Atmospheric chemistry of alcohols

Abstract

The atmospheric chemistry of alcohols, which are widely used as motor vehicle fuels (e.g. ethanol in Brazil) and as industrial solvents, has been reviewed with focus on kinetic data and on reaction mechanisms. Oxidation of alcohols in the atmosphere involves their reaction with the hydroxyl radical (OH). Alcohol-OH reaction rate constants are presented for 33 saturated alcohols including monofunctional and difunctional compounds. The corresponding atmospheric half-lives are one week for methanol and t-butyl alcohol, 2.5 days for ethanol, and 8-15 h for other alcohols. Laboratory studies of alcohol-OH reaction products are described and reaction mechanisms are outlined. Major products are formaldehyde from methanol, acetaldehyde from ethanol, acetone from 2-propanol, 2-butanone and acetaldehyde from 2-butanol and acetone and formaldehyde from t-butyl alcohol. The reaction of OH with alcohols involves H-atom abstraction from C-H bonds; H-atom abstraction from the O-H bond is negligible. The alkyl radicals (R) and α-hydroxyalkyl radicals thus formed react with oxygen. This reaction involves addition for alkyl radicals (R + O2 → RO2) and H-atom abstraction for α-hydroxyalkyl radicals (e.g. ethanol + OH → CH3CHOH, CH3CHOH + O2 → HO2 + CH3CHO). The reaction sequence ethanol → acetaldehyde → peroxyacetyl nitrate (PAN, CH3C(O)OONO2) is described and is relevant to urban air pollution in Brazil. Recommendation is made to carry out additional product studies in order to develop a better understanding of the atmospheric chemistry of alcohols and of their role in on urban and regional air quality.