Distributions and Sources of Methanesulfonic Acid (MSA) over the Tropical Pacific Ocean

Sulfur chemistry in the marine atmosphere is critical to the production of sulfate aerosols, which play an important role in the climate system. Methanesulfonic acid (MSA) is a major yet not well studied oxidation product of dimethyl sulfide (DMS), which is emitted from the ocean. In this study, gas-phase MSA was measured in the lower troposphere over the tropical Pacific on the NCAR C-130 aircraft during the Pacific Atmospheric Sulfur Experiment (PASE). A 1-dimensional chemical transport model (REAM) was used to analyze the vertical profiles of MSA driven by chemistry and turbulent transport. The observed vertical profiles of MSA revealed two remarkable features. First, the measured MSA concentration was enhanced near the ocean surface, decreasing rapidly in the boundary layer from the surface to ~600 m. The model analysis suggests that this sharp gradient cannot be explained by the OH oxidation of DMS or the oxidation by a reasonable level of BrO near the surface. The gradient would imply an unidentified MSA source of 4.0×10 7 molecule/cm2/s close to the ocean surface. Secondly, a large peak of MSA was observed in the lower free troposphere (FT, 1000~2000m). The MSA concentration in the lower FT was an order of magnitude larger than that in the boundary and buffer layers. The anti-correlation between the lower FT MSA concentration and relative humidity (RH) suggests that the enhancement in gas-phase MSA is related to the dehydration of aerosols due to the decreased RH at higher altitudes. The dehydrated aerosols in lower FT lose their capacity to take up gaseous MSA. In addition, our model analysis suggests that a fraction (10-20%) of aerosol-phase MSA must degas from dry aerosols to reproduce the observed vertical profile. The degassing mechanism provides a source of 1.2×10 7 molecule/cm2/s of MSA to the lower free troposphere.