COVID-19疫情期间北京市两次重霾污染过程大气污染物演变特征及潜在源区分析

Evolution and Potential Source Apportionment of Atmospheric Pollutants of Two Heavy Haze Episodes During the COVID-19 Lockdown in Beijing,China

新冠肺炎疫情(COVID-19)期间,执行了严格居家隔离措施,人为排放源急剧降低,但北京仍出现了两次持续重霾污染过程.本研究使用北京市大气污染物、气溶胶数浓度和气象要素数据,结合气团轨迹模式(HYSPLIT),计算了潜在源贡献因子(PSCF)和浓度权重轨迹(CWT),分析了两次重霾污染过程中大气污染物的演变特征及其潜在源区贡献.结果表明,COVID-19期间居家隔离措施对PM_(2.5)和黑碳(BC)的日变化特征影响较大,对CO、NO_(2)、SO_(2)和O_(3)的日变化影响较小.两次重霾污染过程首要污染物均是PM_(2.5),污染过程1主要是以局地污染为主,污染过程2以局地污染和外来输送为主.不同过程下气溶胶数浓度谱分布均为单峰型分布,峰值位于0.3μm,在污染过程中主要是0.2~0.5μm粒径气溶胶数浓度增加,是干净日的3.3~13.6倍.不同过程中BC_(liquid)对BC的贡献为64.8%~85.1%.BC_(liquid)的浓度为:污染过程2(5.04μg·m^(-3))>污染过程1(3.20μg·m^(-3))>干净日(疫情前,2.31μg·m^(-3))>干净日(疫情,0.76μg·m^(-3)).不同过程中PM_(2.5)和BC的PSCF和CWT分布特征不同.PM_(2.5)的PSCF高值区在干净日(疫情前)和干净日(疫情)主要分布在北京的西南方和西部,权重浓度超过30μg·m^(-3);在污染过程1和污染过程2主要分布在北京及其周边地区和西南部,权重浓度均超过90μg·m^(-3).BC的PSCF高值区在干净日(疫情前)、干净日(疫情)和污染过程1主要分布在北京及其周边地区,权重浓度分别超过2.4、0.9和4.5μg·m^(-3);在污染过程2中分布在北京西南部,权重浓度超过5μg·m^(-3).

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 PM_(2.5)and black carbon(BC),while small impacts on the diurnal variations of CO,NO_(2),SO_(2),and O_(3).The primary pollutant was PM_(2.5)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 0.3μm.During the haze episodes,the number concentration in the size range of 0.2-0.5μm increased 3.3-13.6 times that of the clean days.The mass concentration contributions of BC_(liquid) to BC in different processes were 64.8%-85.1%.This mass concentration of BC_(liquid) ranked in the order of haze 1(5.04μg·m^(-3))>haze 1(3.20μg·m^(-3))>clean day(before COVID-19)(2.31μg·m^(-3))>clean day(COVID-19)(0.76μg·m^(-3)).The characteristics of PSCF and CWT distribution of PM_(2.5)and BC were different in different processes.The PSCF high value areas of PM_(2.5)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 30μg·m^(-3).The PSCF high value areas of PM_(2.5)during haze 1 and haze 2 were mainly distributed in Beijing and its surrounding areas and southwestern,when the weight concentration exceeded 90μg·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,4.5μg·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 5μg·m^(-3).