| Environmental persistent free radicals(EPFRs)are a class of environmentally hazardous substances that have attracted much attention in recent years.It exists widely in the atmospheric environment and may increase the risk of various respiratory diseases.Therefore,it is of great significance to study the pollution characteristics and generation mechanism of EPFRs.In this study,a rapid atmospheric EPFRs detection method using a quartz tissue cell was developed.Compared with the traditional quartz tube detection method and solvent extraction method,this method has the advantages of repeatable sample detection,fast detection speed,and small detection error.The detection method was used to systematically study the EPFRs concentration levels,species characteristics,and sources of EPFRs in PM2.5 in Xi'an in 2017.The results showed that the EPFR concentration in PM2.5 in Xi'an in 2017 was 9.8×1011-6.9×1014 spins/m3.The highest concentration of EPFRs occurred in winter with an average concentration of 2.1×1014 spins/m3.The lowest concentration of EPFRs occurred in autumn,with an average concentration of 7.0×1013 spins/m3.According to the annual average atmospheric concentration of EPFRs,the amount of EPFRs inhaled by people in Xi'an is equivalent to approximately 5 cigarettes per person per day and approximately 23 cigarettes per person per day in winter when haze occurs.The results of the study on the EPFRs characteristics show that the EPFRs in PM2.5 in Xi'an are mainly C-center organic radicals that are primarily non-decaying types,accounting for approximately 75%and 85%of total concentration of EPFRs in autumn and winter,respectively.In addition,taking Linfen as an example,the particle size distribution characteristics of EPFRs were also studied.It was found that there was a significant difference between EPFRs in fine particles(<2.1 ?m)and EPFRs in coarse particles(2.1-10 ?m).The EPFRs concentration in coarse particles is higher than that in fine particles in summer,but the opposite in winter.The source of EPFRs was explored by correlation analysis.Significant positive correlations were found between EPFRs and SO2,NO2 and the thermally derived OC3 and OC4 carbonaceous components.The results suggested that coal-fired and traffic may be important sources of EPFRs in PM2.5 in Xi'an.In addition,EPFRs are significantly positively correlated with O3 in summer,suggesting that some EPFRs may also originate from secondary processes.PM2.5 generated from the combustion of various biomass fuels and coal was collected,and the types and decay characteristics of EPFRs in these samples were also studied.The results show that the decay characteristics and g-factor of EPFRs in PM2.5 produced by fuel combustion are very similar to actual atmospheric samples,which indicates that the combustion source has an important contribution to atmospheric EPFRs.The physical and chemical properties and formation mechanism of EPFRs in PM2.5 in Xi'an were explored.In this study,solvent extraction was used to fractionate aerosol components with different polarities and solvent resistance in PM2.5.Most of the total EPFRs in PM2.5 were derived from solvent-resistant organic matter(88%),which likely consisted of graphene oxide(GO)analogues.This study showed that atmospheric EPFRs are not mainly formed from metal oxides but rather from nonextractable organics and suggested that previous studies may have missed the major part of EPFRs in atmospheric particulates if they focused only on solvent-extractable or metallic oxide-formed EPFRs.In addition,this study also explores the contribution of secondary processes to atmospheric EPFRs and their formation mechanisms.The results show that secondary EPFRs can be generated by irradiating atmospheric particulate matter with visible light,and their lifetime is only 30 minutes to 1 day,which is much shorter than the lifetime of the original EPFRs in PM.The secondary EPFRs produced by PM may reach 15%-60%of the original EPFRs.Extractable organic substances are the main substances(about 55%)that generate secondary EPFRs,and they are mainly some humic substances(HULIS).Simulation results show that secondary EPFRs produced by extractable and non-extractable PM components are similar to phenolic compounds and PAHs,respectively.In addition,this study also found that oxygen molecules play an important role in the photochemical generation and decay of EPFRs.Reactive oxygen capture experiments showed that the original EPFRs may contribute to singlet oxygen generation,while the secondary EPFRs generated by photoexcitation may not produce singlet oxygen or hydroxyl radicals.Finally,combined with the main results of this study and the main findings in this field,the main generation mechanisms of atmospheric EPFRs are summarized and future research directions are prospected.There are three main mechanisms:graphene oxide-like substances produced by the polymerization of organic matter,EPFRs produced by high-temperature action of aromatic organic substances and metal oxides,and EPFRs formed by secondary reactions.Among these three mechanisms,graphene oxide-like substances are the main EPFRs generation mechanism(contributing more than 70%).In future research,the research should focus on the health risks of EPFRs in the actual atmospheric environment. |
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