Connecting Brown Carbon Composition and Physicochemical Properties of Aqueous Urban PM 2.5 to their Photosensitized Production of Singlet Oxygen and Organic Triplet Excited States

Oxidants, including singlet oxygen (¹O₂*) and organic triplet excited states (³C*) formed from the photoexcitation of brown carbon (BrC), drive many chemical processes in atmospheric waters. However, due to the chemical complexity of atmospheric BrC, many questions remain about the specific BrC chromophores and physicochemical properties that primarily control ¹O₂* and ³C* production. In this study, we present a framework for apportioning photosensitizers and predicting the production of ¹O₂* and ³C* based on measurable physicochemical properties of BrC. This is achieved by combining photochemical experiments with absorbance and fluorescence measurements and statistical modeling of a year-long data set of PM2.5 extracts from Hong Kong SAR, China. Parallel Factor and Non-negative Matrix Factorization analyses of the fluorescence data revealed that highly oxygenated organic aerosols were the main contributors to ¹O₂* production, whereas less oxygenated organic aerosols were the main contributors to ³C* production. Next, we developed Orthogonal Partial Least Squares-Multiple Linear Regression models that successfully predicted ¹O₂* and ³C* steady-state concentrations ([¹O₂*]ss and [³C*]ss) and quantum yields ( ΦO2*1 and Φ³C*) from standard optical measurements. These models revealed that while [¹O₂*]ss and [³C*]ss depended on parameters that reflected the quantities of BrC chromophores, ΦO2*1 and Φ³C* were influenced by the specific types (i.e., quality) of BrC chromophores present. Overall, this combined approach provides a powerful tool for identifying key BrC chromophore components and specific physicochemical properties that drive ¹O₂* and ³C* production.