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In spring, ozone (O 3) pollution frequently occurrs in eastern China, but key drivers remain uncertain. In this study, interannual variations in springtime ozone in Shanghai, China, from 2013 to 2021, were investigated to assess the health impacts and the effectiveness of recent air pollution control measures. A combination of ground-level measurements of regulated air pollutants, lidar observations, and backward trajectories of air masses was used to identify the key drivers for enhancing springtime O 3. The results show that external imports of O 3 driven by atmospheric circulation are notable sources of springtime surface O 3. For example, the downward transport from the free troposphere could contribute to over 50% of surface O 3 in the morning. The surface O 3 mixing ratios in spring exhibited an upward trend of 0.93 ppb yr−1 (p < 0.05) from 2013 to 2021. The change in meteorological variables, particularly the increase in air temperature, could explain nearly 87% of the springtime O 3 upward trend. The change in anthropogenic emissions of precursors only contributed to a small fraction (<13%) of the increase in springtime O 3. The cumulative exposure of urban residents to O 3 in spring also exhibited a significant upward trend (111 ppb yr−1, p < 0.05). With the rapid increase in surface O 3 , premature respiratory mortality attributable to O 3 exposure has fluctuated at approximately 2933 deaths per year since 2016, even though the total deaths from respiratory diseases have significantly declined. Long-term exposure to high O 3 concentrations is a significant contributor to premature respiratory mortality. [Display omitted] • The springtime O 3 in Shanghai significantly increased from 2013 to 2019. • The springtime O 3 uptrend is mainly caused by an increase in air temperature. • Transport driven by atmospheric circulation is a notable source of surface O 3. • Nearly 18% of the annual respiratory mortality is attributed to O 3 exposure. [ABSTRACT FROM AUTHOR]

Vertical measurements of ozone (O 3) within the 3000-m lower troposphere were obtained using an O 3 lidar to investigate the contribution of the interactions between the transport and boundary layer processes to the surface O 3 levels in urban Shanghai, China during July 23–28, 2017. An extremely severe pollution episode with a maximum hourly O 3 mixing ratio of 160.4 ppb was observed. In addition to enhanced local photochemical production, both downward and advection transport in the lower troposphere may have played important roles in forming the pollution episode. The O 3 -rich air masses in the lower free troposphere primarily originated from central China and the northern Yangtze River Delta (YRD) region. The downward transport of O 3 from the lower free troposphere may have an average contribution of up to 49.1% to the daytime (09:00–16:00 local time) surface O 3 in urban Shanghai during the pollution episode (July 23–26, 2017). As for the advection transport, large amounts of O 3 were transported outward from Shanghai in the planetary boundary layer under the influence of southeasterly winds during the field study. In this condition, the boundary-layer O 3 that was transported downward from the free troposphere in Shanghai could be transported back to the northern YRD region and accumulated therein, leading to the occurrence of severe O 3 pollution events over the whole YRD region. Our results indicate that effective regional emission control measures are urgently required to mitigate O 3 pollution in the YRD region. Image 1 • The surface O 3 episode in Shanghai was closely associated with downward transport. • Contributions of downward transport to surface O 3 varied between 12.6% and 78.3%. • O 3 was transported to Shanghai from the northern YRD in the lower free troposphere. • O 3 was transported to the northern YRD from Shanghai in the boundary layer. [ABSTRACT FROM AUTHOR]