Abstract. 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 (16–22 December, Stage 1), the near-surface PM2.5 concentration in the BTH region increased from 24.2 to 289.8 µg 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 (23–27 December, Stage 2), the average
concentration of PM2.5 was 107.9 µg 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 µg m−3 during Stage 1
and −41.5 µg m−3 during Stage 2. The contributions of
aerosol 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 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 µg m−3 during
Stage 1, which could be mainly attributed to the
contributions 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 primary
reason for the enhancement in the near-surface PM2.5 increase when aerosol
radiative forcing was considered.