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 (O-3) pollution. In this study, the Weather Research and Forecasting model and Community Multiscale Air Quality model (CMAQ) were used to simulate atmospheric O-3 concentrations in Beijing. To investigate the formation mechanisms and source contributions of O-3 pollution in different regions of Beijing, process analysis and the integrated source apportionment method within the CMAQ were applied to O-3 concentrations in the summer of 2018. The process analysis results showed that vertical diffusion was the major contributor to O-3 concentrations at all receptor sites in Beijing, at > 65.94 mu g/(m(3)center dot hr). Gas-phase chemical reactions consumed a significant amount of O-3 in urban and inner suburban areas (> -5.57 mu g/(m(3)center dot hr)), while near-surface chemical reactions made positive contributions in outer suburban areas (> 4.72 mu g/(m(3)center dot hr)). The O-3 formation chemical reactions indicated that NO titration, which removes O-3 at night-time, mainly occurred in urban areas. The weaker chemical reactions occurring near the surface in outer suburbs suggested that suburban-area O-3 was produced in the upper atmospheric layers and was transported vertically to the lower layers. The O-3 source apportionment results showed that boundary contributions were the dominant contributor to O-3 pollution in Beijing (> 40%). The contribution of non-local emissions to O-3 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 O-3 formation in different areas of Beijing and informs the implementation of O-3 control plans. (c) 2022 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.