Tracer-Based Source Apportionment Of Fine Organic Aerosols In Tianjin,China

1.Introduction.Organic aerosols（OA）account for about 20-90%of fine aerosols that potentially control the physical and chemical properties of aerosols in the atmosphere.OA can severely impact the Earth’s climate system and can also adversely affect human health.They are emitted directly from natural and anthropogenic sources,such as fossil fuel combustion,biomass burning,soil suspension,biological species and agricultural waste（primary OA（POA））,and formed by reactions of volatile organic compounds（VOCs）that are emitted from a variety of sources including vegetation,biomass burning and fossil fuel combustion with O₃,OH and NO₃ radicals and subsequent nucleation and/or condensation onto pre-existing aerosol particles in the atmosphere（secondary OA（SOA））.In Asia,high aerosol loadings including the fine(PM_2.5)OA are commonly observed in the atmosphere due to enhanced industrial activities in recent decades.Anhydrosugars,sugars and sugar alcohols are major species of primary organic compounds in the atmosphere.They generated from vegetation and microbial activities,and can be regarded as markers of primary biogenic aerosol particles such as vegetation leaves,pollen,fungi and bacteria.SOA accounts up to 60%of OA globally and even more on a regional scale.SOA formed from biogenic emissions is believed to be even larger than that from anthropogenic sources.Currently,the loading of isoprene-derived SOA（SOA_I）is believed to be larger than that from other precursors in the atmosphere.Monoterpene-（SOA_M）and sesquiterpene-derived SOA（SOA_S）are also considered to be significant contributors to SOA loading in the atmosphere.Classes of anthropogenic organic compounds in PM_2.5including hopanes,polycyclic aromatic hydrocarbons（PAHs）and phthalates also play an important role in the atmosphere.Hopanes are pentacyclic hydrocarbons of the triterpene group,which are considered to derive primarily from bacteriohopanols in bacterial membranes and to produce in sediments over geological time and can be regarded as special bio-markers of petroleum and coal.PAHs have severe influence on human health due to their mutagenic and carcinogenic consequences and have been considered to be the main toxic pollutants in PM_2.5.Plastics are common polymeric materials produced and applied around the world.Due to carcinogenic and endocrine disrupting properties of phthalates,they can also pose potential harm to human health,and hence much attention has been paid on them in recent times.2.Objectives.In order to better understand the characteristics and seasonal variations of OA in PM_2.5 and their sources over North China,the PM_2.5 samples have been collected in Tianjin,a typical mega-city,located in the northern region of China with a dense population of about 15 million and covered with several large industrial areas as well as agricultural fields.Tianjin is suffering from high aerosol loading and often with severe haze events that needs to be studied for their chemical characterization urgently,in order to make policy decisions to control the air pollutant emissions over this region.To characterize the chemical composition,abundance,temporal and seasonal variations of Tianjin aerosols,carbonaceous components including organic carbon（OC）,elemental carbon（EC）and water soluble organic carbon（WSOC）,POA tracers including anhydrosugars and sugars/sugar alcohols,hopanes,and PAHs,and SOA tracers derived from isoprene,α/β-pinene,β-caryophyllene and aromatics（toluene）were measured in PM_2.5 samples collected at an urban site and a background site over Tianjin,North China from July 5 to October 30,2018.3.Experimental.The sampling was performed on the rooftop of a 7-storey building at Tianjin University（TJU）Weijin Road campus located in Nankai District（ND;n=39）and a 6-storey building in TJU Peiyang Park campus in Haihe Educational Park（HEP;n=20）,during summer to autumn（5 July to 30 October）2018.OC and EC were measured using the Sunset Laboratory OC/EC Analyzer（Sunset Laboratory Inc.,USA）and WSOC was measured using TOC analyzer(TOC-V_csh,Shimadzu,Japan).Whilst,organic tracers were determined using gas chromatography coupled with mass spectrometry（GC-MS）.4.Carbonaceous components.Concentrations of EC and OC ranged from 0.11μg C m^-3 to 0.54μg C m^-3 with an average of 0.28±0.13μg C m^-3 and 1.37-8.89μg C m^-3(average 3.01±1.64μg C m^-3)at ND（n=39）and 0.09-0.81μg C m^-3(0.44±0.21μg C m^-3)and 0.85-9.86μg C m^-3(3.86±2.20μg C m^-3)at HEP（n=20）,respectively,during the campaign.While WSOC ranged between 1.14-6.47μg C m^-3(2.27±1.08μg C m^-3)at ND and 0.66-6.11μg C m^-3(2.62±1.34μg C m^-3)at HEP.The average relative abundance of WSOC in OC was found to be 78.0±12.3%at ND and 72.1±17.8%at HEP.Such high abundance of WSOC should be due to either enhanced emission from biomass burning and/or secondary formation from VOCs over Tianjin.5.Organic tracers5.1 Anhydrosugars,sugars and sugar alcohols.Total concentrations of anhydrosugar compounds ranged from 1.01-28.3 ng m^-3with an average of 6.38±6.30ng m^-3in summer and 7.45-234 ng m^-3with an average of 73.8±68.4 ng m^-3 in autumn at ND and ranged from 0.53-10.8 ng m^-3and 32.6-223 ng m^-3,with averages of 3.92±3.34 ng m^-3 and 129±51.9 ng m^-3at HEP in summer and autumn,respectively.Strong correlations（R²=0.93）between galactosan and mannosan were found at both the sites,which indicate their similar source and formation pathways.Concentrations of sugars and sugar alcohols ranged from 0.87-8.17 ng m^-3 with an average of 3.80±2.26 ng m^-3 in summer and 5.35-46.1 ng m^-3(16.7±10.9 ng m^-3)in autumn at ND and varied from0.76-10.4 ng m^-3(3.56±2.81 ng m^-3)in summer and 16.8 to 45.8 ng m^-3(25.2±8.68ng m^-3)in autumn at HEP.Fructose and sucrose were the most abundant species among all sugar species in this campaign.Good correlations（R²=0.96 at ND and R²=0.84 at HEP）found between fructose and glucose suggest that they should have been originated from same biogenic source（s）.Glycerol was the most abundant sugar alcohol species followed by erythritol and inositol,and showed a good relationship with levoglucosan,suggesting their potential emission from biomass burning.Arabitol and mannitol,specific tracers of fungal spores,showed strong correlation that confirms they should have been generated from similar sources in Tianjin aerosols.All the anhydrosugars and sugars/sugar alcohols showed higher concentration levels in autumn than those in summer.Most species at HEP were found to be more abundant than those at ND in autumn.Levoglucosan was found to be the most abundant compound among all the species studied,accounting for about 72%,followed by mannosan.Higher levels of levoglucosan in autumn than those in summer indicates an enhanced emission of OA from biomass burning during autumn,especially at background region of Tianjin.5.2 SOA tracers.All the biogenic SOA tracers,except pinic acid（PA）,were found to be abundant at HEP as compared to ND during the campaign.It is likely because the emission of biogenic VOCs must be higher at the background site,where human activities are less and most of the area is covered with dense vegetation.Concentrations of total isoprene SOA tracers were twice those ofα/β-pinene in summer,while they were the opposite in autumn,followed byβ-caryophyllinic acid and 2,3-dihydroxy-4-oxopentanoic acid（2,3-DHOPA）,the only tracers derived fromβ-caryophyllene and toluene,respectively.The isoprene andα/β-pinene SOA tracers were abundant in summer whereasβ-caryophylinic acid and 2,3-DHOPA dominated in autumn.Temporal variations and linear relations of SOA tracers with ambient temperature and molecular markers（levoglucosan and hopanes）together with the air mass trajectories implied that the SOA derived from terrestrial vegetation and marine biota and biomass burning was high in summer and autumn,respectively,while that derived from fossil fuel combustion and its processing during long-range atmospheric transport were significant in both seasons.The estimated SOA derived from aromatics was found to be most abundant,followed by that from sesquiterpenes,monoterpenes and isoprene,respectively.5.3 Hopanoid compounds.A sum of eleven hopanoid compounds determined ranged from 0-0.25 ng m^-3(average 0.04±0.06 ng m^-3)in summer and 0.02-4.55 ng m^-3(0.89±1.05 ng m^-3)in autumn at ND and 0.00-1.98 ng m^-3(0.24±0.61 ng m^-3)in summer and 0.86-3.29 ng m^-3(1.89±0.83 ng m^-3)in autumn at HEP.All hopanes,except for C_27βand C_31βa,showed very consistent temporal variations with each other at both the sites in both seasons.They peaked during July 23-26 in summer and October18-21 in autumn,except for few cases.Hopanes started to increase from early October and peaked during mid-October and then slightly decreased toward the month end.The higher concentration levels of hopanes in autumn than in summer suggest the contribution of emissions from fossil fuel combustion might be higher in autumn.Alternatively,the clean oceanic air masses arrived in Tianjin might have diluted the aerosol burden during summer time.Besides,the concentration of all hopanes at HEP were higher than those at ND irrespective of season,which indicates that the contribution of fossil fuel emissions was larger at HEP,probably due to additional input of fossil fuel combustion emissions at industries located in suburban Tianjin to HEP,which is unlikely at ND because it located in up wind region.Besides,according to the diagnostic ratios of hopanes,which are closer to those in vehicular exhausts than in coal burning emissions.Such results indicate that hopanes are mainly generated from the combustion of petroleum products rather than the coal combustion emission in Tianjin.5.4 Polycyclic aromatic hydrocarbons.Concentrations of total PAHs ranged from 0.52-2.61 ng m^-3,with an average of 1.48±0.68 ng m^-3in summer and 3.90-20.2ng m^-3(average 12.4±6.10 ng m^-3)in autumn at HEP.At ND,they ranged from 0.02-3.99 ng m^-3(0.71±0.82 ng m^-3)in summer and 0.12-13.4 ng m^-3(4.89±4.06 ng m^-3)in autumn.Such seasonal variations of PAHs during the campaign indicate that the contribution strengths of OA were different in summer and autumn.The variations in PAHs found to be similar at both sites,which indicate that they might have derived from similar emission sources and significantly influenced by long-range transported air masses rather than the local sources in Tianjin region.The high loading of PAHs in autumn is probably caused by enhanced biomass burning and coal combustion emissions that should have been enriched in the air masses originated from Mongolia and northeastern parts of China.In contrast,such influence must be insignificant in summer because the air masses arrived in Tianjin were originated from the oceanic regions.The diagnostic ratios are close to that of coal combustion emission,further suggesting that PAHs in Tianjin are mainly derived from coal combustion in autumn.5.5 Phthalates.Concentrations of total phthalates ranged from 26.9-112 ng m^-3 with an average of 57.2±20.0 ng m^-3in summer and was 25.0 to 116 ng m^-3(average 63.3±28.2 ng m^-3)in autumn at ND.They ranged from 2.30-11.8 ng m^-3(6.85±3.35 ng m^-3)in summer,and 25.8-87.6ng m^-3(56.0±17.0 ng m^-3)in autumn.Phthalates showed higher levels at ND those at than HEP,except for dimethyl.Di-（2-ethylhexyl）was found to be the most abundant species,accounting for～77%,followed by di-n-butyl,di-iso-butyl and diethyl at both the sites.Concentrations of di-（2-ethylhexyl）found to be higher in summer than those in autumn at ND,whereas those of the other phthalates were in autumn than in summer.Di-iso-butyl and di-n-butyl showed very consistent temporal variations with each other at both sites in both the seasons.Besides,they showed strong correlations,suggesting that they were emitted from the plasticsover the Tianjin region,especially in summer.6.Source apportionment6.1 PMF model analysis.Source apportionment of OA has been carried out using all the marker species by positive matrix factorization（PMF）model.Seven factors were resulted at ND during the whole campaign period:summer to autumn.Factor 1 was highly loaded by PAHs species with 55.3%,suggesting the contributions of fossil fuel combustion emission.Factor 2 was dominated by sugars（37.9%）and sugar alcohols（27.5%）,suggesting the contribution of fungal spores and plant materials.Factor 3 was dominated by isoprene SOA tracers,with the high percentage of 84.7%,suggesting the contribution of SOA formation.Factor 4 represented the contribution of DHOPA（56.4%）,indicating the source of aromatics SOA formation.Factor 5 was dominated by anhydrosugars（71.3%）,suggesting the contribution of biomass burning.Factor 6 was loaded by phthalates（61.6%）,representing the plastic emission source.Factor 7 was dominated by hopane species（64.2%）,suggesting the emissions mainly from coal combustion.6.2 SOC Calculation.A tracer-based method was used to apportion the POA and SOA at ND and HEP in Tianjin.The contribution of total SOC to OC and WSOC varied between 4.16%and 23.7%and 6.28-30.7%,respectively,in which biogenic SOC accounted for about 50%in summer and 40%in autumn,indicating significant loading of SOA over Tianjin,North China.7.Outcome.As noted above,I investigated the characteristics and loading of primary and secondary organic aerosols tracers and insights on the sources of POA and SOA in Tianjin,China.Also,the seasonal changes in carbonaceous aerosols as well as in their sources from summer to autumn.Based on the obtained results,I found that fossil fuels combustion,biomass burning and plastic emissions are the main anthropogenic sources of PM_2.5 over the Tianjin region,particularly in autumn.In addition,vegetation emissions and enhanced secondary formation processes are important,particularly in summertime.Thus,this study provided the baseline data of carbonaceous aerosols,particularly POA and SOA tracers and their source contributions in summer and autumn.Furthermore,this study warrants the need to control the fossil fuels and biofuels combustion and the use of plastics over the Tianjin region,in order to prevent the environmental pollution and thus improve the air quality.