Measurements of particulate methanesulfonic acid above the remote Arctic Ocean using a High Resolution Aerosol Mass Spectrometer

Methanesulfonic acid (MSA) is an important product from the oxidation of dimethyl sulfide (DMS), and thus is often used as a tracer for marine biogenic sources and secondary organic aerosol. MSA also contributes to aerosol mass and potentially to the formation of cloud condensation nuclei and new particles. However, measurements of MSA at high temporal resolution in the remote Arctic are scarce, which limits our understanding of its formation, climate change impact and regional transport. Here, we applied a validated quantification method to determine the mass concentration of MSA and non-sea salt sulfate (nss-SO4) in PM2.5 in the marine boundary layer, using a high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) during a research cruise to the Arctic and North Atlantic Ocean, between 55 °N and 68 °N (26th May to 23th June 2022). With this method, the concentrations of MSA in the remote Arctic marine boundary layer were determined for the first time. Results show that the average MSA concentration was 0.025±0.03 μg m-3, ranging from <0.01 to 0.32 μg m-3. The lowest MSA level was found towards the north leg of the cruise (near Sisimut (67 °N)) where was dominant influenced by air mass over sea ice from the northern polar region, and the highest MSA concentrations were observed over the Atlantic open ocean. The diurnal cycles of gas MSA, particulate MSA and nss-SO4 peaked at afternoon followed 1 hour later than that of solar radiation, which suggests that photochemical process via the OH-initiated pathway is an important mechanism for the conversion of DMS into MSA above the remote ocean. The mass ratio of MSA to nss-SO4 (MSA/nss-SO4) presents a temperature dependence, which indicates that the addition branching pathway favors MSA formation, while thermal decay of intermediate radicals could be a possible pathway for sulfate formation. Finally, we conclude the reference values of MSA/nss-SO4 for the remote marine atmosphere as 0.22-0.25 by combining air mass cluster analysis and the group classification results.