Contrasting impacts of policy interventions and population mobility on organic aerosols and oxygenated volatile organic compounds in Beijing

Organic aerosol (OA) is a dominant component of fine particulate matter in megacities, yet disentangling the impacts of emission changes from meteorological variability remains a major challenge for air quality management. This study investigates the responses of OA and its gaseous precursors to short-term emission variations in Beijing across six major events from 2014 to 2022, categorized into three policy interventions (the 2014 APEC Summit, the 2015 Victory Day Parade, and the 2022 Winter Olympic Games) and three population mobility events (Chinese New Year holidays in 2019, 2020, and 2021). Using machine-learning-based meteorological normalization, we found that policy interventions involving coordinated local and regional emission controls effectively reduced secondary OA (SOA) by 46–64%, with emission reductions contributing 63–74% of this decrease. In contrast, local-only emission reductions during population mobility events resulted in negligible OA decreases or even increases, as meteorological conditions played a dominant role in driving pollutant accumulation. High-resolution measurements during the 2022 Winter Olympic Games (WOG) revealed that precursor emission controls not only reduced SOA mass but also significantly suppressed aging processes. Specifically, aging-induced SOA enhancement decreased from 0.5 to 0.6 to 0.1–0.3 μg m� 3 during the WOG period, while the characteristic aging time was reduced from 2.6 to 5.5 to 1.5–3.9 days. Our findings demonstrate that regional coordinated controls are essential for effectively mitigating secondary pollution, whereas the benefits of shortterm local emission changes are frequently offset by unfavorable meteorology.