Seizi Koga

Modeling the methanesulfonate to non-sea-salt sulfate molar ratio and dimethylsulfide oxidation in the atmosphere

A photochemical box model is used to examine the latitudinal and seasonal variations of the methanesulfonate (MSA, CH3SO3−) to non-sea-salt sulfate (nss-SO42−) molar ratio in NOx-poor environments of the remote marine atmosphere. Reasonable agreement between observed and modeled results was obtained. The unimolecular decomposition rates of both CH3SO2 and CH3SO3 are assumed to strongly depend on air temperature. These radicals are thought to be produced through the hydrogen abstraction reaction and the addition reaction of dimethylsulfide (DMS, CH3SCH3) with both OH and NO3. In this model, the production of MSA is assumed to occur through the OH addition reaction of DMS along with the hydrogen abstraction reaction. If the MSA production through the OH addition reaction is neglected, the predicted MSA to nss-SO42− molar ratios are not in agreement with the latitudinal and seasonal variations observed in the atmosphere. Assuming a MSA production yield of 10% through the OH addition reaction in addition to the further oxidation of CH3SO3 without breaking the C-S bond, the model calculations can reproduce MSA/nss-SO42− molar ratios similar to the observed latitudinal and seasonal variations. Since the reaction of DMS with OH is the most important sink of DMS in the summer the addition and abstraction reactions appear to control the MSA production during this season. At middle and high latitudes during the winter, DMS is mainly oxidized by reaction with NO3. Therefore MSA in the winter may primarily be produced from the further oxidation of CH3SO3. It appears that competition between the decomposition and the further oxidation of CH3SO3 is a determining factor of the winter MSA/nss-SO42− molar ratio.

Influence of large-scale atmospheric circulation on marine air intrusion toward the East Antarctic coast

Marine air intrusions into Antarctica play a key role in high-precipitation events. Here we use shipboard observations of water vapor isotopologues between Australia and Syowa on the East Antarctic coast to elucidate the mechanism by which large-scale circulation influences marine air intrusions. The temporal isotopic variations at Syowa reflect the meridional movement of a marine air front. They are also associated with atmospheric circulation anomalies that enhance the southward movement of cyclones over the Southern Ocean. The relationship between large-scale circulation and the movement of the front is explained by northerly winds which, in association with cyclones, move toward the Antarctic coast and push marine air with isotopically enriched moisture into the inland covered by glacial air with depleted isotopic values. Future changes in large-scale circulation may have a significant impact on the frequency and intensity of marine air intrusion into Antarctica.