Secondary Organic Aerosol from the Oxidation of Highly Substituted Phenols by Aqueous Hydroxyl Radical

Abstract:

Phenolic compounds emitted in biomass burning can react in the gas-phase or partition into condensed phases, such as cloud, fog drops, and particle water, where they can be oxidized to form aqueous secondary organic aerosol (aqSOA). To better understand the impact of phenolic compounds on aqSOA, we examined the aqueous oxidation of highly substituted phenols by hydroxyl radical (OH). Previous work has demonstrated that simple phenols rapidly react with OH and efficiently form aqSOA in cloud/fog conditions. However, these compounds do not partition strongly enough to the aqueous phase to be an important source of aqSOA in aerosol liquid water (ALW). In contrast, more highly substituted phenols might be a significant source of aqSOA in ALW. To address this, we measured the oxidation kinetics of 5 highly substituted phenols by OH at pH 2 and 5 and the corresponding yields of aqSOA. Additionally, we measured light absorption by the aqSOA and its photobleaching during continued oxidation. We find that the phenol-OH reactions proceed rapidly, with rate constants of 10⁹ – 10¹⁰ M^-1 s^-1, and form aqSOA efficiently. The resulting aqSOA photobleaches with continued oxidation, with lifetimes of light absorption on the order of a few hours typically. Finally, we will use our data to assess the importance of highly substituted phenols as a source of aqSOA in aerosol liquid water in regions with frequent biomass burning.