Molecular Distributions,sources, Size Distributions And Formation Mechanisms Of Organic Aerosols From Typical Megacities And Background Site In China

This paper characterized the seasonal characteristics,temporal and spatial distribution and change trends of organic aerosols collected in five typical cities(Shanghai,Guangzhou,Xi'an,Chengdu and Urumqi)and a background site(Q inghai Lake)in China.We analyzed the changes of organic aerosols during the period of the dust storm in Xi'an and the project “shifting coal to natural gas” in Urumqi,respevtively,to explore the impacts of natural factors and human activities on the physical and chemical properties of atmospheric aerosols.The main conclusions are summarized as follows:Concentrations and compositions of PAHs and oxygenated PAHs(OPAHs)in four size ranges of ambient particles(<1.1,1.1-3.3,3.3-9.0 and >9.0 ?m)collected in Xi'an and Guangzhou,two megacities of China,during the winter and summer of 2013 were measured and compared with those in 2003.The TSP-equivalent concentrations of ?14PAHs in Xi'an and Guangzhou are 57±20 and 18±23 ng m-3 in winter,5-10 times higher than those in summer.PAHs in both cities are dominated by 5-and 6-ring congeners in summer due to the predominance of traffic emissions,but dominated by 4-and 5-ring congeners in winter due to enhanced emissions from domestic coal burning and increased gas-to-particle phase partitioning of the semi-volatile PAHs.Seven OPAHs were detected in winter while only one OPAH was detected in summer.TSP-equivalent ?7OPAHs during winter are 54±15 and 23±32 ng m-3 in Xi'an and Guangzhou and dominated by 5-ring OPAHs.Size distribution results showed that 4-and 5-ring PAHs and OPAHs in Xi'an and Guangzhou are the dominant species in fine particles(<3.3 ?m)during winter.Relative abundances of 3-ring PAHs and OPAHs increased along with an increase in particle sizes,accounting for from about 1% of the total PAHs or OPAHs in the smallest particles(<1.1 ?m)to more than 90% of the total in the largest particles(> 9.0?m).The toxicity of PAH assessment indicated that atmospheric particles in winter are much more toxic than those in summer and fine particles are more toxic than coarse particles.Compared to those in 2003,fine particulate PAHs and OPAHs in both cities during winter decreased by 50-90%,most likely due to the replacement of coal by natural gas in the country.Polar organic tracers of isoprene and monoterpene photooxidation in TSP(the sum of 9 Anderson size-segregated aerosols)collected in Q inghai Lake(as background site)and five cities of China were measured using gas chromatography/mass spectrometry(GC/MS).All the biogenic secondary organic aerosol(BSOA)tracers showed higher concentrations in the cities than in the background site with higher concentrations in summer than in winter.Moreover,concentrations of BSOA positively correlated with sulfate(R2=0.52-0.9),which suggests sulfate could enhance SOA formation.S ize distribution measurements showed that most of the determined BSOA tracers are enriched in the fine mode(<1.1?m)except cis-pinic acid and cis-pinonic acid,both presenting a bimodal pattern.In summer,the average ratio of isoprene to monoterpene oxidation products(Riso/mono)was much less in the urban aerosols than those in aerosols from mountains and forests of China.Carbon fractions of isoprene and monoterpene tracers to OC increased during summer and the contributions of monoterpene SO A tracers were much higher than those of isoprene SOA tracers,suggesting that monoterpene is a more significant precursor for BSOA than isoprene in urban areas of China.TSP and 9-stage size-segregated samples are simultaneously collected in Xi'an during the spring of 2013 and some primary and secondary organic aerosols are detected by using the TMS derivatization and GC/MS quantification technique.During the dust storm and non-dust storm periods,n-alkanes are detected as the dominant compound class,followed by fatty acids.The anthropogenic primary and secondary organic aerosols display similar variation patterns,which is lower in the dust storm event and higher in the non-dust period,indicating they are mostly originated from local sources,e.g.coal burning and biomass combustion emissions.In contrast,BSOA are largely originated from long-range transport such as Gobi desert plants.Trehalose concentration was very low during the nonevent time,but much higher in the dust storm period and strongly correlated with Ca and Na2SO4,because it is largely derived from Gebi desert surface soil.Their changes in molecular compositions,mass ratios and size distributions further indicate that high molecular weight(HMW)organic aerosols during the dust strom period are mostly originated from plants in Gobi regions and dominated in coarse particles.Phenanthrene(Phe)and di-n-butyl phthalates(DBP)show a bimodal size distribution during the nonevent and a dominant coarse mode in the presence of dust because of their high vapor presures.O n the contrary,benzo(e)pyrene(BeP)and bisphenol(BPA)display a same fine mode size distribution during the non-dust strom and the dust strom periods due to their non-valatile nature.Secondary organic compounds in Xi'an are predominantly enriched on the coarse particles in the presence of the dust storm due to the coagulation or/and a partitioning into the aqueous-phase of dust particles surface.Size-segregated aerosols were also collected in Urumqi,a megacity in northwest China,during two heating seasons before(heating season ?: January – March 2012)and after the project “shifting coal to natural gas”(heating season II: January – March 2014),and determined for n-alkanes,PAHs and oxygenated PAHs to investigate the impact of the replacement of coal by natural gas on organic aerosols in the urban atmosphere.Our results showed that compared to those in heating season I concentrations of n-alkanes,PAHs and OPAHs(the sum of PM1.1,PM1.1-3.3 and PM3.3-9)decreased by 74%,74% and 82% in heating season II,respectively.Source apportionment analysis showed that coal combustion,traffic emission and biomass burning are the major sources of the determined organics during the heating seaso ns in Urumqi.Traffic emission is the main source for n-alkanes in the city.Coal combustion is the dominant source of PAHs and OPAHs in heating season ?,but traffic emission become their major source in heating season ?I.Relative contributions of coal combustion to n-alkanes,PAHs and OPAHs in Urumqi decreased from 21-75% in heating season I to 4-21% in heating season II due to the replacement of coal with natural gas for house heating.Health risk assessment further showed that compared with that in heating season I the number of lung cancer related to PAHs exposure in Urumqi decreased by 73% during heating season II due to the project implementation.O ur results suggest that replacing coal by clean energy sources for house heating will significantly mitigate air pollution and improve human health in China.