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

Ozone (O-3) pollution in China tends to become increasingly severe despite recent emission reductions. O(3 )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 O-3 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) O-3 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 O-3 precursors and weakens the vertical mixing of O-3 and its precursors, thus favoring heavy O-3 pollution and posing higher health risks; whereas, the synoptic patterns with low-pressure induce the opposite changes in O-3 concentration and health risks. The uniform-pressure pattern is accompanied by district spatial variations of O-3 via favoring O-3-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 O-3 concentration and the resultant health risks over the past 40 years. Our findings provide scientific evidence from synoptic and climatic views for forecasting O-3 and its health risks and for mitigating O-3 pollution in mountain-basin areas.