Research On Secondary Organic Aerosol At Urban Beijing Based Oin The Potential Aerosol Mass Reactor

Secondary organic aerosol(SOA)is a key component of fine particle(PM2.5),and it has significant influence on climate change,regional precipitation,visibility and human health.Up to now,the knowledge about sources of SOA and formation mechanisms has not been fully understood.The results of laboratory studies disagree well to model simulation,and cannot elucidate the pollution conditions in field observation.As China's capital,in recent years,Beijing has experienced frequently varying level haze pollution in which SOA play an important role.It is of great importance to study the SOA formation potential in actual atmosphere,which will help us to cognize the formation and conversion mechanisms of SOA and its impact to fine particulate pollution.This study focus on the variation patterns and factors of SOA formation potential in urban Beijing based on potential aerosol mass reactor(PAM reactor)from May 2016 to November 2017.The air sample is oxidized in PAM reactor by hydroxyl radicals that come from the reaction between ozone and H2O when 254 nm UV exist.The hydroxyl radicals can affect the aging time which is an important factor and denote the photo-oxidation time equivalent to the hydroxyl radicals in the actual atmosphere.The influence of aging time on the SOA formation mainly focus on the SOA formation potential.In the experiment of field observation in summer,the hourly average SOA formation potential vary between 3.9 and 9.4 ?g/m3.The diurnal variation of SOA formation potential show the trend of higher value at night and lower value in the daytime,and reach the lowest value at 16:00 in a day based on all the field observation data.The trend of SOA formation potential is consistent with that of the benzene,a typical urban volatile organic compound(VOC),and show inverse correlation to the concentration of ozone in actual atmosphere.The results show that strong photo-oxidation reaction lead to the consumption of precursor VOCs,which have important influence on the SO A formation potential besides impact from the change in boundary layer height.Compared to the similar studies reported in the developed areas,the SOA formation potential of urban Beijing is higher.SOA formation potential was studied with different aging time.It was found that the optimum aging time(i.e.the aging time corresponding to the maximum SOA formation)in Beijing varied between 2 and 4 days,and it varied between 0.8 and 6 days in Los Angeles.In addition,the SOA formation potential of Beijing and Los Angeles can reach 35.0 ?g/m3 and 4.0 ?g/m3,respectively.Different SOA formation potential suggested different pollution levels and characterizations in the two cities.By carrying out the experiments at different pollution level,the results show that concentration of precursor VOCs can tremendously affect the SOA formation.The SOA formation potential and optimum aging time increased from 7.1 to 30.2 ?g/m3 and 2 days to 4 days,respectively.This result was verified by the toluene oxidation experiment carried out in PAM reactor.In summary,the pollution level of VOCs in Beijing is more serious than developed cities such as Los Angeles,which resulted in high SOA potential and might contribute to the long duration haze events in Beijing.