[A-AS12] 大気化学

[AAS12-12] 雲核生成の鍵となるジメチルスルホキシドとOHラジカルの不均一反応の研究

*江波 進一¹ (1.京都大学白眉センター)

キーワード：エアロゾル、光化学的エイジング、気候変動、不均一反応、海洋化学

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 SO₄^2- + CH₃SO₃^- (methane sulfonate) on water surfaces. R = CH₃SO₃^-/SO₄^2- ratios therefore indicate the extent of DMSO heterogeneous oxidation if R_het = CH₃SO₃^-/SO₄^2- 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 R_het = 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 SO₄^2-) in the oxidation of DMSO in the gas-phase versus the R_het ~ 2.7 value determined at the air-water interface means that R = CH₃SO₃^-/nss-SO₄^2- 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.