Characteristics And Source Apportionment Of Atmospheric Volatile Organic Compounds In Traditional Industrial Cities Of Guanzhong Plain

Baoji,as a typical traditional industrial city in the Guanzhong Plain,is located in the key air pollution control areas in China.The three directions of it are high altitude,and it is difficult for pollutants to spread out after entering.In recent years,ozone（O₃）pollution in Baoji has become prominent,and volatile organic compounds（VOCs）are the key precursors for the formation of O₃,and it is very necessary to study VOCs to provide effective support for the scientific formulation of air pollution control countermeasures in traditional industrial cities with complex terrain like Baoji.In this study,manual sampling was carried out in traffic zone（D1）,comprehensive zone（D2）,industrial zone（D3）and scenic zone（D4,control sites）from July 2021～December2022.We analyzed 117 VOCs using gas chromatography-mass spectrometry/hydrogen ion flame detector（GC-MS/FID）and high-performance liquid chromatography（HPLC）.We analyzed the pollution characteristics and reactivity of VOCs under different time,space and air quality levels by combining ozone generation potential（OFP）,and introduced the reaction parameters and ventilation coefficients to modify the source apportionment method（M-PMF）of VOCs.Using the online VOCs data of the comprehensive zone,based on a variety of models and methods,the causes of O₃pollution in the key season（June）were analyzed from three aspects:meteorological conditions,precursors and regional transport,and the main results were as follows:（1）The ratio of VOCs in different seasons and functional areas reached a significant level,The components with the largest proportion of ratio were alkanes and oxygenated volatile organic compounds（OVOCs）,while the key components of OFP were OVOCs and alkenes,and the components that contributed most to OFP were formaldehyde,acetaldehyde,isoprene,1-pentene,ethylene,propylene,cyclopentane and isobutane.In terms of different seasons,the highest ratio of VOCs in summer was47.47ppbv,and the strongest activity was 105.06ppbv.In terms of different functional areas,the ratio of VOCs at industrial zone（D3）was the highest 60.27ppbv,the largest OFP.The ratio composition has its own characteristics,D1 isopentane,cyclopentane and ethane were rich,which greatly affected by gasoline vehicle exhaust and gasoline and LPG evaporation,D2 acetone and 2-butanone concentration was high,D3isopropanol and vinyl acetate and other industrial solvents were rich,D4 dominant species were isoprene,acetaldehyde,acetone and acetylene.From the different air quality conditions,when the air quality was good,moderate,and lightly polluted,the VOCs ratio were 53.26,43.92,and 47.35ppbv,respectively,the OFP was 104.64,95.57,and 100.24ppbv,respectively.Compared with the good air quality,when the air quality was moderate and lightly polluted,due to the photochemical reaction,the consumption of components such as alkenes and aromatic hydrocarbons increased,and a lot of O₃and OVOCs were produced.（2）Aiming at the problem that VOCs were easy to react and affected by diffusion conditions during the analysis of traditional PMF（T-PMF）,the input concentration of VOCs was corrected by using reaction parameters and ventilation coefficients,and the source apportionment method（M-PMF）was optimized,and applied to the source apportionment of VOCs in Baoji.Both M-PMF and T-PMF were input to 41 species,and 6 factors were determined.Compared with T-PMF,the ratio of aromatic hydrocarbons and alkenes increased by 57.84%and 66.67%in M-PMF,respectively,which could compensate for the ratios lost by the reaction.The analysis results of M-PMF showed that the main sources of VOCs in Baoji were gasoline vehicle exhaust and gasoline and LPG evaporation,industrial emissions and biomass and coal combustion emissions account for 23.9%,20.1%and 20.5%,respectively.The simulation effect was evaluated,the intercept range of M-PMF was 0～0.77,and the mean value of 0.15was less than T-PMF.In M-PMF,species with a slope greater than 0.8 were 3 times higher than in T-PMF.The temporal variation trend of the contribution of M-PMF and T-PMF factors was consistent with the characteristics of source emission intensity.（3）In summer,the O₃ formation in Baoji was in the VOCs control zone,the easterly wind,high temperature and lower pressure conditions were more conducive to O₃ formation,and was affected by easterly and southerly transmission.According to smog production model（SPM）and observation-based model（F0AM）,the O₃ formation in June was in the VOCs control zone for 23 days and transition zone for 7 days,with the highest sensitivity to alkenes and aromatic hydrocarbons.According to M-PMF,VOCs were important precursors,mainly from biomass and coal combustion（24.6%）,gasoline vehicle exhaust and gasoline and LPG evaporation（21%）and biogenic emissions（18.9%）.According to the O₃ budget analysis,the maximum formation rate of O₃ on the pollution day occurred at 13:00,and the average value was 9.88ppbv·h^-1.According to the conditional probability function（CPF）,the concentration of O₃,total volatile organic compounds（TVOCs）and OVOCs was high when the wind direction was easterly,and the alkane was high when the wind was westerly.According to the backward trajectory analysis,when the air quality was moderate and lightly polluted,the easterly trajectories form Xi’an and Xianyang and the south trajectories from Qinling mountain accounted for the largest proportions,48.03%and 65.5%,respectively,indicating that the transport of pollutants in eastern cities and the abundant biogenic VOCs in southern Qinling mountain would promote the occurrence of O₃pollution.