Methanesulfonic and sulfuric acids are major contributors to tropical Indo-Pacific aerosol

Abstract. In the marine environment dimethylsulfide (DMS) is the most abundant sulfur-containing trace gas. It serves as a key precursor to new particle formation and growth via its oxidation products, sulfuric acid (SA, H₂SO₄) and methanesulfonic acid (MSA, CH₃SO₃H). Here, we present measurements of MSA and SA in the Indo-Pacific region during the CAFE-Pacific (Chemistry of the Atmosphere Field Experiment in the Pacific) campaign in January–February 2024. The measurements were conducted on board the HALO (High Altitude and LOng-range) aircraft using nitrate mass spectrometry. We observe gas-phase concentrations of up to 4 × 10⁷ cm^-3 MSA and 6 × 10⁷ cm^-3 SA in the marine boundary layer. In the lower free troposphere, the MSA/SA ratio increases with altitude in agreement with the temperature-dependent DMS oxidation. At higher altitudes, adiabatic heating and subsequent evaporation of acidic particles within the instrument inlet enable the detection of both particle- and gas-phase MSA and SA. A detailed analysis of two flights shows that marine deep convection can lead to DMS transport from the boundary layer to the upper troposphere and subsequent particle formation and growth after approximately 10–20 hours of OH exposure aligning with the DMS lifetime determined by kinetic modelling. We frequently observe MSA concentrations significantly exceeding those of SA, suggesting that free-tropospheric particles – particularly over the Indo-Pacific Warm Pool – may be dominated by MSA. Our results imply that marine convection represents an important source of airborne particles in the upper tropical troposphere, one of the most pristine regions of Earth's atmosphere.