ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-9-14995-2009 A three-dimensional model study of methanesulphonic acid to non sea salt sulphate ratio at mid and high-southern latitudes Castebrunet H. 1 Martinerie P. 1 Genthon C. 1 Cosme E. 2 Laboratoire de Glaciologie et Géophysique de l'Environnement, UMR 5183, 38402 Saint Martin d'Hères cedex, France Laboratoire des Ecoulements Géophysiques et Industriels, UMR 5519, 38402 Saint Martin d'Hères, France 13 07 2009 9 4 14995 15043 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/9/14995/2009/acpd-9-14995-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/9/14995/2009/acpd-9-14995-2009.pdf

The Antarctic and sub-Antarctic methanesulphonic acid (MSA) to non sea salt sulphate (nssSO<sub>4</sub>) ratio is simulated with the Laboratoire de Météorologie Dynamique Atmospheric General Circulation Model including an atmospheric sulphur chemistry module. Spatial variations of the MSA/nssSO<sub>4</sub> ratio in different regions have been suggested to be mostly dependent on temperature or sulphur source contributions. Its past variations in ice cores have been interpreted as related to the DMS precursor source location. Our model results are compared with available field measurements in the Antarctic and sub-Antarctic regions. This suggests that the MSA/nssSO<sub>4</sub> ratio in the extra-tropical south hemisphere is mostly dependent on the relative importance of various DMS oxidation pathways. In order to evaluate the effect of a rapid conversion of dimethyl sulphoxide (DMSO) into MSA, not implemented in the model, the MSA+DMSO to nssSO<sub>4</sub> ratio is also discussed. Using this modified ratio, the model mostly captures the seasonal variations of MSA/nssSO<sub>4</sub> at mid and high-southern latitudes. In addition, the model qualitatively reproduces the bell shaped meridional variations of the ratio, which is highly dependent on the adopted relative reaction rates for the DMS+OH addition and abstraction pathways, and on the assumed reaction products of the MSIA+OH reaction. MSA/nssSO<sub>4</sub> ratio in Antarctic snow is fairly well reproduced except at the most inland sites characterized with very low snow accumulation rates. Our results also suggest that atmospheric chemistry plays an important role in the observed decrease of the ratio in snow between coastal regions and central Antarctica. The still insufficient understanding of the DMS oxidation scheme limits our ability to model the MSA/nssSO<sub>4</sub> ratio. Specifically, reaction products of the MSIA+OH reaction should be better quantified, and the impact of a fast DMSO conversion to MSA in spring to fall over Antarctica should be evaluated. Direct measurements of MSA and nssSO<sub>4</sub> dry deposition velocities on Antarctic snow would improve our ability to model MSA and nssSO<sub>4</sub> in ice cores.

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