Aqueous Secondary Organic Aerosol (aqSOA) Formation By Radical Reactions: Model Studies Comparing the Role of OH Versus Organic Radicals

Abstract:

Chemical reactions in the aqueous phase are thought to significantly contribute to ambient aerosol mass under specific conditions. Results from many laboratory studies suggest that these reactions are efficiently initiated by the OH radical and lead to high molecular weight compounds (oligomers). Recent laboratory experiments have shown that methyl vinyl ketone (MVK) can form oligomers in high yield in aqueous solutions similar to aerosol water. Additional experiments have shown that the direct photolysis of pyruvic acid can generate organic radicals that initiate similar oligomer products upon oxidation of MVK (Renard et al., submitted).

Sources of the OH radical in the aerosol aqueous phase include the direct uptake from the gas phase, Fenton reactions and, to a smaller extent, direct photolyses of hydrogen peroxide and nitrate. Recent model studies imply that under many conditions, aqSOA formation might be oxidant-limited since these OH(aq) sources are not sufficient to provide a continuous OH supply. This limitation can be (partially) removed if additional radical sources in the multiphase system are considered. Exemplary, we include the direct photolysis of aqueous pyruvic acid as a proxy for possible other radical sources. Model results will be shown and consequences for aqSOA formation and processing under ambient conditions will be discussed.