Transport and boundary layer interaction contribution to extremely high surface ozone levels in eastern China.

Vertical measurements of ozone (O) within the 3000-m lower troposphere were obtained using an O lidar to investigate the contribution of the interactions between the transport and boundary layer processes to the surface O levels in urban Shanghai, China during July 23-28, 2017. An extremely severe pollution episode with a maximum hourly O 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-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 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 in urban Shanghai during the pollution episode (July 23-26, 2017). As for the advection transport, large amounts of O 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 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 pollution events over the whole YRD region. Our results indicate that effective regional emission control measures are urgently required to mitigate O pollution in the YRD region.