Cloud processing of DMS oxidation products limits SO₂ and OCS production in the Eastern North Atlantic marine boundary layer

Abstract. Dimethyl sulfide (DMS) is the major sulfur species emitted from the ocean. The gas-phase oxidation of DMS by hydroxyl radicals proceeds through the stable, soluble intermediate hydroperoxymethyl thioformate (HPMTF), eventually forming carbonyl sulfide (OCS) and sulfur dioxide (SO₂). Recent work has shown that HPMTF is efficiently lost to marine boundary layer (MBL) clouds, thus arresting OCS and SO₂ production and their contributions to new particle formation and growth events. To date, no long-term field studies exist to assess the extent to which frequent cloud processing impacts the fate of HPMTF. Here we present six weeks of measurements of cloud fraction and the marine sulfur species, methanethiol, DMS, and HPMTF, made at the ARM Research Facility on Graciosa Island, Azores, Portugal. Using an observationally constrained chemical box model, we determine that cloud loss is the dominant sink of HPMTF in this region of the MBL during the study, accounting for 79–91 % of HPMTF loss on average. When accounting for HPMTF uptake to clouds, we calculate a campaign average reduction in DMS-derived MBL SO₂ and OCS of 52–60 % and 80–92 % for the study period. Using yearly measurements of site- and satellite-measured 3-dimensional cloud fraction and DMS climatology, we infer that HPMTF cloud loss is the dominant sink of HPMTF in the Eastern North Atlantic during all seasons, and occurs on timescales faster than what is prescribed in global chemical transport models. Accurately resolving this rapid loss of HPMTF to cloud has important implications for constraining drivers of MBL new particle formation.