Heterogeneous responses of secondary organic aerosol sources to long-term emission reductions in megacity Shanghai, China.

Stringent clean air policies have driven a sharp decline in atmospheric organic aerosol (OA) concentrations in eastern China over the past decade. However, long-term evolution of OA sources, especially secondary OA (SOA), and its underlying drivers remain poorly understood. In this study, dozens of organic molecular tracers in atmospheric fine particulate matter (PM2.5) were measured in urban Shanghai during summer and winter of 2015, 2019, and 2021. The total concentration of organic tracers decreased over this period, with primary tracers declining significantly while secondary species exhibited varying trends. Using a tracer-based positive matrix factorization model, five SOA source factors were identified, including biogenic SOA, aromatic SOA, short-chain aliphatic organosulfates (OSs), and secondary nitrate and sulfate factors, the latter two likely representing SOA formed via aqueous-phase processes. Six primary OA (POA) sources were also resolved. Secondary sources dominated the OA mass, with the first three factors being the largest contributors. During the observation period, contributions from biogenic SOA and aliphatic OSs declined annually, primarily driven by reduced atmospheric oxidizing capacity and aerosol heterogeneous reactivity. In contrast, aromatic SOA contributions increased, especially in summer, likely due to higher SOA yields following substantial reduction in nitrogen oxides. Simulations with the Community Multiscale Air Quality model further indicate that SOA trends were primarily meteorologically driven from 2015 to 2019 but became emission-dominated during 2019-2021. These findings reveal the heterogeneous responses of SOA sources to emission reductions in a Chinese megacity and highlight the importance of formulating source-specific mitigation strategies.