Model analysis of vertical exchange of boundary layer ozone and its impact on surface air quality over the North China Plain

In addition to photochemical production and horizontal regional transport, surface O₃ concentration can also likely be affected by vertical transport, which is not well known so far. The process analysis was conducted by using the Regional Atmospheric Modeling System Community Multiscale Air Quality (RAMS-CMAQ) model to investigate photochemical production and the vertical transport mechanism of boundary-layer O₃ during a typical O₃ pollution episode in the North China Plain (NCP), and further quantify the contribution of vertical transport to surface O₃. The diurnal variations of vertical budgets of O₃ and NO₂ in the boundary layer at multiple sites showed that there were substantial differences in the vertical distribution of O₃ production and transport between urban and suburban/rural areas. In urban areas, surface O₃ is consumed by titration reaction to generate NO₂, which is then transported to the upper boundary layer and produces O₃ by photochemical reaction. With the development of the boundary layer, the upper-layer O₃ stored in the residual layer at nighttime can be transported vertically to the surface as the turbulent diffusion intensifies the next morning. While in suburban and rural areas, the vertical transport is relatively weaker because the photochemical formation of O₃ occurs in the whole boundary layer, although it decreases slightly with the altitude. Model simulation showed that 20.6-27.9% of urban surface O₃ changes in the morning (09:00-10:00 LST) was attributable to the downward transport from the residual layer, while it is 15.0-22.1% at suburban site. The vertical transport from above the boundary layer contributed 24.0-63.6% to daytime urban surface O₃ changes, which was weak in suburban areas. Differences and similarities in O₃ formation and transport mechanism in urban and suburban regions revealed here highlight the importance of earlier control and regional collaboration.