Secondary aerosol formation from dimethyl sulfide – improved
mechanistic understanding based on smog chamber experiments
and modelling
Abstract. Dimethyl sulfide (DMS) is the dominant biogenic sulphur compound in the ambient atmosphere. Low volatile acids from DMS oxidation promote the formation and growth of sulphur aerosols, and ultimately alter cloud properties and Earth's climate. We studied the OH-initiated oxidation of DMS in the Aarhus University research on aerosols (AURA) smog chamber and the marine boundary layer (MBL) with the aerosol dynamics, gas- and particle-phase chemistry kinetic multilayer model ADCHAM. Our work involved the development of a revised and comprehensive multiphase DMS oxidation mechanism, both capable of reproducing smog chamber and atmospheric relevant conditions. The secondary aerosol mass yield in the AURA chamber was found to have a strong dependence on the reaction of methyl sulfinic acid (MSIA) and OH at low relative humidity (RH), while the autoxidation of the intermediate radical CH3SCH2OO forming hydroperoxymethyl thioformate (HPMTF) proved important at high RH. The observations and modelling strongly support that a liquid water film existed on the Teflon surface of the chamber bag, which enhanced the wall loss of water soluble intermediates and oxidants DMSO, MSIA, HPMTF, SO2, MSA, SA and H2O2. The effect caused a decrease in the secondary aerosol mass yield obtained at both dry (0–12 % RH) and humid (50–80 % RH) conditions. Model runs reproducing the ambient marine atmosphere indicate that OH comprise a strong sink of DMS in the MBL, although less important than halogen species Cl and BrO. Cloudy conditions promote the production of SO42− particular mass (PM) from SO2 accumulated in the gas-phase, while cloud-free periods facilitate MSA formation in the deliquesced particles. The exclusion of aqueous-phase chemistry lowers the DMS sink as no halogens are activated in the sea spray particles, and underestimates the secondary aerosol mass yield by neglecting SO42− and MSA PM production in the particle phase. Overall, this study demonstrated that the current DMS oxidation mechanisms reported in literature are inadequate in reproducing the results obtained in the AURA chamber, whereas the revised chemistry captured the formation, growth and chemical composition of the formed aerosol particles well. Furthermore, we emphasise the importance of OH-initiated oxidation of DMS in the ambient marine atmosphere during conditions with low sea spray emissions.