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

Fine-particle pollution associated with haze threatens human health, especially in the North China Plain region, where extremely high PM2.5 concentrations are frequently observed during winter. In this study, the Weather Research and Forecasting with Chemistry (WRF-Chem) model coupled with an improved integrated process analysis scheme was used to investigate the formation and evolution mechanisms of a haze event over the Beijing-Tianjin-Hebei (BTH) region in December 2015; this included an examination of the contributions of local emissions and regional transport to the PM2.5 concentration in the BTH area, and the contributions of each detailed physical or chemical process to the variations in the PM2.5 concentration. The mechanisms influencing aerosol radiative forcing (including aerosol direct and indirect effects) were also examined by using process analysis. During the aerosol accumulation stage (1622 December, Stage 1), the near-surface PM2.5 concentration in the BTH region increased from 24.2 to 289.8 mu g m(-3), with the contributions of regional transport increasing from 12 % to 40 %, while the contribution of local emissions decreased from 59 % to 38 %. During the aerosol dispersion stage stage (23-27 December, Stage 2), the average concentration of PM2.5 was 107.9 mu g m(-3), which was contributed by local emissions (51 %) and regional transport (24 %). The 24 h change (23:00 minus 00:00 LST) in the near-surface PM2.5 concentration was +43.9 mu g m(-3) during Stage 1 and - 41.5 mu g m(-3) during Stage 2. The contributions of aerosol chemistry, advection, and vertical mixing to the 24 h change were +29.6 (+17.9)mu g m(-3), -71.8 (-103.6) mu g m(-3), and - 177.3 (-221.6) mu g m(-3) during Stage 1 (Stage 2), respectively. Small differences in the contributions of other processes were found between Stage 1 and Stage 2. Therefore, the PM2.5 increase over the BTH region during the haze formation stage was mainly attributed to strong production by the aerosol chemistry process and weak removal by the advection and vertical mixing processes. When aerosol radiative feedback was considered, the 24 h PM2.5 increase was enhanced by 4.8 mu g m(-3) during Stage 1, which could be mainly attributed to the contributions of the vertical mixing process (+22.5 mu g m(-3)), the advection process (-19.6 mu g m(-3)), and the aerosol chemistry process (+1.2 mu g m(-3)). The restrained vertical mixing was the primary reason for the enhancement in the near-surface PM2.5 increase when aerosol radiative forcing was considered.