| To investigate the pollution characteristics and sources of atmospheric organic aerosols in the background region of the Yangtze River Delta(YRD),China,PM2.5 and size-resolved aerosols were collected in summer(from May 30 th to August 15 th,2018)and winter(from December 27 th,2018 to January 25 th,2019)in Chongming Island,China and determined for n-alkanes and PAHs by using a GC-MS technique.The main conclusions are summarized as follows:(1)In summer,the concentration of PM2.5 during the whole sampling period was 33±21 ?g·m-3,among which 35% of the collected PM2.5 samples were of a particle loading larger than the 1st grade of China National Air Quality Standard(35 ?g·m-3).In winter,the concentration of PM2.5 was 59±41 ?g·m-3 during the campaign,among which 34% of the measured PM2.5 was larger than the 2st grade of China National Air Quality Standard(75 ?g·m-3),suggesting that PM2.5 pollution in winter in Chongming Island was more serious.(2)In summer,the concentration of n-alkanes in PM2.5 in Chongming Island was 26±44 ng·m-3,with C29 being the most abundant molecule.In the period with PM2.5 higher than 35 ?g·m-3,which was classified as the summertime pollution period,concentration of n-alkanes in PM2.5 was 11 times higher than that in the period with PM2.5 less than 15 ?g·m-3,which was classified as the summertime cleaning period.C31 was the second most abundant n-alkane in the polluted period but the largest one in the clean period.Diagnostic ratios showed that most of n-alkanes in PM2.5 were derived from fossil fuel combustion,mainly originating from vehicle and industrial emissions from North China Plain(NCP)and YRD during the polluted period,and from ship emissions during the cleaning period.The concentration of n-alkanes in winter was 70±51 ng·m-3 with C25 being the highest one.In the period with PM2.5 higher than 75 ?g·m-3,which was classified as the wintertime pollution period,concentration of n-alkanes was 3.7 times higher than that in the period with PM2.5 less than 35 ?g·m-3,which was classified as the wintertime cleaning period.Fossil fuel combustion sources during the polluted and cleaning periods contributed to 85% of the total n-alkanes,which mainly came from YRD during the pollution period,but mainly from Shandong and the northern part of Jiangsu Province in the cleaning period.(3)Concentration of PAHs in PM2.5 of Chongming Island was 0.76±1.0 ng·m-3 during the summer campaign,among which concentration of PAHs in the polluted period was 16 times higher than that in the cleaning period.In the polluted period benzo(b)fluoranthene(Bb F)was the most abundant congener and benzo(ghi)perylene(Bghi P)was the second highest.From the polluted period to the cleaning period,the relative abundances of 4-ring PAHs increased,while 5,6-ring PAHs decreased.In the polluted period 5,6-ring PAHs were the dominant,accounting for 64% of the total,while in the clean period 3,4-ring PAHs were the most abundant,accounting for 54% of the total.PMF analysis further showed that during the polluted period vehicle exhausts and industrial emissions were the largest sources of PAHs,contributing to 51% of the total.In contrast,during the cleaning period ship emissions were the largest source,contributing to 45% of total PAHs.In winter,the concentration of PAHs in PM2.5 was 8.4±4.8 ng·m-3,on average.PAHs showed a highest concentration(13 ng·m-3)during the wintertime polluted period,2.4 times higher than that in the wintertime cleaning period.Unlike the summer case,in winter Bb F was the dominant PAH,but the second most abundant congener was fluoranthene(Fla)or chrysene(Chry).During the winter campaign 4-ring PAHs were the highest,accounting for 40% of the total,and the second highest was 5-ring PAHs,accounting for 37% of the total.There was no significant difference in the molecular composition of PAHs among the clean,transition and polluted periods,all showing a molecular composition characteristic of coal and biomass combustion.PMF analysis further showed coal burning and exhausts emissions were the largest sources of PAHs,accounting for 61% during the polluted period.In the clean period ship emissions and other sources contributed to 26% of total PAHs.(4)n-Alkanes in Chongming Island during the campaign mainly dominated in the coarse mode(>2.1 ?m)in summer but in the fine mode(<2.1 ?m)in winter with fossil fuel combustion being the major source in the two seasons.In summer,plant wax derived n-alkanes(WNA)presented a unimodal size distribution with 70%~82% of the whole WNA in the coarse mode.However,WNA presented a unimodal size distribution in winter with a peak in the fine mode in the polluted period and a bimodal size distribution in the clean and transition periods,peaking in the fine and coarse modes,respectively.Fossil fuel combustion derived n-alkanes(FFNA)showed a bimodal size distribution pattern in the two seasons with 45%~67% of the total in the coarse mode during the summer and 71%~80% of the total in the fine mode during the winter.(5)In summer,high molecular weight(HMW)-PAHs(i.e.,5,6-ring PAHs)mainly distributed in the fine mode.In contrast,low molecular weight(LMW)(i.e.,3,4-ring PAHs)PAHs mainly distributed in the fine mode during the polluted period but in the coarse mode during the transition period and clean periods.Relative abundance of LMW PAHs in the coarse mode was higher in the clean period than that in the transition and polluted periods,due to the volatilization of LMW PAHs from the fine mode and the subsequent condensation onto the coarse mode.(6)Local biomass burning event(Event 1 in summer,from May 30 th to June 8th)and regional transport event(Event 2 in summer,from July 23 th to August 1th)were identified as two types of pollution events in summer in Chongming Island.During Event 1,levoglucosan concentration(33±34 ng·m-3)was an order of magnitude higher than that during the clean period,suggesting that Event 1 was caused by biomass burning from Chongming Island.Relative abundance of plant wax derived n-alkanes(%WNA)reached 53% in Event I,significantly higher than that during the clean period(33%).During Event 2 in summer,n-Alkanes were mainly derived from regional transport from YRD with CPI1(1.4)significantly lower than that in Event 1(4.4).Regional transport was the major causation of pollution events in winter.During the wintertime Event 1(from January 1th to 3th),the concentration and proportion of levoglucosan were relatively lower,indicating that the impact of biomass burning was insignificant and fossil fuel combustion was the major source.During the wintertime Event 4(from January 21 th to 24th),which was caused by the pollution transported from the upwind regions such as Jiangsu Province,the concentration of organic matter was the highest among the four wintertime pollution episodes,the concentration and proportion of levoglucosan were higher than in the clean period,showing that there was a significant biomass burning activity.Both biomass burning and fossil fuel combustion were the major sources to PAHs,while fossil fuel combustion was the major source for n-alkanes in winter in Chongming Island. |
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