Nitrate-driven extreme winter PM2.5 pollution in Shanghai, China

The intensity and frequency of severe fine particulate matter (PM 2.5 ) pollution events in Shanghai have reduced following strict emission controls. However, an unexpected resurgence of PM 2.5 occurred during 2023–2024 winter (peak 162.7 μg m −3 , highest since 2018–2019), yet its characteristics and underlying drivers remain unclear. Using observations, the Community Multiscale Air Quality (CMAQ) model, and machine learning (eXtreme Gradient Boosting [XGBOOST], SHapley Additive exPlanations [SHAP]), this study revealed that nitrate (NO 3 − ) dominated PM 2.5 composition (33%) and three of four pollution events, primarily from local emissions (16.8%) and adjacent transport (39.4%), with volatile organic compounds (VOCs), relative humidity, and ammonia (NH 3 ) as key factors. Sensitivity analyses demonstrated NH 3 reductions effectively reduced NO 3 − and PM 2.5 in Shanghai, while VOCs controls showed greater efficacy for PM 2.5 at low reduction levels, though regional effectiveness varied. These findings highlight the importance of VOCs and NH 3 reductions to mitigate NO 3 − and PM 2.5 pollution in Shanghai and eastern China.