Modulations of synoptic and climatic changes on ozone pollution and its health risks in mountain-basin areas

Abstract Ozone (O3) pollution in China tends to become increasingly severe despite recent emission reductions. O3 is sensitive to atmospheric conditions, but its modulations by changing synoptic systems and climate are unclear, especially in mountain-basin areas such as the Sichuan Basin (SCB). This study examines the impacts of typical synoptic systems and their secular changes on O3 pollution and its health risks in mountain-basin areas, by taking SCB as an example. Seven dominant synoptic patterns are identified and three typical synoptic patterns with high- (low-) pressure are associated with high (low) O3 concentrations over entire SCB, while the pattern with uniform-pressure is accompanied by heavy (light) pollution in western (eastern) SCB. Under the synoptic patterns with high-pressure, clear weather with fewer clouds, strong solar radiation, weak convection, and hot and dry atmosphere enhances the photochemical reactions of O3 precursors and weakens the mixing of O3 and its precursors, thus favoring heavy O3 pollution and posing higher health risks; whereas, the synoptic patterns with low-pressure induce the opposite changes in O3 concentration and health risks. The uniform-pressure pattern is accompanied by district spatial variations of O3 via favoring O3-related physical and chemical processes in eastern SCB and inhibiting that in the west. Under climatic changes, increasing synoptic pattern with high-pressure and decreasing synoptic pattern with low-pressure over SCB significantly increased O3 concentration and the resultant health risks over the past 40 years. Our findings provide scientific evidence from synoptic and climatic views for forecasting O3 and its health risks and for mitigating O3 pollution in mountain-basin areas.