VOCs Pollution Characteristics,Source Analysis And Health Evaluation In The Southern Suburbs Of Xi’an

Volatile Organic Compounds（VOCs）are complex and diverse organic pollutants commonly existing in atmospheric environments.It is a key precursor to the formation of ozone（O₃）and Secondary Organic Aerosols（SOA）,which is closely related to environmental air quality,public health and climate change.In recent years,VOCs concentration in urban areas has been increasing,forming the SOA,which has become an important factor affecting air quality in winter.Human activities and unfavorable meteorological factors such as high humidity and static stability in winter are important reasons for the rise of near-surface VOCs in our country.However,the change of VOCs composition has an important impact on the generation of O₃,especially the presence of aldehydes and ketones in VOCs.Due to the low resonance stability of C=O bonds and C-C bonds in the molecular structure of aldehydes and ketones,aldehydes and ketones are easy to be excited by light and attacked by free radicals,thus promoting the generation of O₃.Guanzhong region is an important population gathering place in the west of China.The concentration of VOCs volatile organic compounds in this region has been increasing in recent years,especially the deterioration trend of VOCs pollution in Xi’an and its surrounding areas.Therefore,it is of great significance to study the reactivity and source analysis of VOCs in atmospheric environment for the control of urban air pollution and the protection of residents’health.In this study,LFGGC-2013 online gas chromatography analysis system was used to collect VOCs samples in the southern suburbs of Xi’an from December 2020 to November 2021,and 55 VOCs species were analyzed.Combined with meteorological parameters and ambient air quality data of the observation sites of the“National Urban Atmospheric Environment Monitoring Network”,the component characteristics,chemical activity,and source characteristics of VOCs in the southern suburbs of Xi’an were studied.On this basis,orthogonal matrix factorization（PMF）and US EPA health risk assessment model were introduced to study the local sources of VOCs and the non-carcinogenic risks and carcinogenic risks to human health,and the main conclusions were drawn as follows:（1）VOCs volume fraction was significantly higher at night than during the day,The daily variation trend of NO_x is basically consistent with that of VOCs,NO_x peaks in the morning and evening peak,while the variation trend of O₃ is opposite to that of VOCs,showing a unimodal distribution.The mean volume fraction of VOCs was 41.63±12.09ppb,VOCs showing trends in Winter>Spring>Autumn>summer.The L_OH,OFP and SOA_FP were 9.63±2.23 s^-1,205.02±53.13 ppb and 2.597±1.919μg/m³ respectively.During the observation period of Xi’an City,the 60%concentration target of O₃ emission reduction can be achieved under the scenarios of NO_x:VOCs=0:1 and 1:3,When NO_x:VOCs=0:1,O₃ emission reduction rate is the fastest.（2）Characteristic ratio method（T/B）shows that winter,spring,and autumn are significantly affected by motor vehicle emissions.In summer,the industrial activity is strong,solvent coatings contribute significantly to VOCs.According to the PMF model,the main VOCs sources in Xi’an are:solvent volatile source,petrochemical source,diesel tail gas source,gasoline tail gas source,natural source,coal source,coating process source and oil and gas volatile source.Winter coal source（37.30%）contributed the most to VOCs,spring solvent volatile source（19.88%）and natural source（19.57%）contributed the most to VOCs,summer natural source（24.13%）contributed the most to VOCs.Autumn solvent volatile source（20.50%）and diesel tail gas source（19.43%）had a greater impact on the contribution of VOCs.The backward trajectory clustering results show that the observation period of the southern suburb of Xi’an is mainly affected by the long-distance transmission in the northwest direction of Inner Mongolia Autonomous region and Ningxia Hui Autonomous Region,and the local air mass is affected by the short distance.Regional joint prevention and control is an important strategy to effectively cope with the air pollution challenge.（3）According to US EPA health risk assessment,the average value of non-carcinogenic risk during the observation period was 6.59E-02,Spring（8.53E-02）>Winter（8.35E-02）>Autumn（6.18E-02）>In summer（3.35E-02）,the toxic VOCs in this region were no more than carcinogenic risk.The carcinogenic risk of benzene exceeded the safety threshold（1E-06）in winter（1.22E-06）,spring（2.32E-06）and autumn（1.74E-06）,and there was a carcinogenic risk.The carcinogenic risk of ethylbenzene was 1.09E-06 in winter,which exceeded the safety threshold（1E-06）,indicating was a carcinogenic risk.The total carcinogenic risk value was higher than the US EPA safety threshold in four seasons,there is a potential carcinogenic risk.