Characteristics of boundary layer ozone and its effect on surface ozone concentration in Shenzhen, China: A case study.

In late September 2019, the longest and most extensive ozone (O) pollution process occurred at Pearl River Delta. Base on the observational data, surface-level O, vertical distribution characteristics boundary layer O as well as its effect on surface-level O are thoroughly analyzed. The O lidar results showed similar vertical O profiles both in pollution episodes and clean periods, from which a high O concentration layer between 300 and 500 m and a sub-high O concentration layer between 1300 and 1700 m (near the top of the mixing layer) can be found. Besides, the downward O transport paths from the high/sub-high O concentration layers could be observed along with the boundary layer evolution: At nighttime, large amounts of O were effectively stored into the residual layer (RL). Due to the upward development of Mixing layer (ML) in early morning, atmospheric vertical mixing carried the O inside the RL down to the surface, which led to a rapid increase in the surface-level O. The sub-high O layer began the downward mixing at noon, and became well-mixed after the boundary layer was fully developed in the afternoon, by which the near surface O pollution deteriorated again. Further analysis of the heavy O pollution episodes show that, the high O concentration inside the RL contributed 54% ± 6% of the surface-level O at 9:00 LT and the average contribution of O in the sub-high concentration layer to the surface-level O at 14:00 LT was 26% ± 9%. Based on the quantitative analysis of the observational data, this paper focus to reveal the importance of the contribution of O inside the RL and near the top of the ML to the surface O.