The Effects of pH on Aqueous Phase SOA and Brown Carbon Formation and Transformation


Student thesis: Doctoral Thesis

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Award date13 Sept 2023


The pH of the atmospheric aqueous phase adds complexity to the atmospheric chemistry and needs to be considered when investigating Secondary organic aerosol (SOA) and brown carbon (BrC) formation and transformation. This thesis demonstrates a detailed study of the aqueous phase SOA and BrC formation and transformation and emphasizes the impacts of the pH of the aqueous phase, aiming to provide new molecular and mechanical insights into the atmospheric chemistry of organic aerosols.

This thesis started with the study of the formation and evolution of brown carbon during aqueous phase nitrate-mediated photooxidation of phenolic compounds (PhCs) which are a class of aromatic compounds that are known brown carbon precursors. How aqueous-phase inorganic nitrate photolysis can drive the formation of BrC by facilitating the photooxidation of organic compounds was explored. Upon exposure to UV light in the presence of nitrate, PhCs underwent prompt reaction, leading to the formation of brown carbon. Higher initial nitrate concentrations led to faster reaction rates. The major products produced during the initial stages of photooxidation were formed by the nitration of PhCs. Moreover, the impacts of pH on SOA and BrC evolution was investigated. During the nitrate-mediated photooxidation, the effects of pH was not universal and depended on the initial phenolic compound. During the copper-catalyzed Fenton-like reactions, higher pH led to greater initial formation of BrC for all the studied PhCs at higher pH which was likely due to the formation of more hydroxylated products and polymers. Aside from laboratory investigations, the abundance and fractional solubilities of different metals in size-fractionated aerosols collected in Hong Kong were measured and the potential effect of aerosol pH on the metal solubilities was investigated. The average metal fractional solubilities spanned a wide range for both fine (7.8% to 71.2%) and coarse (0.4% to 47.9%) aerosols. Sulfate was found to be strongly associated with both the concentrations of water-soluble Cr, Fe, Co, Cu, Pb, and Mn in fine aerosols, and it was revealed that these strong associations were due to sulfate providing both the acidic environment and liquid water reaction medium needed for the acid dissolution process.

This thesis delivers important insights into how the atmospheric aqueous phase pH influences the formation and transformation of SOA and brown carbon, contributing to better modelling prediction of SOA and brown carbon formation/transformation for areas with high levels of inorganic nitrate and/or sulfate.