Chemical composition differences in gas- and particle-phase organic and DMS-derived oxidation products between a pristine marine environment and a wildfire-influenced marine air mass

Marine dimethyl sulfide (CH₃SCH₃, DMS) is a major source of natural gas-phase sulfur emissions. Moreover, DMS oxidation products such as methane sulfonic acid (CH₃SO₃H, MSA) and sulfuric acid (H₂SO₄) are known to influence the formation of cloud condensation nuclei (CCN). Recently, a stable intermediate formed from DMS oxidation, hydroperoxymethyl thioformate (HOOCH₂SCHO, HPMTF), has been shown to sometime exceed mixing ratios of 100 ppt in the marine boundary layer. This product would thus be able to delay the formation of sulfate aerosols and could have a significant impact on cloud formation. In order to investigate these species, a cruise field campaign in the Atlantic Ocean was organized in 2025 to measure pristine marine air. However, we also sampled air masses coming from the Canadian wildfires that occurred early June 2025, which allowed for comparison between pristine marine air and biomass burning (BB) periods. Oxidation products were measured using a time-of-flight chemical ionization mass spectrometer (Vocus 2R-ToF-CIMS) coupled with a Filter Inlet for Gas and Aerosols (FIGAERO inlet), allowing measurements of both gas- and particle-phase chemical composition from one instrument. This FIGAERO CIMS alternated between iodide and bromide reagent ions. Based on the measurements with the iodide reagent ion, the relative distribution between DMS-derived sulfur containing species and oxygenated organic compounds (CHO) remained similar across the two periods, both for gas- and particle-phase. Indeed, the abundance of these species significantly increased by similar factors (2.5 and 2.2 times higher for sulfur containing species and organics, respectively) during the BB period for particle-phase compounds. Particle-phase MSA and H₂SO₄ were 2.7 and 1.5 times higher, respectively, during the BB period compared to the pristine marine environment. Similarly, many particle-phase CHO species were enhanced during the BB period. Such species include C₆H₁₀O₅ (levoglucosan, 23 times higher), C₄H₄O₆ (13 times higher), C₆H₈O₆ (12 times higher), C₃H₄O₅ (11 times higher) and C₂H₂O₄ (3 times higher). Contrariwise, HPMTF, both in gas- and particle-phase, was more abundant during the pristine period compared to the BB period. As HPMTF is known to be removed by cloud uptake, this could indicate that biomass burning periods, loaded with sulfate aerosol, could result in higher cloud coverage, which would lead to higher HPMTF sink. Indeed, the irradiance measured from the ship was lower during the BB period compared to the usual pristine period irradiance. This could indicate that HPMTF was either less produced from marine DMS during this period due to lesser irradiance, or experienced higher sink due to enhanced cloud coverage, or both. This work shows that biomass burning can significantly change the abundance of DMS-derived compounds. As wildfires become more common due to global warming, it is important that these changes be considered for accurate modelling and predictions.