Abstract
Abstract. The Antarctic and sub-Antarctic methanesulphonic acid (MSA) to non sea salt sulphate (nssSO4) ratio is simulated with the Laboratoire de Météorologie Dynamique Atmospheric General Circulation Model including an atmospheric sulphur chemistry module. Spatial variations of the MSA/nssSO4 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/nssSO4 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 nssSO4 ratio is also discussed. Using this modified ratio, the model mostly captures the seasonal variations of MSA/nssSO4 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/nssSO4 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/nssSO4 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. A better understanding of BrO source processes is needed in order to include DMS + BrO chemistry in global models. Completing the observations of DMS, BrO and MSA at Halley Bay with DMSO measurements would better constrain the role of BrO in DMS oxidation. Direct measurements of MSA and nssSO4 dry deposition velocities on Antarctic snow would improve our ability to model MSA and nssSO4 in ice cores.
Highlights
Natural methanesulphonic acid (MSA) and non sea salt sulphate are formed by the oxidation of dimethylsulphide (DMS), produced in the surface ocean by phytoplankton species and subsequently released into the atmosphere
This work aimed at better understanding the processes which influence seasonal and spatial variations of the MSA to nssSO4 ratio (R) in air and snow at mid and high-southern latitudes
The model successfully simulates seasonal variations of R in the Antarctic atmosphere if a fast conversion process of dimethyl sulphoxide (DMSO) into MSA is taken into account
Summary
Natural methanesulphonic acid (MSA) and non sea salt sulphate (nssSO4) are formed by the oxidation of dimethylsulphide (DMS), produced in the surface ocean by phytoplankton species and subsequently released into the atmosphere. Legrand et al (1992) have proposed to use R as a marker of the latitude of DMS emissions which produced the MSA and nssSO4 measured in Antarctic ice. An optimal use of R as a marker on a global scale requires that factors such as temperature, light intensity, or oxidant concentrations are well understood (Bates et al, 1992). Taking into account only DMS sources, calculated R values are 3 times higher than observed values, and a significant bias is obtained at coastal Antarctic stations This over-estimation may be explained by the direct production of MSA from DMS in the chemistry scheme (DMSO is not represented) and the coarse model resolution (Gondwe et al, 2004). In relation with Antarctic ice core data and the past sulphur cycle, we take into account the recently published seasonality of R over the Antarctic continent and discuss MSA to nssSO4 ratios in snow
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