Simulations of seasonal variations of sulfur compounds in the remote marine atmosphere |
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| Authors: | Seizi Koga Hiroshi Tanaka |
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| Institution: | (1) National Institute for Resources and Environment, 16-3 Onogawa, 305 Tsukuba, Japan;(2) Institute for Hydrospheric-Atmospheric Sciences, Nagoya University, Chikusa-ku, 464-01 Nagoya, Japan |
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| Abstract: | A photochemical box model is used to simulate seasonal variations in concentrations of sulfur compounds at latitude 40° S. It is assumed that the hydroxyl radical (OH) addition reaction to sulfur in the dimethyl sulfide (DMS) molecule is the predominant pathway for methanesulfonic acid (MSA) production, and that the rate constant increases as the air temperature decreases. Concentration of the nitrate radical (NO3) is a function of the DMS flux, because the reaction of DMS with NO3 is the most important loss mechanism of NO3. While the diurnally averaged concentration of OH in winter is a factor of about 8 smaller than in summer, due to the weak photolysis process, the diurnally averaged concentration of NO3 in winter is a factor of about 4–5 larger than in summer, due to the decrease of DMS flux. Therefore, at middle and high latitudes in winter, atmospheric DMS is mainly oxidized by the reaction with NO3. The calculated ratio of the MSA to SO2 production rates is smaller in winter than in summer, and the MSA to non-sea-salt sulfate (nssSO4
2-) molar ratio varies seasonally. This result agrees with data on the seasonal variation of the MSA/nssSO4
2- molar ratio obtained at middle and high latitudes. The calculations indicate that during winter the reaction of DMS with NO3 is likely to be a more important sink of NOx (NO+NO2) than the reaction of NO2 with OH, and to serve as a significant pathway of the HNO3 production. If dimethyl sulfoxide (DMSO) is produced through the OH addition reaction and is heterogeneously oxidized in aqueous solutions, half of the nssSO4
2- produced in summer may be through the oxidation process of DMSO. It is necessary to further investigate the oxidation products by the reaction of DMS with OH, and the possibility of the reaction of DMS with NO3 during winter. |
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| Keywords: | photochemical box model marine atmosphere DMS oxidation NO3 radical MSA/nssSO4
2- molar ratio seasonal variation |
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