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

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.