Method Development And Applications Of Compound-specific Radiocarbon Analysis Of Dicarboxylic Acids In Aerosols

Organic aerosols（OA）typically comprise 20-90%of atmospheric fine particulate mass,and their crucial role in health and climate effects are increasingly recognized by public attention.Among the large spectrum of OA,water-soluble organic compounds have gained great scientific interest due to their important role in cloud-aerosol interactions and consequent climate effects.Low molecular weight（LMW）dicarboxylic acids（hereafter,diacids）are one of the most abundant components of the atmospheric water-soluble organic aerosols.Due to their high water-solubility and hygroscopic properties,diacids can act as cloud condensation nuclear（CCN）and ice nuclear（IN）and thus have a significant impact on climate by reflecting solar radiation.LMW diacids can be emitted from primary sources such as fossil fuel combustion and biomass burning;however,they are largely produced by secondary processes and hence can act as tracers of secondary organic aerosols（SOA）.In particular,oxalic acid（C₂）,the dominant compound of LMW diacids,is a major/benchmark of atmospheric aqueous phase photochemical reaction.A better quantitative source diagnostic of diacids would help to reduce the uncertainties in the assessment of OA,and thereby provide highly valuable insights into air pollution evolution and aerosol-climate interactions.Various approaches have been employed to provide information about the sources and atmospheric processes of diacids,such as the molecular distribution of diacids,relationships with typical source markers（e.g.,levoglucosan,methanesulphonic acid）and stable carbon isotope ratios of diacids.Nevertheless,tracking the precursor sources and atmospheric formation of diacids remains challenging,particularly owing to the lack of powerful technical tools.Radiocarbon(¹⁴C)is a unique tool to unambiguously distinguish the relative contributions of biomass-derived and fossil fuel sources.Compound-specific radiocarbon analysis（CSRA）could offer more precise information on the sources and atmospheric evolution of concerned organic compounds.In this study,we developed a new method to enable a high-purity isolation of individual diacids in aerosol samples,which is suitable for AMS quantification.The established method was successfully applied in ambient environments at several typical urban and background regions in China.The first part of this dissertation presents a novel method for CSRA of dicarboxylic acids(C₂-C₁₂)in atmospheric aerosols,which experienced strict quality control/quality analysis.Specifically,the method starts with a dibutyl ester derivatization technique to purify dicarboxylic acids from the complex sample matrix,followed by separation and harvesting of single compounds with preparative capillary gas chromatography（PCGC）in sufficient amounts for off-line natural abundance radiocarbon(¹⁴C)analysis with accelerator mass spectrometry（AMS）.We optimized the whole analytical procedures include extraction,derivatization,molecular isolation and collection,combustion,and graphitization to obtain highest method recoveries and lowest introduction of exogenous carbon.After optimized the derivatization reaction condition,PCGC performance,and solvent removal times,the full-method recovery is 60%for oxalic acid and up to 70%for other diacids.The method–inducedδ¹³C isotope fractionation was less than±0.5‰during derivatization and PCGC isolation.The study demonstrated that the two main dead carbon contamination sources（i.e.,residual solvent and column bleed）was below the detection limit and not influencing the results.The quantified total blanks introduced by chemical oxidation,PCGC purification,drying,combustion,and graphitization were in the range of 0.8±0.4μg of Modern-C and 0.2±0.1μg of Fossil-C.The corrected F_m values agreed well with the expected ones within an error of±1 SD.For oxalic acid with the largest measurement uncertainty,a sample size of>100μg of C yielded uncertainty of 1SD with total propagated error of<0.02 F_m.For diacids with higher molecular weight,the increase in uncertainty caused by decreasing sample size was not significant.A minimum size of 50μg C of ambient dicarboxylic acids is needed for credible ¹⁴C measurement.These results demonstrate that the established method can be employed to preserve the carbon isotopic signals of diacids from ambient aerosols.The successful development of a dual-isotope method for diacids opens a new analytical dimension for investigating the sources and evolution of climate-and health-affecting atmospheric SOA.As a case study,we use dual carbon isotopic fingerprints((Δ¹⁴C-δ¹³C)of specific SOA components such as dicarboxylic acids in the Pearl River Delta（PRD）region of south China.The sampling site was positioned in Heshan Atmospheric Environmental Monitoring Superstation located in southwest PRD.The backward trajectory analyses indicate that aerosol samples collected in late spring and summer are influenced by the air masses originated from South China Sea,whereas fall,winter and early spring samples were associated with pollution outflow from the industrialized and heavily populated PRD.Hence,we categorized the sampling campaign into two characteristics based on air mass transport regime,i.e.,“coastal background”and“urban outflow”.The distinct seasonality of the monsoon system was reflected in the seasonally-varying concentrations of major chemical compositions in PM_2.5 particle.With strongly continental outflows during fall to early spring,higher concentration of dicarboxylic acids,oxocarboxylic acids,α-dicarbonyls and anthropogenic water-soluble inorganic ions were observed in the PRD-downwind receptor site.The enrichment ofδ¹³C-C₂revealed that coastal background samples are influenced by photochemical aging through atmospheric transport from warm ocean.In contrast,the PRD-origin samples,which have negativeδ¹³C-C₂ values,is associated with the increased production of C₂through VOC oxidation and aqueous-phase SOA formation.This process is mainly controlled by the aerosol water content.Radiocarbon-based source apportionment suggest that fossil fuel（55%）is the dominant contributor of C₂ in urban outflow,followed by biomass burning（23%）and biogenic emission（22%）.On the other hand,biogenic emission（61%）is the dominant contributor of C₂ in urban outflow,followed by fossil fuel（33%）and biomass burning（6%）.During urban outflow,the f_fossil-C₂were lower than f_fossil-ωC₂（69%）and f_fossil-Mgly（67%）,but higher than f_fossil-C₃（33%）and f_fossil-C₄（36%）.This result further demonstrate that large fossil precursors contribute in the formation of C₂ through aqueous process.Because C₂ is a major or ultimate product of aqueous phase photochemical reactions in the atmosphere,it is likely that aqueous phase processes would significantly contribute to the formation of SOA through fossil precursors.In the third part of the paper,we investigated concentration as well as dual carbon isotopic signatures of WSOC and diacids across five regions in China.In addition,we compared the temporal and spatial variation of WSOC and diacids between these five regions.LMW dicarboxylic acids are the most abundant components in water-soluble organic aerosols,which account for 3.5-13.2%（7.4±2.8%）of WSOC.Spatially,lowest concentration(25 ng m^-3)of diacids were found in Beijing（BJ）,whereas the highest concentration(1269 ng m^-3)of diacids were found in Wuhan（WH）.C₂ is the most abundant diacids among all sampling sites,which account for 82.0±6.1%of total diacids.Seasonally,the concentration of diacids in summer were higher than those in winter samples for BJ,which should be attributed to strict emission control policy in BJ during winter.On the contrary,higher diacids concentration were observed in winter for southern cities,possibly due to the enhanced combustion activities and strongly continental outflow during winter.WSOC were enriched inδ¹³C during winter than summer,likely reflect enhanced corn residue burning and coal combustion.In contrary,C₂ were depleted inδ¹³C during winter than summer.This may be because C₂ was mainly produced from secondary formation.Enhanced photochemical aging process results the enrichment ofδ¹³C-C₂ in summer,whereas enhanced aq SOA formation process results the depletion ofδ¹³C-C₂ in winter.In winter,WSOC was more depleted in ¹³C and ¹⁴C compared to C₂,whereas C₂ was more depleted in ¹³C and ¹⁴C during summer.The comparison between WSOC and C₂ demonstrate that non-fossil components in WSOC are more easily to further oxidative aging to small molecules such as C₂.Pollution source-derived hygroscopic particles and fossil precursors facilitate the aq SOA formation processes,which results in negativeδ¹³C values and higher fossil contribution to C₂.