Assessing the formation and evolution mechanisms of severe haze pollution in Beijing−Tianjin−Hebei region by using process analysis

Fine–particle pollution associated with haze threatens human health, especially in the North China Plain, where extremely high PM2.5 concentrations were frequently observed during winter. In this study, the WRF–Chem model coupled with an improved integrated process analysis scheme was used to investigate the formation and evolution mechanisms of a haze event happened over Beijing–Tianjin–Hebei (BTH) in December 2015, including examining the contributions of local emission and outside transport to the absolute PM2.5 concentration in BTH, and the contributions of each detailed physical or chemical process to the variations in the PM2.5 concentration. The influence mechanisms of aerosol radiative forcing (including aerosol direct and indirect effects) were also examined by using the process analysis. During the aerosol accumulation stage (December 20–22, Stage_1), the average near–surface PM2.5 concentration in BTH was 250.0 µg m−3, which was contributed by local emission of 42.3 % and outside transport of 36.6 %. During the aerosol dispersion stage (December 23–27, Stage_2), the average concentration of PM2.5 was 107.9 µg m−3. The contribution of local emission increased to 50.9 %, while the contribution of outside transport decreased to 24.3 %. The 24–h change (23:00LST minus 00:00LST) in the near–surface PM2.5 concentration was +50.4 µg m−3 during Stage_1 and −41.5 µg m−3 during Stage_2. Contributions of aerosol chemistry process and vertical mixing process to the 24–h change were +43.8 (+17.9) µg m−3 and −161.6 (−221.6) µg m−3 for Stage_1 (Stage_2), respectively. Small differences in contributions from other processes were found between Stage_1 and Stage_2, such as advection process, cloud chemistry process, and so on. Therefore, the PM2.5 increase over BTH during haze formation stage (Stage_1) was mainly attributed to strong production by aerosol chemistry process and weak removal by vertical mixing process. When aerosol radiative feedback was considered, the 24–h PM2.5 increase was enhanced by 9.6 µg m−3 during Stage_1, which could be mainly attributed to the contributions of vertical mixing process (+39.8 µg m−3), advection process (−38.6 µg m−3) and aerosol chemistry process (+5.1 µg m−3). The restrained vertical mixing could be the primary reason for the enhancement in near–surface PM2.5 increase when aerosol radiative forcing was considered.