudes in winter, the DMS oxidation mainly proceeds through the reaction with NO3, rather than the reaction with OH(Fig. 1), because the NO3 concentration increases with decreasing the DMS flux. Therefore, the ratio of the production rate of MSA to SO2 is smaller in winter than that in summer.
This is a reason why the MSA/nss-SO42- molar ratio varies seasonally(Figure. 2). If the OH addition reaction is the predominant pathway for the MSA production, the results of calculations can well explain the increase of the MSA/nss-SO42- molar ratio toward high latitudes and the latitudinal distribution of SO2 concentration in the summer season, and the seasonal variation of the MSA/nss-SO42- molar ratio at the middle and high latitudes.


Selected Publication
1) Simulations of seasonal variations of sulfur compounds in the remote marine atmosphere., J. Atmos. Chem., 23, 163-192, 1996.

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