The role of NO x in Co-occurrence of O 3 and PM 2.5 pollution driven by wintertime east Asian monsoon in Hainan.

Clarifying the driving forces of O ₃ and fine particulate matter (PM _2.5 ) co-pollution is important to perform their synergistic control. This work investigated the co-pollution of O ₃ and PM _2.5 in Hainan Province using an observation-based model and explainable machine learning. The O ₃ and PM _2.5 pollution that occurs in winter is affected by the wintertime East Asian Monsoon. The O ₃ formation shifts from a NO _x -limited regime with a low O ₃ production rate (P _O3 ) in the non-pollution season to a transition regime with a high P _O3 in the pollution season due to an increase in NO _x concentrations. Increased O ₃ 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 ₃ production. Machine learning reveals that NO _x promotes O ₃ and PM _2.5 co-pollution during the pollution period. The P _O3 shows an upward trend at the observation site from 2018 to 2022 due to the inappropriate reduction of volatile organic compounds (VOCs) and NO _x in the upwind areas. Our results suggest that a deep reduction of NO _x should benefit both O ₃ and PM _2.5 pollution control in Hainan and bring new insights into improving air quality in other regions of China in the future.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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