From Emission-Dominated Mitigation to Meteorology-Modulated Secondary Pollution: Decadal Insights from Beijing's...

Short-term emission controls during major events provide quasi-experimental opportunities to evaluate urban air pollution responses to anthropogenic interventions. However, their effectiveness under declining emission baselines requires reevaluation. Here, we investigated aerosol chemical composition and volatile organic compounds (VOCs) during the 2025 Beijing Victory Day Parade, comparing findings directly with the 2015 Parade. Despite effective traffic emission reductions in 2025, a decrease of 41.6% in NO₂ and 14.8% in aromatic VOCs, Fine particulate matter (PM_2.5) unexpectedly increased by 20.9%. This contrasted sharply with 2015, when comparable controls reduced PM_2.5 from 55.7 to 27.0 μg m^-3. Machine learning attribution showed that nearly 90% of the PM_2.5 reduction in 2015 was driven by anthropogenic emission controls, whereas increases in PM_2.5, particularly sulfate and secondary organic aerosol (SOA) in 2025 were primarily associated with adverse meteorological conditions. Elevated temperature and humidity enhanced secondary formation, thereby offsetting the benefits of primary emission controls. VOCs analysis further showed that, despite the reduction in anthropogenic aromatics, biogenic and oxygenated VOCs, and temperature-sensitive emissions including volatile chemical products (VCPs)increased substantially during the 2025 control period. These changes shifted SOA formation from the photochemical aging of anthropogenic precursors in 2015 toward the aqueous-phase and heterogeneous processing of biogenic and other temperature-sensitive precursors in 2025. Our results reveal a decadal transition in Beijing's short-term PM_2.5 response from emission-dominated PM_2.5 mitigation to meteorology-modulated secondary aerosol formation under a lower-emission regime. Future event-based air-quality management should integrate conventional emission reductions with meteorology-informed VOC controls targeting temperature-sensitive sources and secondary aerosol formation.