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Abstract. A rapid development of a severe air pollution event in Beijing, China, at the end of November 2015 was investigated withunprecedented observations collected during the field campaign of the Study of Urban Rainfall and Fog/Haze(SURF-15). Different from previous statistical analyses of air pollution events and their correlations with meteorologicalenvironmental conditions in the area, the role of turbulent mixing in the pollutant transfer was investigated indetail. The analyses indicate that the major pollution source associated with high particulate matter of diameter2.5  µm ( PM2.5 ) was from south of Beijing. Before the day of the dramatic PM2.5 increase, thenighttime downslope flow from the mountains to the west and north of Beijing reduced the surface PM2.5 concentration northwest of Beijing. The nighttime surface stable boundary layer (SBL) not only kept the relativelyless-polluted air near the surface, it also shielded the rough surface from the pollutant transfer by southwesterly winds abovethe SBL, leading to the fast transport of pollutants over the Beijing area at night. As the daytime convective turbulentmixing developed in the morning, turbulent mixing transported the elevated polluted air downward even though the weaksurface wind was from northeast, leading to the dramatic increase of the surface PM2.5 concentration in theurban area. As a result of both turbulent mixing and advection processes with possible aerosol growth from secondaryaerosol formation under the low-wind and high-humidity conditions, the PM2.5 concentration reached over700  µg m−3 in the Beijing area by the end of the day. Contributions of the two transporting processes tothe PM2.5 oscillations prior to this dramatic event were also analyzed. The study demonstrates the importantrole of large-eddy convective turbulent mixing in vertical transfer of pollutants and the role of the SBL in not onlydecoupling vertical transport of trace gases and aerosols but also in accelerating horizontal transfer of pollutantsabove.

Observational analyses of dramatic developments of a severe air pollution event in the Beijing area