Keywords:Aerosol, Photochemical aging, Climate change, heterogeneous reaction, marine chemistry
The oxidation of dimethyl sulfide (DMS) emitted from ocean (~ 45 Tg S per year) is a global source of cloud condensation nuclei. Hydrophobic DMS is mostly oxidized in the gas-phase into H2SO4(g) + DMSO(g) (dimethyl sulfoxide), whereas water-soluble DMSO is oxidized into SO42- + CH3SO3- (methane sulfonate) on water surfaces. R = CH3SO3-/SO42- ratios therefore indicate the extent of DMSO heterogeneous oxidation if Rhet = CH3SO3-/SO42- for DMSO(aq) + ·OH(g) were determined. Here, products and intermediates of the oxidation of aqueous DMSO initiated by gas-phase hydroxyl radicals, OH(g), at the air-water interface were directly detected by mass spectrometry in a novel setup under various experimental conditions. Exposure of millimolar DMSO aqueous microjets to ~ 10 ns OH(g) pulses from the 266 nm laser flash photolysis of O3(g)/O2(g)/H2O(g)/N2(g) mixtures yielded an array of interfacial intermediates/products, including CH3SO3- and HSO4-, that were unambiguously and simultaneously identified in situ by mass spectrometry. We determined Rhet = 2.7 from the heterogeneous OH-oxidation of DMSO on aqueous aerosols for the first time. The nearly quantitative production of H2SO4(g) (that leads to SO42-) in the oxidation of DMSO in the gas-phase versus the Rhet ~ 2.7 value determined at the air-water interface means that R = CH3SO3-/nss-SO42- variations in the aerosol, particularly in remote locations, should arise from the competition between the gas-phase versus the heterogeneous DMSO oxidation pathways. The present study reveals that interfacial OH-oxidation processes play a more significant role in the generation and growth of atmospheric aerosol over ocean than previously envisioned.