Characteristics and Sources of Carbonaceous Species in PM2.5 in Urban and Roadside Sites in Beijing in Autumn and Winter

CAO Yang; WANG Qin; JING Kuan; FU Jiaming; GUO Yu; LIU Baoxian; AN Xinxin

doi:10.19672/j.cnki.1003-6504.1214.23.338

Abstract：In order to explore the characteristics and sources of carbonaceous species at different locations in Beijing in autumn and winter,online continuous observations of organic carbon(OC),element carbon(EC),PM2.5,O3,CO,NOx and meteorological factors were carried out simultaneously at urban and roadside sites in September and December 2020.The results showed that air quality was good during the study period,the ρ(OC),ρ(EC),ρ(PM2.5)were(5.66±3.54),(1.59±1.31),(32.9±25.1)μg/m3 at roadside site,respectively,which were 32%,70%and 33%higher than urban site.PM2.5 pollution was heavier at roadside,vehicles may be a major contributor to the carbonaceous species especially EC.In autumn,there was no significant difference in OC concentration between two sites,and EC was 47%higher at roadside site,ω(total carbona-ceous matter(TCM=1.4×OC+EC))in PM2.5 were 29%and 25%at urban and roadside sites.In winter,OC and EC at roadside site were 54%and 83%higher than those at urban site,ω(TCM)were 27%and 31%at urban and roadside sites.PM2.5 and gaseous pollutants were higher in winter than in autumn,and higher at roadside than at urban site,except O3.Under weak wind conditions,the concentration of carbonaceous species at urban and roadside sites were 1.3～1.6 and 2.0～3.8 times of that under non-weak wind conditions.Therefore,it is crucial to take emission reduction measures under weak wind conditions to improve air quality.There were little difference between OC/EC ratios under weak wind conditions at two sites,OC/EC were 2.56 and 3.35 in autumn and winter.Local emission was the main influence factor under weak wind conditions,OC/EC were 2.99 and 1.86 at urban and roadside sites.The ρ(secondary organic carbon(SOC))and SOC/OC,calculated using the minimum OC/EC(OC/EC)min in different meteorological conditions,were 1.99 μg/m3 and 47%at urban site,3.13 μg/m3 and 55%at roadside site.In autumn SOC concentration was lower but accounted for a higher proportion of OC.In terms of diurnal variation,OC and EC kept low concentrations and fluctuated slightly throughout the day under non-weak wind conditions.Under weak wind conditions,OC at the two sites presented a bimodal distribution,with peaks around 12:00 and 19:00.Due to the atmospheric boundary layer changes and pollution emissions,the noon peak was not obvious and the concentra-tion was prominent in the mid-night in winter.EC curve at urban site showed bimodal peaks in the morning and evening rush-hour with low concentration,EC showed a"V"shaped distribution and concentration continued to be high around mid-night.In autumn,the correlation between PM2.5 and carbon components was weak,and roadside vehicles contributed more to carbon particles.The correlation was strong in winter,so attention should be paid to combustion emission transmission in the south and southeast of the region.