| In the past four decades,industrial production and urban expansion have developed rapidly,which benefited from the China's reform and open-up policies,but the costs of these achievements have also been enormous.As the economic development model still follows the traditional industrialization path with low resource utilization and large pollution emissions,the process of economic growth is accompanied by increasingly serious ecological degradation and environmental pollution.Moreover,human being is also threatened by a large amount of air pollutants at the same time.The volume fraction of volatile organic compounds?VOCs?in the atmospheric environment have rose continually,and the composition and activity characteristics are also more complex.In addition,VOCs are common precursors that can produce particles and O3.Therefore,VOCs has attracted people's attention worldwide.Nowadays,the 19th National Congress of the Communist Party of China has clearly stated that it is necessary to continuously implement air pollution prevention and control actions,establish a perfect VOCs pollution prevention and management system,complete the reduction of VOCs emissions determined by the"13th Five-Year Plan",and thus promote continuous improvement in 2020.Therefore,in-depth exploration of volatile organic compounds in ambient air has application value and significance for controlling regional air quality.Chongqing is an important industrial city in the southwest region of China,and the air pollution of Chongqing is serious,but the research results on VOCs in Chongqing are still scarce,so this gap needs to be filled through long-term observation and accumulation.Therefore,in this study,Beibei urban district of Chongqing City was selected as the research area.Atmospheric samples were regularly collected from 2012to 2017 to analyze the volume fraction,composition characteristics,seasonal variation characteristics of VOCs.Secondly,ozone generation potential?OFP?was calculated to find out the photochemical activity characteristics of VOCs.Thirdly,toluene/benzene ratio method,xylene/ethylbenzene ratio method and principal component analysis method were used to analyze the source of VOCs.Finally,the correlations between TVOCs?Total Volatile Organic Compounds?and particulate matter(PM2.5),ozone?O3?,and nitrogen oxides?NOx?were performed.This study can provide a scientific basis for improving air quality in Beibei urban district,Chongqing.The main findings are as follows:?1?During the entire observation period from 2012 to 2017,the volume fraction of atmospheric TVOCs in Beibei urban district of Chongqing varied from 0.25×10-9 to112.02×10-9,with an average volume fraction of 31.64×10-9.The composition characteristics of TVOCs were that alkane,olefins,halogenated hydrocarbons and aromatic hydrocarbons accounted for 36.81%,12.61%,14.36%and 35.59%,respectively.The six annual TVOCs emission contribution rates in the atmosphere were basically the same as the emission contribution rate during the entire observation period.Moreover.alkanes had the highest contribution to TVOCs in Beibei urban district,Chongqing,followed by aromatic hydrocarbons,and the lowest contribution was olefin.The overall seasonal distribution characteristics of VOCs components during the whole observation period were that the volume fraction of alkanes were highest in winter and lowest in summer,the volume fraction of olefins were highest in autumn and lowest in spring,the volume fraction of halogenated hydrocarbons were highest in spring and lowest in summer,the volume fraction of aromatic hydrocarbons were highest in summer and lowest in winter.In addition,aromatic hydrocarbons were the most abundant in spring and summer,followed by alkanes.In autumn and winter,alkanes were dominant,followed by aromatic hydrocarbons.In general,the volume fraction of VOCs in Beibei urban district of Chongqing was higher than that of foreign cities,but lower than that of most domestic cities and lower than that of most other districts of Chongqing.?2?Through a comparison between the mixing ratio of different VOCs components and the contribution rates of ozone generation from 2012 to 2017 in Beibei urban district,Chongqing,it was found that alkanes generally showed a higher component mixing ratio and the lowest ozone generation contribution rate,so its photochemical reaction activity was relatively low.The mixing ratios of olefin components were all below 20%,but the contribution to ozone generation was basically maintained at about40%,so its photochemical reaction activity was relatively strong.Therefore,these components generally presented the characteristics of olefins>aromatic hydrocarbons>alkanes.Among the monomer VOCs components,isoprene,m/p-xylene,propylene,trans-2-butene and some other components all had a higher contribution to the ozone generation potential.According to the OFP values in different seasons,except for 2012,the other years showed the highest OFP values in summer(maximum:170.72×10-9),followed by autumn and spring,and the lowest in winter(minimum:62.47×10-9).?3?The annual toluene to benzene ratio description from 2012 to 2017 proved that the main source of pollution from atmospheric VOCs in Beibei urban district of Chongqing was motor vehicle exhaust.However,in 2016,industrial activities may also be sources of VOCs pollution with small effect.The principal component analysis results of VOCs showed that the main source of pollution in Beibei urban district of Chongqing was motor vehicle exhaust?45.43%?,followed by various industrial production activities?28.58%?.Vegetation emissions had little impact on it,only accounting for 4.66%.The correlation analysis results of TVOCs,O3,PM2.5,and NOx showed that TVOCs were significantly negatively correlated with O3 and positively correlated with PM2.5,NOx was negatively correlated with O3.The results of the health risk assessment of VOCs in Beibei urban district of Chongqing indicated that there was no non-carcinogenic risk of VOCs to adults in this study area,but the value of carcinogenic risk was slightly higher than the standard value,indicating that VOCs in this study area had potential carcinogenic risk,of which benzene was the main carcinogen pollutant.?4?This study also put forward some reasonable suggestions for the control of VOCs in Beibei urban district of Chongqing,which mainly focued on pollution source control,online supervision and policy guarantee.Petrochemical industry,solvent use and other industrial activities and vehicle emissions were the most important sources of VOCs emissions in Beibei urban district,Chongqing,Therefore,it is possible to reduce the pollution of VOCs by promoting the prevention and control of pollution from motor vehicle exhaust and volatilization leakage,strengthening the installation of high-concentration forecasting devices at monitoring points and monitoring points in industrial areas.At the same time,online monitoring of VOCs pollution sources and improvement of relevant legal regulations can also be useful. |
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