The role of NO in Co-occurrence of O and PM pollution driven by wintertime east Asian monsoon in Hainan.

Clarifying the driving forces of O and fine particulate matter (PM) co-pollution is important to perform their synergistic control. This work investigated the co-pollution of O and PM in Hainan Province using an observation-based model and explainable machine learning. The O and PM pollution that occurs in winter is affected by the wintertime East Asian Monsoon. The O formation shifts from a NO-limited regime with a low O production rate (P) in the non-pollution season to a transition regime with a high P in the pollution season due to an increase in NO concentrations. Increased O and atmospheric oxidation capacity promote the conversion from gas-phase precursors to aerosols. Meanwhile, the high concentration of particulate nitrate favors HONO formation via photolysis, in turn facilitating O production. Machine learning reveals that NO promotes O and PM co-pollution during the pollution period. The P shows an upward trend at the observation site from 2018 to 2022 due to the inappropriate reduction of volatile organic compounds (VOCs) and NO in the upwind areas. Our results suggest that a deep reduction of NO should benefit both O and PM pollution control in Hainan and bring new insights into improving air quality in other regions of China in the future.