Significant Underestimation of Gaseous Methanesulfonic Acid (MSA) over Southern Ocean

Methanesulfonic acid (MSA), derived from the oxidation of dimethylsulfide (DMS), has a significant impact on biogenic sulfur cycle and climate. Gaseous MSA (MSA g ) has been often ignored in previous studies due to its quick conversion to particulate MSA (MSA p ) and low concentrations. MSA g, MSA p, and nss-SO 4 2– were observed simultaneously for the first time with high-time-resolution (1 h) in the Southern Ocean (SO). The mean MSA g level reached up to 3.3 ± 1.6 pptv, ranging from ∼24.5 pptv in the SO, contributing to 31% ± 3% to the total MSA (MSA T ). A reduction of the MSA to nss-SO 4 2– ratios by about 30% was obtained when MSA g was not accounted for in the calculation, indicating that MSA g was very important in the assessment of the biogenic sulfur contributions in the atmosphere. Mass ratios of MSA to nss-SO 4 2– increased first and then decreased with the temperature from −10 to 5 °C, with a maximum value at the temperature of −3 °C. Positive correlations between MSA g to MSA T ratios and temperature were presented, when the temperature was higher than 5 °C. This study highlights the importance of MSA g for understanding the atmospheric DMS oxidation mechanism and extends the knowledge of MSA formation in the marine atmosphere.