Organic Aerosols In PM_2.5 In Urban Chongqing:Seasonal Variation,Source Apportionment And Long-range Transport

As important components of atmospheric fine particulate matter(PM_2.5),carbonaceous aerosols mainly include organic carbon（OC）and elemental carbon（EC）.Due to the significant impact on climate change,human health and environmental quality,carbonaceous aerosols have increasingly become the forefront of international atmospheric environmental research and the key to control urban air pollution.Polycyclic aromatic hydrocarbons（PAHs）,n-alkanes,unresolved complex mixtures（UCM）,steranes and hopanes are important components of organic aerosols,widely present in atmospheric environment,which are usually used as important biomarkers to identify the source of carbonaceous aerosols.Among them,PAHs have become hot target pollutants in environmental field due to their significant carcinogenic,teratogenic,and mutagenic properties.N-alkanes,UCM,steranes and hopanes are typical biomarkers to analyze the migration,transmission and source of atmospheric particles,and their composition and distribution can reflect important geochemical information of the pollution sourceThis study was based on four seasonal PM_2.5 samples（n=77）collected on the roof of the State Key Laboratory of Coal Mine Disaster Dynamics and Control in Chongqing University Campus A from October 2015 to August 2016.These samples were analyzed for OC,EC,USEPA 16 PAHs,n-alkanes,UCM,steranes and hopanes to understand their pollution levels and variation characteristics,quantified the pollution sources,and compared pollution characteristics between non-episodic and episodic days to analyze the mechanism of pollution episodes.The study was expected to provide basic scientific data for further development of air pollution forecasting and early warning,and regional governance of atmospheric environment.The results showed that the annual average concentrations of OC,EC,char,soot,?PAHs,?n-alkanes and UCM in PM_2.5 in Shapingba District of urban Chongqing were 20.66μg/m³,6.16μg/m³,5.42μg/m³,0.74μg/m³,38.29 ng/m³,328.69 ng/m³ and 2.52μg/m³,respectively.The concentrations of OC,?PAHs,?n-alkanes and UCM were highest in winter and lowest in summer.While the concentration of EC,char and soot was highest in autumn and lowest in winter.Seven carbon components based on the TOR method were applied to carry out PMF source analysis,a total of 3 factors were identified,including the mixed source of biomass/coal combustion and road dust（52.7%）,gasoline vehicle emission source（22.9%）,and diesel vehicle emission source（24.4%）.Motor vehicle exhaust was the main source of carbonaceous aerosols in autumn,spring and summer.In winter,it was mainly affected by the combined sources of coal/biomass combustion and road dust.Five pollution episodes were observed during the sampling period.The increased concentrations of carbonaceous aerosols from the episodes in autumn might be due to the increasements in the traffic of gasoline vehicles in local and surrounding cities.The winter pollution episode might be affected by the combined effects of local coal/biomass combustion emissions and the input from surrounding rural areas.The increased contribution of the mixed sources of biomass/coal combustion and road dust in spring episode might be related to the long-range transport of sand dust from northwest China.Among USEPA 16 PAHs,the concentrations of Bb F（7.17 ng/m³）,Icd P（5.89 ng/m³）and Bghi P（5.05 ng/m³）were the highest.5-6 rings PAHs accounted for the highest proportion（64.6%）of the total PAHs,followed by 4 rings PAHs（27.8%）and 2～3 ring PAHs（7.6%）.The annual average value of the toxic equivalent quantity（TEQ）of PAHs was 6.46 ng/m³,and the average annual value of incremental lifetime cancer risk to minors and adults through respiratory exposure were.27×10^-6 and 0.73×10^-6,respectively.PM_2.5-bound PAHs in winter had a potential carcinogenic risk to human health.The PMF source analysis of 16 PAHs identified 3 factors,among which motor vehicle exhaust emission（39.28%）was the main source,followed by coal/biomass combustion source（31.74%）and evaporation/diagenesis sources（28.99%）.The main sources in autumn,spring and summer were vehicle exhaust emission and evaporation/diagenesis sources.PAHs in winter mainly came from coal/biomass combustion.The increased contribution of vehicle exhaust emission sources（47.2%）in autumn episodic days might be related to the increasement of vehicle traffic in local and surrounding cities.The contribution of biomass/coal combustion sources in winter episodic days（62.89%）was higher than that of non-episodic days（56.93%）,which may be affected by the increasing local coal/biomass combustion emissions and the input from surrounding rural areas.The contributions of vehicle exhaust emission source（49.73%）and biomass/coal combustion sources（20.53%）increased in spring episodic days,which may be affected by the combined influence of meteorological conditions,long-range transport of sand dust,increased vehicle emission and biomass/coal combustion.A total of 28 n-alkanes were detected in PM_2.5.The carbon number was distributed in C₁₁-C₃₈,C₂₉as the main peak carbon.The CPI of n-alkanes was 0.79-1.52,and the average percentage of plant wax contribution was 20.90%.The U:R ratio was lowest in summer,which might be driven by temperature and light,leading to vigorous life activities of microorganisms and plants.The PMF source analysis of 28 n-alkanes identified 4 factors,including fossil fuel combustion sources（23.45%）,fossil fuel residue sources（29.1%）,plant wax emission sources（26.1%）and biomass combustion sources（21.33%）.Fossil fuel combustion sources had a higher contribution in spring（34.82%）and winter（32.05%）,mainly due to increased pollution emissions and adverse meteorological conditions.The contribution of fossil fuel residue sources（49.30%）in summer was significantly higher than that of other three seasons,mainly owing to the driving of light and temperature conducive to the volatilization of fossil fuel residues into the atmosphere.The contribution of higher plant wax emission sources in winter（42.14%）was obviously higher than that of other three seasons,which might be affected by both meteorological conditions and increased emissions of pollutants.Relating to the burning of crop wastes such as straws by farmers,the contribution of biomass combustion sources was highest in autumn（47.42%）.The CPI and Wax C_n of n-alkanes were higher than non-episodic days of corresponding seasons in pollution episodes,the U:R ratio in episodic days was lower.The contribution of higher plant wax emission sources in autumn and winter pollution episodes were slightly higher than that of non-episodic days.The increase of n-alkanes concentration on polluted episodes were mainly affected by the increase in plant wax emissions.The increase of n-alkanes in spring pollution episodes were related to fossil fuel and biomass burning activities.13 hopanes and 12 sterane were detected in PM_2.5 samples,and the carbon number was distributed in C₂₇-C₃₃(without C₂₈)and C₂₇-C₂₉,respectively.The main peak carbon for hopanes was C30αβ,followed by C29αβ.For steranes,C29ααα（20S）was the main peak carbon in autumn,spring and summer,while C27ααα（20R）as the main peak in winter.The total content of hopanes and steranes were both ordered as:winter>autumn>spring>summer.The annual average values of Ts/Tm,C30αβ/C31αβ（22R）and C31αβ（22S）/（22S+22R）of hopanes were 1.15±0.20,5.26±0.75 and 0.59±0.01,respectively,indicating input of high-maturity petroleum hydrocarbons such as motor vehicle exhaust emissions.The annual average values of C29αββ/（ααα+αββ）and C29ααα（20S）/（20S+20R）of sterane were 0.40 and 0.53,respectively,mainly indicating the input of fossil fuel residues with high thermal maturity.The Ts/Tm and C30αβ/C31αβ（22R）of hopanes in pollution episode 1 and 3 were higher than the corresponding non-episodic days,which may be related to the increasement of vehicle exhaust emissions.The Ts/Tm and C30αβ/C31αβ（22R）of hopanes in other three pollution episodes were all lower than the corresponding non-episodic days,which might be affected by the combustion of low-maturity coal/biomass.For steranes,the C29ααα（20S）/（20S+20R）of pollution episode 1 and 4 were slightly higher than that of non-episodic days,implying that the contribution of high-maturity petroleum hydrocarbon input increased partially.The C29ααα（20S）/（20S+20R）of pollution episode5 was slightly lower than that of non-episodic days,which might be affected by the combustion of low-maturity coal/biomass.Analysis of 72-h back trajectories of air mass from an altitude of 500m found that southwest wind prevailed in summer and autumn,the air masses mainly originated from southwest China（including Yunnan,Guangxi,Guizhou,Hunan and Sichuan province）.In spring and winter,the northwest wind prevailed,the air mass trajectory mainly came from western China（including Tibet,Yunnan,Xinjiang and Qinghai province）.PSCF analysis showed that the potential source areas of OC,EC,ΣPAHs andΣn-alkanes were mainly concentrated in southeastern Sichuan,western Chongqing and their adjacent areas,while the potential source areas for UCM mainly distributed in southeastern Sichuan.