Aerosol radiative effects and feedbacks on boundary layer meteorology and PM2.5 chemical components during winter haze events over the Beijing-Tianjin-Hebei region
Authors: Li Jiawei , Han Zhiwei , Wu Yunfei , Xiong Zhe , Xia Xiangao , Li Jie , Liang Lin , Zhang Renjian 
Published: July 2020

Abstract. An online coupled regional climate–chemistry–aerosol model (RIEMS-Chem) was
developed and utilized to investigate the mechanisms of haze formation and
evolution and aerosol radiative feedback during winter haze episodes in
February–March 2014 over the Beijing-Tianjin-Hebei (BTH) region in China.
Model comparison against a variety of observations demonstrated a good
ability of RIEMS-Chem in reproducing meteorological variables, planetary boundary layer (PBL) heights,
PM2.5, and its chemical components, as well as aerosol optical
properties. The model performances were remarkably improved for both
meteorology and chemistry by taking aerosol radiative feedback into account.
The domain-average aerosol radiative effects (AREs) were estimated to be −57 W m−2 at the surface, 25 W m−2 in the atmosphere, and −32 W m−2 at the top of atmosphere (TOA) during a severe haze
episode (20–26 February), with the maximum hourly surface ARE reaching −384 W m−2 in southern Hebei province. The average feedback-induced changes
in 2 m air temperature (T2), 10 m wind speed (WS10), 2 m relative humidity
(RH2), and PBL height over the BTH region during
the haze episode were −1.8 ∘C, −0.5 m s−1, 10.0 %, and −184 m, respectively. The BTH average changes in PM2.5 concentration due to
the feedback were estimated to be 20.0 µg m−3 (29 %) and 45.1 µg m−3 (39 %) for the entire period and the severe haze episode,
respectively, which demonstrated a significant impact of aerosol radiative
feedback on haze formation. The relative changes in secondary aerosols were
larger than those in primary aerosols due to enhanced chemical reactions by
aerosol feedback. The feedback-induced absolute change in PM2.5
concentrations was largest in the haze persistence stage, followed by those in the
growth stage and dissipating stage. Process analyses on haze events in
Beijing revealed that local emission, chemical reaction, and regional
transport mainly contributed to haze formation in the growth stage, whereas
vertical processes (diffusion, advection, and dry deposition) were major
processes for PM2.5 removals. Chemical processes and local emissions
dominated the increase in PM2.5 concentrations during the severe haze
episode, whereas horizontal advection contributed to the PM2.5 increase
with a similar magnitude to local emissions and chemical processes during a
moderate haze episode on 1–4 March. The contributions from physical and
chemical processes to the feedback-induced changes in PM2.5 and its
major components were explored and quantified through process analyses. For
the severe haze episode, the increase in the change rate of PM2.5 (9.5 µg m−3 h−1) induced by the feedback in the growth stage was
attributed to the larger contribution from chemical processes (7.3 µg m−3 h−1) than that from physical processes (2.2 µg m−3 h−1), whereas, during the moderate haze episode, the increase in the
PM2.5 change rate (2.4 µg m−3 h−1) in the growth stage
was contributed more significantly by physical processes (1.4 µg m−3 h−1) than by chemical processes (1.0 µg m−3 h−1). In general, the aerosol–radiation feedback increased the
accumulation rate of aerosols in the growth stage through weakening vertical
diffusion, promoting chemical reactions, and/or enhancing horizontal
advection. It enhanced the removal rate through increasing vertical
diffusion and vertical advection in the dissipation stage, and had little
effect on the change rate of PM2.5 in the persistence stage.