Oxidation of methanol by hydroxyl radicals in aqueous solution under simulated cloud droplet conditions

The results of a detailed mechanistic study of aqueous-phase OH-oxidation o f methanol are presented. Analysis of reaction products by specific chromat ographic methods revealed that hydrated formaldehyde is not the only stable primary reaction product. Formic acid and/or formate ion are also stable p rimary molecular reaction products of methanol OH-oxidation. The branching ratios for their formation are highly pH dependent. lit pH = 7, hydrated fo rmaldehyde is the dominant molecular reaction product (ratio 4.5 : 1 for hy drated formaldehyde: formate ion), whereas at pH = 2, formic acid is the do minant product (ratio 3.7:1 for formic acid:hydrated formaldehyde). At all pH studied, the sum of the primary stable products represents 49 ( +/- 11)% of methanol removal, in agreement with the amount of OOCH2 OH radicals for med relative to methanol removal 48 ( +/- 2)%. The formation of primary for mic acid at pH = 2 is attributed to OOCH2 OH self-reaction, and the strong pH effect is attributed to the base-catalyzed decomposition of OOCH2OH lead ing to the formation of hydrated formaldehyde. Evaporation and/or an additi on reaction between CH2OH and HO2 radicals leading to the formation of hydr oxymethyl hydroperoxide is proposed to explain the missing yields. The impl ications of this mechanism to atmospheric chemistry are discussed, (C) 2000 Elsevier Science Ltd. All rights reserved.