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 of methanol are presented. Analysis of reaction products by specific chromatographic methods revealed that hydrated formaldehyde is not the only stable primary reaction product. Formic acid and/or formate ion are also stable primary molecular reaction products of methanol OH-oxidation. The branching ratios for their formation are highly pH dependent. At pH=7, hydrated formaldehyde is the dominant molecular reaction product (ratio 4.5 : 1 for hydrated formaldehyde : formate ion), whereas at pH=2, formic acid is the dominant 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 OOCH2OH radicals formed relative to methanol removal 48(±2)%. The formation of primary formic acid at pH=2 is attributed to OOCH2OH self-reaction, and the strong pH effect is attributed to the base-catalyzed decomposition of OOCH2OH leading to the formation of hydrated formaldehyde. Evaporation and/or an addition reaction between CH2OH and HO2 radicals leading to the formation of hydroxymethyl hydroperoxide is proposed to explain the missing yields. The implications of this mechanism to atmospheric chemistry are discussed