Formation processes and source contributions of ground-level ozone in urban and suburban Beijing using the WRF-CMAQ...

Beijing faces the challenge of high levels of ozone (O₃) pollution. In this study, the Weather Research and Forecasting model and Community Multiscale Air Quality model (CMAQ) were used to simulate atmospheric O₃ concentrations in Beijing. To investigate the formation mechanisms and source contributions of O₃ pollution in different regions of Beijing, process analysis and the integrated source apportionment method within the CMAQ were applied to O₃ concentrations in the summer of 2018. The process analysis results showed that vertical diffusion was the major contributor to O₃ concentrations at all receptor sites in Beijing, at > 65.94 µg/(m³·hr). Gas-phase chemical reactions consumed a significant amount of O₃ in urban and inner suburban areas (> -5.57 µg/(m³·hr)), while near-surface chemical reactions made positive contributions in outer suburban areas (> 4.72 µg/(m³·hr)). The O₃ formation chemical reactions indicated that NO titration, which removes O₃ at night-time, mainly occurred in urban areas. The weaker chemical reactions occurring near the surface in outer suburbs suggested that suburban-area O₃ was produced in the upper atmospheric layers and was transported vertically to the lower layers. The O₃ source apportionment results showed that boundary contributions were the dominant contributor to O₃ pollution in Beijing (> 40%). The contribution of non-local emissions to O₃ levels was significantly greater in the outer suburbs than in urban and inner suburban areas due to topography. This study increases the understanding of the complex processes of O₃ formation in different areas of Beijing and informs the implementation of O₃ control plans.