Radical Cloud Chemistry: Oxidation of Organic Compounds in Atmospheric Water

Abstract

Atmospheric aging of organic compounds by photo-chemical oxidation can occur rapidly in the aqueous phase in aerosol particles, cloud droplets, and fog droplets and can result in changes to their chemical composition and absorptivity. Despite their capacity to significantly alter the properties of organic aerosol, aqueous phase reactions have not gained as much attention as other atmospheric aging pathways. The goal of this thesis was to further our understanding of photoreactions of atmospherically-relevant organic compounds in cloud water. This goal was accomplished through three projects, which investigated factors influencing the production of hydroxyl (OH) radicals in aqueous phase reactions and the lifetime, oxidation mechanism, and fate of light absorbing molecules that contribute to brown carbon. In the first project, the impact of dissolved organic compounds, specifically from α-pinene ozonolysis secondary organic aerosol (SOA) material, on the Fenton reaction, a major source of OH radicals in cloud and fog water, was investigated. OH radical production was significantly suppressed in the presence of SOA material, likely due to complexation of the iron involved in the Fenton reaction. In the second project, aqueous photo-oxidation kinetics of three light-absorbing nitrophenol compounds often associated with wildfire smoke were examined and were found to have an in-cloud lifetime on the order of hours. Photo-oxidation initially caused an absorption enhancement that corresponded to products with added oxygenated functional groups. Further photo-oxidation led to products that do not absorb visible light, reducing their contribution to atmospheric warming. In the third project, representative and chemically complex brown carbon was generated in the laboratory from wood smoke. Light absorption increased from aqueous photoreaction and OH oxidation and was linked to oligomerization and functionalization of aromatic compounds, similar to the nitrophenol chemistry. While extended OH oxidation led to a decrease in absorbance, brown carbon in wood smoke is likely to have a longer lifetime than estimated by individual components. Overall, this work demonstrated key outcomes of organic chemical transformations resulting from aqueous atmospheric aging under relevant cloud conditions and provided implications for their effect on the climate system.