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In the past decades, China has witnessed high air pollution associated with rapid economic development, although regulatory efforts have alleviated the situation since 2013. Haze events characterized by high particulate matter (PM) levels in China are not only of enormous magnitude but also represent a distinct chemical regime. Once driven by direct emissions, these high-PM episodes are now more affected by secondary aerosol, especially secondary organic aerosol (SOA). This Review synthesizes the state of the science of SOA formation in urban China, specifically (i) how the dominance of anthropogenic precursors affects SOA formation, (ii) what are the prevailing SOA formation mechanisms, and (iii) how important are the multipollutant and multiphase processes in SOA formation and evolution. We also highlight essential directions for future studies. 

Brown carbon (BrC) has a substantial direct radiative effect, but current estimates of its impact on radiative balance are highly uncertain due to a lack of measurements of its light-absorbing properties, such as mass absorption efficiency (MAE). Here, we present a new analytical paradigm based on a Bayesian inference (BI) model that takes multiwavelength aethalometer measurements and total carbon data to resolve the concentrations of black carbon and BrC, and MAEs of BrC on a sample-by-sample basis. Hourly MAEs, unattainable in previous studies, can now be calculated, enabling the first-time observation of the darkening-bleaching dynamics of BrC in response to photochemical transformation. We demonstrate the application of this BI model to analyze measurements collected over one year (2021–2022) in Hong Kong. Diel variations in MAE370 nm of BrC reveal a darkening-to-bleaching transition occurring between 8 and 10 o’clock when the solar irradiance ranges from 30 to 400 W m–2. Furthermore, we consistently observe an increase in MAE370 nm of BrC with nitrogen oxide concentrations, suggesting the enhanced formation of nitrogenous organics. This BI model-based data analysis would bring forth a breakthrough in amassing observation data of BrC and its MAEs in diverse ambient environments and with high time resolution.