ABSTRACT
To control the spread of the 2019 novel coronavirus(COVID-19), China imposed rigorous restrictions, which resulted in great reductions in pollutant emissions. However, two heavy haze pollution episodes still occurred in Beijing. In this study, we use the air pollutants, aerosol number concentration, and meteorological elements data in Beijing, combined with the HYSPLIT model, to calculate the potential source contribution factor(PSCF) and concentration weight trajectory(CWT), and analyze the characteristics of evolution and potential source apportionment of atmospheric pollutants during the two episodes. The COVID-19 lockdown restrictions had great impacts on the diurnal variations of PM2.5 and black carbon(BC), while small impacts on the diurnal variations of CO, NO2, SO2, and O3. The primary pollutant was PM2.5 during the two haze pollution episodes, and the haze1 episode was mainly local pollution, while haze 2 was mainly local and external transportation pollution. The spectrum of aerosol number concentration was unimodal under different processes, with the peak located at 0.3 µm. During the haze episodes, the number concentration in the size range of 0.2-0.5 µm increased 3.3-13.6 times that of the clean days. The mass concentration contributions of BCliquid to BC in different processes were 64.8%-85.1%. This mass concentration of BCliquid ranked in the order of haze 1(5.04 µg·m-3) >haze 1(3.20 µg·m-3) >clean day(before COVID-19) (2.31 µg·m-3) >clean day(COVID-19) (0.76 µg·m-3). The characteristics of PSCF and CWT distribution of PM2.5 and BC were different in different processes. The PSCF high value areas of PM2.5 on the clean day(before COVID-19) and the clean day(COVID-19) were mainly distributed in the southwest and western of Beijing, and the weight concentration exceeded 30 µg·m-3. The PSCF high value areas of PM2.5 during haze 1 and haze 2 were mainly distributed in Beijing and its surrounding areas and southwestern, when the weight concentration exceeded 90 µg·m-3. The PSCF high value areas of BC were mainly distributed in Beijing and its surrounding areas on clean days(before COVID-19), clean days(COVID-19) and haze 1, with weighted concentrations exceeding 2.4, 0.9 and, 4.5 µg·m-3, respectively. The PSCF high value areas of BC on haze 2 was distributed in the southwest of Beijing, and the weight concentration exceeded 5 µg·m-3.