What it is
This Review synthesizes the state of the science on secondary organic aerosol (SOA) formation in urban China, where severe haze events combine an enormous magnitude of particulate matter with a chemically distinct regime. It explains that high-PM episodes, once driven mainly by direct emissions, are now shaped more by secondary aerosol, especially SOA, even as regulation has alleviated pollution since 2013. The authors organize the field around three questions: how the dominance of anthropogenic precursors affects SOA formation, what the prevailing formation mechanisms are, and how important multipollutant and multiphase processes are in SOA formation and evolution.
Why it matters
China's haze is characterized as a distinct chemical regime, so understanding and controlling it requires more than importing frameworks built for other regions. By pinpointing the shift from primary emissions to secondary organic aerosol and the central role of anthropogenic precursors, the review defines where air-quality science and future controls need to focus now that direct-emission sources are already being regulated.
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Filed underAtmospheric chemistry and aerosols, Air Quality and Health Impacts, Air Quality Monitoring and Forecasting