Characterization And Identification Of Sources Of Fine PM_2.5 Carbonaceous Aerosols In Tianjin,North China

Atmospheric aerosols can affect the visibility,ecosystem and materials,human health,Earth's climate system and heterogeneous chemistry.Aerosol affects the ecosystem directly through deposition?both wet and dry?and indirectly by affecting the soil.Compared to direct effects,indirect effects are usually more significant because they can alter nutrient cycling and inhibit plant nutrient uptake,which primarily depends on the chemical composition of the deposited aerosol particles.The effects of aerosols on human health have become a major concern in the current scenario.Epidemiological studies have correlated elevated levels of particulate matter results in increased hospital admissions with decreased pulmonary function,heart disease,or aggravation of chronic conditions and also the increased mortality.Aerosols play a pivotal role in the Earth's climate system and atmospheric chemistry.They affect the Earth's radiative balance and hydrological cycle and thus exert an impact on climate.Aerosols can also cause an indirect effect by acting as cloud condensation nuclei?CCN?,also provide catalytic surfaces for many important chemical reactions.Therefore,atmospheric aerosols acquired greater importance worldwide due to their significant local,regional,and global impacts.Carbonaceous aerosols are comprised of elemental carbon?EC?and many organic compounds that collectively considered as organic carbon?OC?or organic matter.They account for a major fraction?5-9%?EC and 20-50%OM of fine(PM_2.5)particle mass in mid-latitudes including China.Basically speaking,EC is derived from incomplete combustion of fossil fuel and biomass burning.OC can be directly emitted into the atmosphere?primary?from anthropogenic and natural?soil dust and biota?sources and can also be formed by oxidation of reactive organic gases of both the origins and subsequent gas-to-particle conversion in the atmosphere?secondary?.According to solubility in water,OC is further classified into two groups:water-soluble OC?WSOC?and water-insoluble OC?WIOC?.It is generally believed that WSOC is mainly derived by secondary formation but biomass burning also contributes significantly.WIOC is thought to be mainly from primary origins with a substantial contribution from fossil fuel combustion and marine emissions,biomass burning and secondary formation are also significant.Carbonaceous aerosols also have adverse effects on environment,climate,agriculture yields,public health and visibility and also play an important role in chemistry of the atmosphere,they received great attention over past two decades.EC is a strong absorber of sunlight and causes warming at the top of the atmosphere and cooling at the Earth's surface,whereas OC is highly reflective to sunlight and cause cooling effect.WSOC can alter the hygroscopic properties of atmospheric aerosols and thus enhance the capability of particles to act as CCN that should result in the indirect radiative forcing.It is composed of a substantial fraction of dicarboxylic acids and related compounds,produced by oxidation processes of gaseous precursors transported over long distances to remote areas and subjected for further photochemical processing?aging?in the atmosphere,referred as secondary OA?SOA?.Dicarboxylic acids,oxocarboxylic acids and?-dicarbonyls account for a dominant fraction of WSOC that accounts for 10-40%in total organic carbon.Diacids and related compounds with high abundance of oxalic?C₂?acid have been recognized as ubiquitous constituents in atmospheric aerosols in various environments such urban,rural,mountain,polar and marine.It possesses low vapor pressure and carboxyl group,are involved in fog formation and acid precipitation.They participate in various physical and chemical reactions in the atmosphere.The primary sources of diacids and related compounds include anthropogenic and biological sources,the former mainly include gasoline,diesel and other fossil fuel combustion process,whereas the latter include biomass burning and biofuel combustion.In addition,plant debris,pollen spores,fungal spores,and sea spray are considered to be significant primary sources.The VOCs such as isoprene and monoterpenes emitted from biogenic sources including biomass burning and aromatic hydrocarbons from vehicular exhausts and various industrial emissions have been considered as the main precursors of diacids and related compounds.However,photochemical oxidation of various unsaturated and saturated fatty acids is an important formation pathway for it.The removal of diacids and related compounds from the atmosphere is mainly through wet and/or dry deposition and chemical degradation.Wet deposition is recognized as an effective means of elimination.Up to now,due to the complexity of pretreatment and the easy introduction of impurities to the experimental process,actual concentration of organic dicarboxylic acid and related components are underestimated.In China,diacids and related compounds are only observed in few sites,such as Xi'an,Chengdu,Hong Kong,Beijing these urban cities as well as Mt Tai,which is mostly due to emissions from household combustion of coal,cooking and biogenic combustion activities and other anthropogenic primary emissions.They also reported that water-soluble diacids were characterized by higher concentrations in the south while lower concentrations in the north during winter.In recent years,dicarboxylic acids and related compounds,stable carbon isotope analysis techniques have been successfully used to study the aging extent of aerosol and its potential photochemical transformation mechanism.However,Tianjin aerosols have been studied only for inorganic ions and few days observation for diacids and related compounds has also been reported.The measurements of ¹³?C_TC and organic molecular species are sparse.Here should be noted that Tianjin is a typical metropolis in northeastern China,with a population of nearly 15 million and an area of about 12,000km².It is located in the southeast of Beijing,on the lower reaches of the Haihe River and adjacent to the Bohai Sea.As one of the fastest growing regions in China,Tianjin has become an economic center in North China.In recent years,satellite measurements and field observations indicate that Tianjin has suffered for a long time extremely high aerosol loading and poor visibility.High aerosols in the atmosphere of Tianjin can weaken solar radiation reaching the surface and inhibit the development of atmospheric boundary layers.In addition,studies have shown that high aerosol components in Tianjin It also has a strong impact on photochemical reactions and ozone formation.Therefore,comprehensive studies on fine(PM_2.5)carbonaceous aerosols in Tianjin region are highly necessary to understand their characteristics and secondary transformation and transmission processes and thus to reveal their precise role on human health risks and the regional climate.So this study focus on the carbonaceous components in fine particles of summer and winter aerosol of 2016 for day and night in Tianjin,diacids and related compounds and inorganic ions as well as stable carbon isotope ratios would provide clear evidence for major specific sources and the influence of local emissions and long-range transported air masses in Tianjin,meanwhile,compared with typical cities,oceans and polar regions around the world,thus the role of in-situ secondary formation and photochemical processing as well as the sources of OA will be revealed,furthermore,we have a better understanding of the aerosol pollution and the extent of aging in Tianjin.The results obtained will serve as baseline data,knowledge on secondary processes of OA and air quality in Tianjin,which would help to develop strategies to mitigate PM_2.5 pollution in North China.Details for this research are as follows:Seasonal or diurnal variation of different carbonaceous components and diacids acids and their related compounds:All components showed higher levels in winter.It was found that PM_2.5,OC is about 5 times higher in winter than in summer.Similarly,the EC in winter is four times higher than in summer.Throughout this campaign,the average value of WSOC has increased for three times in winter.The average concentration of water-insoluble carbon was found to be greatly increased in winter compared to water-soluble carbon.Among various carbonaceous compounds,the SOC is nearly 7times in winter higher than that in summer,which indicates that the secondary organic aerosol in winter has a great contribution to winter smog in Tianjin.Summer OC and EC are significantly lower than most cities in China,sometimes lower than southern China.However,the OC observed during the winter stayed at a high level,only slightly lower than Xi'an.The EC observed in Tianjin is lower than that reported in most Chinese cities,but it is comparable to that reported by Taiyuan,indicating that the contribution of carbonaceous aerosols produced by coal combustion in Tianjin should be significant.Compared with previous years,the EC content is greatly reduced,but the OC load is still at a high level,especially in winter.In addition,PM_2.5 reaches its maximum in winter.With such high loading of PM_2.5 and OC,SOC indicates that Tianjin's winter aerosol pollution is very serious,and local or regionally transmitted coal emissions should be the main source,especially in winter.When observing the trend of each bulk component in different seasons in winter and summer,only the secondary organic carbon in summer showed a diurnal trend,and the daytime was higher than the nighttime.It is very likely that strong sunshine and high temperature should promote the formation of summer SOC.In addition,WIOC is a major part of winter OC,WIOC,SOC and WSOC show similar trends during the winter,suggesting that they should come from the same/similar sources or formation way.Organic carbon in winter showed a strong positive correlation with EC,while was weak in summer.A good positive correlation in winter indicates that organic carbon and elemental carbon have a common source.OC should mainly come from heating fossil fuel combustion and/or biomass burning in winter,while the source of OC in summer is more complicated.The concentration of total dicarboxylic acid,total oxoacids and?-dicarbonyl compounds in Tianjin is 2 or 3 times in winter higher than that in summer,respectively.All of the normal saturated diacids,branched diacids,unsaturated diacids and polyfunctional diacids,oxoacids and dicarbonyl compounds exhibited high concentrations in winter,with the exception of ketomalonic acid and 4-ketoheptanoic acid.The total concentration of diacids and related compounds in winter in Tianjin is significantly higher than the average level of fourteen Chinese cities.In addition,the concentration of organic acids is higher than that in Jeju Island.These comparisons indicate that Tianjin aerosols are enriched in dicarboxylic acids and related compounds,especially in winter.Among the molecular distributions of dicarboxylic acids,oxalic acid is still the predominant species,followed by succinic acid,malonic acid and sebacic acid.Among the unsaturated diacids,the concentration of phthalic acid is the highest,followed by terephthalic acid.Both seasons showed abundant phthalic acid and C₉ acid,indicating that biomass burning and anthropogenic sources have an important contribution to Tianjin aerosol.A significant increase in winter for terephthalic acid indicates plastic waste combustion or industrial emission combustion is a great contribution to winter aerosols in Tianjin.The proportion of saturated diacids and polyfunctional diacids in the total diacid and its related compounds in winter is greatly reduced,and the proportion of the other classes of diacids or their related compounds increases as their concentration increases.Most diacids and related compounds show high levels during the day,especially for oxalic acid,malonic acid,succinic acid,phthalic acid,glyoxylic acid,pyruvic acid and glyoxal,which indicated that aerosol are more susceptible to secondary oxidation during the day time.Methylglyoxal showed a higher concentration on summer nights,which may be related to the release of large amounts of isoprene precursors from terrestrial plants during summer night.Total dicarboxylic acid in summer showed a distinctly different trend from the total oxoacid and dicarbonyl compounds with a higher level at daytime than that of night,indicating that dicarboxylic acid in summer is mainly from long range transport,while these three types of compounds showed similar trends in winter,indicating that the secondary photochemical oxidation process are similar.As for the temporary variation of individual species,malonic acid,adipic acid and sebacic acid in short-chain dicarboxylic acids and unsaturated phthalic acid showed significant diurnal changes in mid-July,while oxalic acid and malonic acid,the trend of succinic acid and its similarity,only glutaric acid maintained a constant level,the large peak occurred in winter,which is likely to be carried by the air mass of the day to Tianjin aerosol.Branched saturated dicarboxylic acids have no apparent similar fluctuations,indicating that such diacids should be derived from other precursors or other atmospheric chemical processes.Origins and secondary formation and transformations of carbonaceous components and diacids and related compounds in different seasons:stable isotope is a more accurate method used to identify source of aerosols.The range of stable carbon isotope in summer is equivalent to comparable to the higher carbon isotope ratios reported in the particles emitted from C₃ plant,which indicates that biomass burning is important in summer.The increase of the ratios C₂/?C₂,C₂/Pyr,and C₂/glyoxal in summer of Tianjin indicates the important role of isoprene emissions and subsequent photochemical oxidation to the summer aerosols.In winter,mainly from local or regional transformation of coal combustion and biomass burning,in addition,phthalic acid/adipic acid also showed a high value in winter,again indicated the pollution caused by anthropogenic sources such as coal combustion.The abundance of secondary organic carbon in winter is up to 58%,so the haze in winter in Tianjin is mainly formed by the combination of local coal combustion and secondary oxidation.At the same time,the long-chain dicarboxylic acid(C₆-C₁₂)has accounted for a higher abundance in saturated dicarboxylic acids during winter that is opposite to the short-chain dicarboxylic acid?C₂-C₄?,and correlated well with phthalic acid in winter,which indicates that long-chain dicarboxylic acids are more affected by human activities in winter.In addition,long-chain dicarboxylic acid showed a fairly good correlation with potassium ions in whole campaign which is regarded as a marker of biomass burning,indicating that biomass burning is important for long-chain dicarboxylic acids in both seasons,while short chains should have been produced by fossil fuel combustion and biomass burning or biological emissions as well as significantly produced from photochemical process of gaseous organic species,which is represented by a positive correlation with sulfate?r²=0.7?.The degree of aging of Tianjin aerosol can be evaluated from the correlation between stable carbon isotope ratio and water-soluble organic carbon and the abundance of some total diacids.When the aerosol aged,the ratio of stable carbon isotope increases,and the ratio of stable carbon isotope in summer is positively correlated with water-soluble carbon,indicating that the aerosol in summer has already aged before its'arrival in Tianjin.The total dicarboxylic acid-C normalized by TC in summer ranged from 1.79%to 4.42%,with an average of 2.82%.This average is nearly twice that of winter?range:0.55-3.87%,average:1.79%?In addition,malonic acid/succinic acid has a high average value in summer and has obvious diurnal variation,and maleic acid/fumaric acid shows a lower value,indicating that summer aerosol has subjected to intensive photochemical reaction.Meanwhile,the ratio in the summer of 2016 is comparable to that of the Taishan aerosol,higher than that of Chennai?1.58%?and Tokyo city?0.95%?.but lower than marine aerosols?8.8%?,in addition,the backward air mass trajectory indicates that Tianjin summer aerosols are mainly affected by air masses from the Pacific Ocean and passed through the Bohai Sea,based on the above comparison and meteorological studies suggested that summer aerosol in Tianjin experienced a serious photochemical aging process during long range transport,but less aged than the aerosol of ocean.The formation pathway of oxalic acid in summer is different during the day and night.In the evening,it is produced by the oxidation of methylglyoxal by?C₂ as an intermediate,moreover,mGly is caused by oxidation of isoprene emitted from terrestrial plants.While at day time,oxalic acid is formed by the oxidation of malonic acid and succinic acid.In cold season,aqueous phase secondary formation of oxalic acid is more prominent characterized by the good positive relation by glyoxalic acid and sulfate between oxalic acids well as higher humidity?max:80%?.In addition,the relationship between intermediate,precursors and the final product of oxalic acid indicated that the replenishment of intermediate product is faster than that of consumption to end product.