Surface and free tropospheric sources of methanesulfonic acid over the tropical Pacific Ocean

The production of sulfate aerosols through sulfur chemistry in marine environments is critical to the tropical climate system. However, not all sulfur compounds have been studied in detail. One such compound is methanesulfonic acid (MSA). In this study, we use a one‐dimensional chemical transport model to analyze the observed vertical profiles of gas phase MSA during the Pacific Atmospheric Sulfur Experiment. The observed sharp decrease in MSA from the surface to 600 m implies a surface source of 4.0 × 107 molecules/cm2/s. Evidence suggests that this source is photolytically enhanced in daytime. We also find that the observed large increase of MSA from the boundary layer into the lower free troposphere (1000–2000 m) results mainly from the degassing of MSA from dehydrated aerosols. We estimate a source of 1.2 × 107 molecules/cm2/s to the free troposphere through this pathway. This source of soluble MSA could potentially provide an important precursor for new particle formation in the free troposphere over the tropics, affecting the climate system through aerosol‐cloud interactions. Key Points Observations reveal missing sources of MSA over the tropical Pacific The unknown sulfur source has large impacts on aerosol formation and climate