The ‘two-way feedback mechanism’ between unfavorable meteorological conditions and cumulative PM2.5 mass existing in polluted areas south of Beijing

Abstract

In winter, heavy aerosol pollution episodes (HPEs) occur frequently on the North China Plain (NCP) under unfavorable meteorological conditions. After the aerosol pollution with fine aerosol particles less than 2.5 μm (PM2.5) accumulates to a certain extent in Beijing and its vicinity (BIV), boundary-layer (BL) meteorological conditions worsen due to radiative cooling effects of aerosols, which further facilitate PM2.5 accumulation. This "two-way feedback mechanism" between unfavorable metrological conditions and cumulative PM2.5, particularly PM2.5 explosive growth with mass concentration doubled in several to 10 h, has been found in BIV. To investigate whether the two-way feedback exists on the southern NCP, we selected four representative cities with radiosonde observations, which lie south of Beijing and used PM2.5 data, radiation observations, and radiosonde data from December 2016 to the beginning of January 2017. We found that the two-way feedback existed in these four cities. In each city, HPEs included transport stages (TSs), during which relative strong winds transport pollutants into different regions, and cumulative stages (CSs), in which temperature inversions occur and became striking due to the radiative cooling effects of elevated aerosols under slight or calm winds. This would further cause an increase of PM2.5 concentration. During the CSs, growth of PM2.5 concentration was governed by the two-way feedback, which explained over 70% of the increase. During HPEs, warm and humid advection, topographic wind, longwave radiation at night were conducive to the inversion at the beginning of HPEs, but not dominant with respect to anomalous inversion in CSs. In addition, with the presence of supersaturated layers, aerosol would enter the cloud-fog system, which would reduce observed PM2.5 mass concentration to a certain degree.