The formation mechanism of air pollution episodes in Beijing city: Insights into the measured feedback between aerosol radiative forcing and the atmospheric boundary layer stability.

Abstract

Based on the high-resolution observation of meteorological factors profiles, particulate matter concentration and aerosol radiative forcing (ARF) from 25 August to 17 November 2018 in Beijing, the feedback between ARF and the atmospheric boundary layer (ABL) stability was systematically investigated during air pollution episodes. There was the initial explosive growth in particulate matter (PM) concentration that PM2.5 sharply increased from ~8μgm-3 to ~100μgm-3, with aerosol optical depth (AOD) increasing from ~0.25 to ~0.58. This was the transport phase dominated by the southerly winds. As PM increased, the high aerosol loading scattered more solar radiation cooling the earth-atmosphere system (ARF at the top of the atmospheric column (TOA): from ~5Wm-2 to ~-52Wm-2). Meanwhile, high aerosol loading absorbed more solar radiation and heated the atmospheric layer with ARF at the interior of the atmospheric column (ATM) increasing from ~21Wm-2 to ~42Wm-2. The absorption and scattering effects of aerosol together cooled the surface (ARF at the surface of the atmospheric column (SFC): from ~-16Wm-2 to ~-90Wm-2). Thus, the ABL stability rapidly increased in the following cumulative phase and heavy pollution phase with a strong temperature inversion (inversion depth of ~300-1000m) occurring. In turn, the persistent temperature inversion caused the significant accumulation of moisture (water vapor density of ~5-10gm-3) and pollutants, and PM were prone to physicochemical reactions in the high-humidity environment, further increasing PM. It was the constant feedback effect between ARF and the ABL stability that continually reduced atmospheric environmental capacity and aggravated air pollution (PM2.5 and AOD reaching ~95-125μgm-3 and ~1.38-1.75, respectively). Finally, the feedback was broken by dry, clean and strong north winds appearing in Beijing in the dissipation phase.