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. Fine-particle pollution associated with haze threatens humanhealth, especially in the North China Plain region, where extremely high PM 2.5 concentrations are frequently observed during winter. In this study, theWeatherResearch and Forecasting with Chemistry (WRF-Chem) model coupled with an improved integrated process analysis schemewas used to investigate the formation and evolution mechanisms of a hazeevent over the Beijing–Tianjin–Hebei (BTH) region in December 2015; this included an examination ofthe contributions of local emissions and regional transport to the PM 2.5 concentration in the BTH area, and the contributions of each detailed physical orchemical process to the variations in the PM 2.5 concentration. Themechanisms influencing aerosol radiative forcing (including aerosol directand indirect effects) were also examined by using process analysis.During the aerosol accumulation stage (16–22 December, Stage 1), the near-surface PM 2.5 concentration in the BTH region increased from 24.2 to 289.8  µg m−3 , with the contributions ofregional transport increasing from 12 % to 40 %, while the contributionof local emissions decreased from 59 % to 38 %. During the aerosoldispersion stage (23–27 December, Stage 2), the averageconcentration of PM 2.5 was 107.9  µg m−3 , which wascontributed by local emissions (51 %) and regional transport (24 %).The 24 h change (23:00 minus 00:00 LST) in the near-surface PM 2.5 concentration was +43.9 µg m−3 during Stage 1and −41.5 µg m−3 during Stage 2. The contributions ofaerosol chemistry, advection, and vertical mixing to the 24 h change were +29.6 ( +17.9 )  µg m−3 , −71.8 ( −103.6 )  µg m−3 , and −177.3 ( −221.6 )  µg m−3 during Stage 1(Stage 2), respectively. Small differences in the contributionsof other processes were found between Stage 1 andStage 2. Therefore, the PM 2.5 increase over the BTH region during thehaze formation stage was mainly attributed to strong production by theaerosol chemistry process and weak removal by the advection and vertical mixingprocesses. When aerosol radiative feedback was considered, the 24 hPM 2.5 increase was enhanced by 4.8  µg m−3 duringStage 1, which could be mainly attributed to thecontributions of the vertical mixing process ( +22.5 µg m−3 ),the advection process ( −19.6 µg m−3 ), and the aerosol chemistry process( +1.2 µg m−3 ). The restrained vertical mixing was the primaryreason for the enhancement in the near-surface PM 2.5 increase when aerosolradiative forcing was considered.

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