Summer Ozone Pollution Episodes in South China and Transport Impacts from Southeast Asia

Surface ozone (O 3 ) pollution is a long-standing air quality problem in South China. Despite its relatively low concentrations, summertime O 3 pollution has become more severe in the past decade, with increasingly frequent O 3 pollution events and elevated concentrations across South China. Interestingly, 39 regional high-O 3 episodes occurred under strong summer monsoon influences, which challenged the conventional view that clean air brought by the monsoon solely dilutes air pollution. The WRF-CMAQ simulations revealed that while local emissions primarily drove daytime O 3 formation in South China, emissions from mainland Southeast Asia and shipping substantially contributed to average hourly O 3 in South China by 4 ppbv. Unexpectedly, the Beibu Gulf acted as an O 3 reservoir, where O 3 and its precursors from upwind regions and ship emissions accumulated and reacted, transporting O 3 to South China through southerly winds. These findings redefine the dual role of the summer monsoon, demonstrating that it can both alleviate pollution and facilitate the transboundary transport of air pollutants. Our study underscores the increasing challenge that emissions in Southeast Asia and shipping activities pose to air quality in South China, calling for transboundary cooperation to manage O 3 pollution.