Observation of Brown carbon in PM_2.5 revealing effects of source-dependence and aging on light absorption

Organic aerosol (OA) in atmospheric fine particulate matter (PM_2.5) has significant impacts on human health, the atmospheric environment, and climate change. Light-absorbing OA, referred to as brown carbon (BrC), is non-negligible during atmospheric processes. However, seasonal and day-night variations, as well as the identification of key driving factors for the dynamic changes in BrC optical properties during continuous haze episodes, remain inadequately understood. In this study, we measured BrC light absorption in PM_2.5, comparing water-soluble BrC (WS-BrC) and water-insoluble BrC (WIS-BrC) across haze episodes in different seasons, including dust pollution, revealing distinct seasonal variations in light-absorbing capability, highest in winter and lowest in summer, resulting from changes in mass absorption efficiency. The light-absorbing capability exhibited an obvious day-night change owing to the different yield pathways. In addition, we found that WS-BrC dominated the light absorption coefficient owing to its high concentration. Furthermore, the aging process gradually exerted depression as PM_2.5 accumulated. Excitation-emission-matrix fluorescence analysis evidenced the aging and sourced effects. Statistical and Shapley additive explanations analysis highlighted that nitrogen-containing (i.e., reductive and oxidative nitrogen) secondary formation pathways within daytime photochemistry and nighttime nitrate radical reactions dominantly contributed to WIS-BrC. However, fuel combustion and secondary formation concurrently resulted in WS-BrC. The transformation from WIS-BrC to WS-BrC cannot be neglected during the day. These insights underscore the dynamic nature of BrC light capability because of the differences in sources and atmospheric processes. This study provides a reference for the accurate evaluation of variations in BrC optical properties and informs climate and air quality solutions.