A photochemical origin of acetic acid in the troposphere

Significant concentrations of formic and acetic acids are known to exist in the troposphere. These acids contribute between 16 and 35% of the free acidity in North American precipitation and between 25 and 98% of the free acidity of precipitation in remote areas. Their origin is still relatively uncertain. For formic acid, several gas phase and aqueous phase chemical production mechanisms have been postulated, and seem to explain at least roughly the observed concentrations. However, for acetic acid, the analogous production mechanisms appear to be far too slow, and it has been suggested that this acid may be emitted directly by vegetative matter. Based on recent laboratory and theoretical studies, we have considered the possible gas phase production of acetic acid via the reaction of peroxy acetyl radicals with HO2 and organic peroxy radicals. A highly detailed gas phase chemical mechanism coupled with a simple 1-D model was used to predict organic acid concentrations for one set of conditions (Amazon boundary layer) where extensive field measurements are available for model initialization and comparison. The results indicate that the gas phase photooxidation of non-methane-hydrocarbons can produce amounts of acetic acid in the range of several parts per billion, comparable to the observed values.