Complex interplay between transboundary ozone and domestic emissions shapes surface ozone pollution in China

Background ozone (O3), defined as O3 originating from transboundary transport and domestic natural precursors, has traditionally been viewed as largely unresponsive to domestic anthropogenic emissions, representing an uncontrollable baseline for a nation’s O3 pollution levels. However, this paradigm overlooks the chemical interactions between the cycled oxidants from transboundary O3 and domestic precursors. Here, we developed a novel expanded odd oxygen (Oy) tagged modeling framework to explicitly track the sources and full photochemical cycling of O3 and its radical reservoirs during a typical autumn O3 pollution episode in China. Our results demonstrated that interactions between transboundary O3 and domestic precursors accounted for 44% to 49% of surface O3 levels across Eastern China during the study period. Transboundary O3 played a dual photochemical role, simultaneously promoting O3 formation by serving as a major source of ROx radicals, while also suppressing the ozone-forming potential of domestic precursors through ROx removal and modulation of the OH turnover rate. Consequently, the interplay between background and domestic anthropogenic sources fundamentally shaped the ambient O3 formation regime. This work challenges the prevailing view of a chemically static background, redefining the “controllable” portion of O3 pollution and necessitating a reassessment of mitigation strategies from regional to intercontinental scales.