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. Due to excessive anthropogenic emissions, heavy aerosol pollution episodes(HPEs) often occur during winter in the Beijing–Tianjin–Hebei (BTH) area ofthe North China Plain. Extensive observational studies have been carried outto understand the causes of HPEs; however, few measurements of verticalaerosol fluxes exist, despite them being the key to understanding verticalaerosol mixing, specifically during weak turbulence stages in HPEs. In thewinter of 2016 and the spring of 2017 aerosol vertical mass fluxes weremeasured by combining large aperture scintillometer (LAS) observations,surface PM 2.5 and PM 10 mass concentrations, and meteorologicalobservations, including temperature, relative humidity (RH), and visibility,at a rural site in Gucheng (GC), Hebei Province, and an urban site at theChinese Academy of Meteorological Sciences (CAMS) in Beijing located 100 kmto the northeast. These are based on the light propagation theory andsurface-layer similarity theory. The near-ground aerosol mass flux wasgenerally lower in winter than in spring and weaker in rural GC than inurban Beijing. This finding provides direct observational evidence for aweakened turbulence intensity and low vertical aerosol fluxes in winter andpolluted areas such as GC. The HPEs included a transport stage (TS), anaccumulative stage (AS), and a removal stage (RS). During the HPEs from 25to 31 January 2017, in Beijing, the mean mass flux decreasedby 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 decreasedfurther to 0.00087 mg m −2  s −1 , accounting for approximately one-third ofthe flux in the TSs. A similar reduction from the TSs to ASs was observed inthe HPE from 16 to 22 December 2016 in GC. It can be seen thatfrom the TS to the AS, the aerosol vertical turbulent flux decreased, butthe aerosol particle concentration within the surface layer increased, and it isinferred that in addition to the contribution of regional transport fromupwind areas during the TS, suppression of vertical turbulence mixingconfining aerosols to a shallow boundary layer increased accumulation.

atmospheric chemistry and physics

Aerosol vertical mass flux measurements during heavy aerosol pollution episodes at a rural site and an urban site in the Beijing area of the North China Plain