Modelling study of boundary-layer ozone over northern China - Part I: Ozone budget in summer

Regional photochemical pollution caused by ozone (O3) is serious in northern China during summer. In this study, we combined network observation data with the Fifth-Generation Pennsylvania State/National Centre for Atmospheric Research Mesoscale Model -Community Multiscale Air Quality (MM5-CMAQ) model system to simulate O3 and its precursors'concentrations over northern China in June 2008. Comparisons of the simulations and observations indicate that the model can accurately reproduce the temporal and spatial distributions of temperature, humidity, and wind as well as the evolution of O3 and its precursors over northern China. The monthly mean of the total oxidants (nitrogen dioxide+O3) at 15:00 LT exceeded 90ppbv across the North China Plain, thereby indicating significant photochemical pollution in this area. Vertical diffusion is the main source of the near-ground O3, with contributions of more than 20ppbvh−1 in the urban areas. Dry deposition and chemical reactions are the main sinks for O3, with contributions of more than 20ppbvh−1 and 7ppbvh−1 in the forest and urban areas, respectively. Although vertical diffusion is the main source of near-ground O3, photochemical reactions dominate the O3 concentrations in the boundary layer because of the circulation between the lower and upper boundary layers. Considering that O3 is mainly produced in the upper boundary layer, both nitrogen oxide and volatile organic compounds should be controlled on the North China Plain. The results presented here are intended to provide guidance for redefining strategies to control photochemical pollution over northern China.