Photooxidants are known to drive many atmospheric chemical processes. Previous studies have shown that the photoexcitation of light absorbing organic chromophores (i.e., brown carbon) in atmospheric waters can lead to the promotion of these molecules from their ground states to highly reactive triplet excited states (3C*). These 3C* can subsequently oxidize other organic molecules as well as produce other photooxidants such as singlet oxygen (1O2). Aqueous reactions of 3C* and 1O2 with organic compounds can lead to the formation of low volatility organic matter that constitute secondary organic aerosols (SOA). Understanding the importance of these 3C* and 1O2 reactions necessitates knowing their steady-state concentrations. However, there has been a limited number of measurements of 3C* and 1O2 production from atmospheric particulate matter, and available measurements have mainly been restricted to North America. In this study, we report first ever measurements of the steady-state concentrations of 3C* and 1O2 in illuminated aqueous extracts of fine particulate matter collected during different seasons at two urban sites and one rural site during a year-round study conducted in Hong Kong, South China. The concentrations and absorbance of water-soluble organic carbon were generally found to be the highest at all three sites during the winter season. Similarly, the steady-state concentrations of 3C* and 1O2 were generally the highest during the winter. Interestingly, the steady-state 1O2 concentrations were higher than the steady-state 3C* concentrations but this may be attributed to the 3C* probe used for the measurements. The steady-state concentrations of 3C* and 1O2 were found to be strongly correlated with each other and the water-soluble organic carbon concentrations. Differences in the environments (rural vs. urban) did not appear to affect the steady-state concentrations of 3C* and 1O2.

First measurements of photooxidant production from brown carbon and other chromophores in atmospheric fine particulate matter in Hong Kong, South China

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