Sources Of Aerosols And Aging Mechanisms Of Individual Particles Under Various Atmospheric Polluted Conditions

To date,air pollution is one of the serious environmental problems in China.Although the emission of primary pollutants in China has declined in recent years under a series of emission reduction policies promulgated by the government,air pollution still frequently occurred in China due to adverse meteorological conditions and the transportation of pollutants.At present,the study on air pollution is mainly focused on some special events,while the research on the source and aging mechanism of individual aerosol particles under various atmospheric polluted conditions is still lacking.This study analyzed the physicochemical characteristics of winter haze aerosols in Northeast China and the North China Plain(NCP),the long-range transport particles in urban and background air,and aerosols in a typical megacity(i.e.,Hangzhou)in the Yangtze River Delta(YRD)during haze pollution and during the novel coronavirus disease(COVID-19)pandemic period.Further,the main source and the aging mechanism of aerosols under these atmospheric conditions were explored and compared.During regional haze events in Northeast China and the NCP,organic matter(OM)and secondary inorganic ions are the main components of PM_2.5.81%-96%and 74%-80%of haze aerosols in Northeast China and in the NCP contain tarballs.These large amounts of tarballs are mainly emitted from the residential coal burning in rural areas of Northeast China and the NCP.However,during the long-range transport haze event in Northeast China,OM becomes the dominant component of PM_2.5(53%-60%by mass),and K⁺concentration increases by two times.In addition,the number fraction of K-rich particles mixed with tarballs increases from 4%-5%to 50%-52%.These K-rich particles mixed with tarballs are mainly sourced from long-range transport of agricultural biomass burning.Therefore,compared with the NCP,the contribution of coal and biomass burning to winter haze formation in Northeast China is larger.Transmission electron microscopy(TEM)analysis further shows that heterogeneous reactions are the main aging pathway of haze aerosols in Northeast China and the NCP during winter.In the urban air in winter,the long-range transported aerosols mainly are tarball-containing particles(71%-96%by number),while in the background air in summer,soot-containing particles(47%)are the main ones.The tarballs in the urban air are mainly emitted from coal and biomass burning,while the soot in the background air is mainly from vehicle emissions.The tarball-containing particles undergo complex aging processes during the long-range transport.Heterogeneous reactions may promote the formation of secondary aerosols on the tarballs and further lead to the transformation from partly coated tarballs to totally coated tarballs.The change of the mixing state of tarballs can affect their optical properties and activities as cloud condensation nuclei.TEM and cryogenic TEM analyses show that the soot experiences redistribution during the long-range transport and aging.The phase separation of inorganic salts and organic coatings in individual soot-containing particles can redistribute soot from the inorganic salt core into the organic coating,thus changing their optical properties and climate effects.The soot redistribution depends on the ratio of organic coating thickness to soot size(T_OC/D_S).By comparing soot particles in the urban and background air,we find that soot particles in the urban air are mainly distributed in inorganic salts(88%by number),while soot particles in the background air are mainly embedded in organic coatings(64%).The difference between the soot mixing position in the urban and background areas is mainly ascribed to the discrepancy in the T_OC/D_S ratio.Hangzhou is a typical megacity in the YRD.NO₃^-is the main aerosol component in Hangzhou during the haze period.The higher NO₃^-/SO₄^2-ratio(>2)during the haze period indicates that the contribution of vehicular sources(e.g.,vehicle exhausts)to PM_2.5 is dominant.During the COVID-19 pandemic period,vehicular source emissions significantly decrease in Hangzhou,with a large reduction of 56%-68%in NO₃^-.However,SO₄^2-remains stable and becomes the dominant component of PM_2.5 during the COVID-19 pandemic period,thus the NO₃^-/SO₄^2-ratio decreases and is lower than1.This result indicates that stationary sources are dominant during the COVID-19pandemic period.Therefore,we suggest that NO_x from vehicle emissions should be further reduced to improve air quality of the YRD in the future.WRF-Chem model results show that the increase of HO_x radicals in daytime enhances the gas-phase reactions of SO₂ and NO_x during the COVID-19 pandemic period in Hangzhou,while the increase of O₃ and NO₃ radicals in nighttime promotes the multiphase chemical reactions.Furthermore,TEM analysis shows that heterogeneous reactions may be the main formation mechanism of secondary aerosols during the haze period in Hangzhou.This study explores the differences of aerosol sources and aging mechanisms under various atmospheric conditions.This result can provide scientific bases for controlling different types of air pollution and evaluating the potential climate and human health effects of aerosol particles.