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

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 vertical 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.