Study On The Pollution Characteristics And Partition Mechanism Of Organophosphate Esters In The Atmosphere And Vegetation Of Xi’an

As flame retardants and plasticizers,organophosphate esters(OPEs)have been widely utilized in plastics,textiles,and building materials,and resulted in a sharp increment on production annually.OPEs were ever regarded as effective substitutes for brominated flame retardants(BFRs);however,some current researches showed that their concentrations in air were 1 to 3 orders of magnitude higher than BFRs.Since their half-lives were relatively long,combing their definite toxicity and long-range transport potential,OPEs have increasingly attracted extensive attention and become a class of global organic contaminants.Besides,OPEs were questioned whether they were still safer substitutes by some researchers.Xi’an,as the economic hub of northwest China,has developed industry,dense population and heavy air pollution.The research on organic phosphate esters in the air in this area was limited.In this study,gas phase and particle phase samples of OPEs were collected from the atmospheric environment of Xi’an City in three quarters in 2020 by using a large flow air sampler,and six plant samples were collected.The method of high performance liquid chromatography-tandem triple quadrupole mass spectrometry(HPLCMS/MS)was optimized for the analysis of OPEs in gaseous,particulate,and vegetation samples,and the seasonal variation trend of OPEs was clarified.Principal Component Analysis(PCA)and Potential Source Contribution Factor(PSCF)were used to trace atmospheric OPEs,and PCA and Hierarchical Cluster Analysis(HCA)were used to explore the sources of OPEs in plants.The predictions of four gas-particle models were evaluated,and fugacity models were used to explore the trends and fluxes of OPEs gas-particle exchange.For the first time,the pollution level and distribution of new OPEs in Xi’an City were observed and traced back by PCA and PCSF models.The time variation law of gas-particle exchange of OPEs was analyzed,and the main controlling factors were determined.The specific work is as follows:(1)The concentrations of OPEs in atmospheric gas and particulate phase samples were analyzed by HPLC-MS/MS.The results were as follows:11 kinds of gas phase OPEs were detected,the concentration range was 28.66 pg/m~3～944 pg/m~3,and the average concentration was 325.18 pg/m~3.A total of 26 kinds of particle phase OPEs were detected,with a concentration range of 231.78 pg/m~3 to 191.90 pg/m~3,and the average concentration was 629.42 pg/m~3.AlkylOPE and chlorinated OPEs accounted for the highest proportion in the atmospheric environment,and Tris(1-Chloro-2-propyl)phosphate(TCPP-1)and Tri(n-butyl)phosphate(TBP)were the highest contribution OPEs in the gas phase and particle phase,respectively.The concentration of OPEs in the gas phase was significantly higher in summer than that in winter,while the concentration in the particle phase was the opposite.The temperature was the main factor causing this change.PCA and PSCF models were used to tace atmospheric OPEs,and the sources were inferred to include plastics,rubber,electronic products,automobiles and drug synthesis.In addition,the main source areas in the light pollution period were Baoji and Hanzhong,while in the heavy pollution period,the main source areas were not only Baoji and Hanzhong but also western Henan.(2)The average concentrations of OPEs in Cedar,Photinia,Ilex,Oxalis,Hemerocallis and Moss samples were 23.79 ng/g,16.50 ng/g,23.02 ng/g,21.53 ng/g,17.55 ng/g and 20.75 ng/g,respectively.The enrichment ability of woody plants to OPEs was stronger than that of herbs and mosses.The average contribution of alkyl(49%)and chlorinated(39%)OPEs was the highest in all plants.TMP,TCPP,TCEP and TEP were the major OPEs in plant leaves.The interspecific difference of OPEs enrichment in plants was not obvious.Through the observation of leaf surface morphology by scanning electron microscope,it was found that particulate OPEs could be retained by plant leaves,and the removal efficiency of precipitation for most OPEs was more than 20%.ArylOPEs(TMTP,TPCP,TEHP and TOCP)mainly existed in the particle phase and were removed by rain washing with the foliar dust,with the removal ratio between 2%and 78%.The main removal method of TCEP,TCPP-2 and TBEP might be dissolving in rainwater,with the removal ratio between 1%and 44%.The lipid content was the main control factor for the enrichment of OPEs on the leaf surface.PCA and HCA analysis found that the OPEs in plants mainly came from the release of electronic products,transportation and mechanical devices.(3)The gas-particle distribution characteristics of OPEs were evaluated by gas-particle fraction.The results shown that OPEs were mainly distributed in the particle phase in the atmospheric environment of Xi’an(φ_g=0.43～0.97),while TCEP,TCPP-1,TCPP-2,THEP,TPrP and TnBP are mainly distributed in the gas phase in summer.Correlation analysis was shown that temperature was the main factor affecting the gas-particle partition of OPEs.With the increase of temperature,Kp decreases,and OPEs mainly exist in the gas phase,and mainly in the particle phase with the decreased temperature.(4)This study evaluated the accuracy of J-P model,H-B model and L-M-Y model in predicting gas-particle distribution behavior.It was found that J-P model had a better prediction effect for TCEP,TEP,TPrP and TnBP;H-B model and L-M-Y model had high prediction accuracy for TCPP-1,TCPP-2,TPhP,EHDPP and TDCP.The prediction accuracy of chlorinated,alkyl and aryl OPEs was as follows:L-M-Y model>H-B model>J-P model.By exploring the influence of LogKoa on the accuracy of the prediction model,it is found that the J-P model was suitable for high volatile and low fat-soluble OPEs;H-B model and L-M-Y model could be used for low-volatile and high-liposoluble OPEs.(5)In this study,based on the fugacity model theory,it was found that there were obvious seasonal differences in gas-particle exchange of most OPEs by calculating the gas-particle fugacity ratio of OPEs.In summer and autumn,there was a dynamic balance between the gas and particle phase.In winter,there was a net volatilization from the particle phase to the gas phase.TDCP has shown net volatilization of the particle phase throughout the year,and TPrP has shown dynamic equilibrium in gas-particle partition.The results show that the gas-particle exchange behavior of OPEs was affected by both the chemical properties and the emission sources.The annual exchange flux range of Xi’an was-126.68 kg/d(summer)～255.53 kg/d(winter).In autumn and winter,OPEs in the particle phase were potential secondary sources for the gas phase.