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https://doi.org/10.5194/acp-2020-720
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-720
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  24 Aug 2020

24 Aug 2020

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This preprint is currently under review for the journal ACP.

A foehn-induced haze front in Beijing: observations and implications

Ju Li1, Zhaobin Sun1, Donald H. Lenschow2, Mingyu Zhou3, Youjun Dou1, Zhigang Cheng1, Yaoting Wang1, and Qingchun Li1 Ju Li et al.
  • 1Institute of Urban Meteorology, Beijing, China
  • 2National Center for Atmospheric Research, Boulder, CO, USA
  • 3National Marine Environment Forecast Center, Beijing, China

Abstract. Despite frequent foehns in the Beijing–Tianjin–Hebei (BTH) region, there are only a few studies of their effects on air pollution in this region, or elsewhere. Here, we discuss a foehn-induced haze front (HF) event using observational data to document its structure and evolution. Using a dense network of comprehensive measurements in the BTH region, our analyses indicate that the foehn played an important role in the formation of the HF with significant impacts on air pollution. Northerly warm–dry foehn winds, with low particulate concentration in the northern area, collided with a cold–wet polluted air mass to the south and formed an HF in the urban area. The HF, which is associated with a surface wind convergence line and distinct contrasts of temperatures, humidity and pollutant concentrations, resulted in an explosive growth of particulate concentration. As the plains-mountain wind circulation was overpowered by the foehn, a weak pressure gradient due to the different air densities between air masses was the main factor forcing advances of the polluted air mass into the clean air mass, resulting in severe air pollution over the main urban areas. Our results show that the foehn can affect air pollution through two effects: direct wind transport of air pollutants, and altering the air mass properties to inhibit boundary-layer growth and thus indirectly aggravating air pollution. This study highlights the need to further investigate the foehn and its impacts on air pollution in the BTH region.

Ju Li et al.

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Short summary
We analyzed a haze front event involving warm–dry downslope flow in December 2015 in Beijing, China. The haze front was formed by the collision between a clean-warm–dry air mass flowing from a nearby mountainous region and a polluted-cold–wet air mass over an urban area. We found that the polluted air advanced toward the clean air, resulting in a severe air pollution event. Our study highlights the need to further investigate the warm–dry downslope and its impacts on air pollution.
We analyzed a haze front event involving warm–dry downslope flow in December 2015 in Beijing,...
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