Abstract. Due to excessive anthropogenic emissions, heavy aerosol pollution episodes
(HPEs) often occur during winter in the Beijing–Tianjin–Hebei (BTH) area of
the North China Plain. Extensive observational studies have been carried out
to understand the causes of HPEs; however, few measurements of vertical
aerosol fluxes exist, despite them being the key to understanding vertical
aerosol mixing, specifically during weak turbulence stages in HPEs. In the
winter of 2016 and the spring of 2017 aerosol vertical mass fluxes were
measured by combining large aperture scintillometer (LAS) observations,
surface PM2.5 and PM10 mass concentrations, and meteorological
observations, including temperature, relative humidity (RH), and visibility,
at a rural site in Gucheng (GC), Hebei Province, and an urban site at the
Chinese Academy of Meteorological Sciences (CAMS) in Beijing located 100 km
to the northeast. These are based on the light propagation theory and
surface-layer similarity theory. The near-ground aerosol mass flux was
generally lower in winter than in spring and weaker in rural GC than in
urban Beijing. This finding provides direct observational evidence for a
weakened turbulence intensity and low vertical aerosol fluxes in winter and
polluted areas such as GC. The HPEs included a transport stage (TS), an
accumulative stage (AS), and a removal stage (RS). During the HPEs from 25
to 31 January 2017, in Beijing, the mean mass flux decreased
by 51 % from 0.0049 mg m−2 s−1 in RSs to 0.0024 mg m−2 s−1 in the TSs. During the ASs, the mean mass flux decreased
further to 0.00087 mg m−2 s−1, accounting for approximately one-third of
the flux in the TSs. A similar reduction from the TSs to ASs was observed in
the HPE from 16 to 22 December 2016 in GC. It can be seen that
from the TS to the AS, the aerosol vertical turbulent flux decreased, but
the aerosol particle concentration within the surface layer increased, and it is
inferred that in addition to the contribution of regional transport from
upwind areas during the TS, suppression of vertical turbulence mixing
confining aerosols to a shallow boundary layer increased accumulation.
Measuring the Vertical Profiles of Aerosol Extinction in the Lower Troposphere by MAX-DOAS at a Rural Site in the North China Plain
