Based On High-resolution Online Observations: Composition Characteristics And Sources Of Volatile Organic Compounds In Typical Areas Of Southwest Chin

Volatile organic compounds（VOCs）are major trace elements in the atmospheric troposphere,which are associated with the formation of ozone and aerosols in polluted air and play an important role in the formation of photochemical smog.Biogenic volatile organic compounds（BVOCs）and oxygenated volatile organic compounds（OVOCs）are two important subgroups of VOCs.OVOCs,such as aldehydes,ketones,and alcohols,are mainly produced by the oxidation of biological and anthropogenic hydrocarbons and other organic matter,and also by direct emissions from vegetation,biomass burning,fossil fuel combustion,and industry.They would be removed through OH oxidation,photolysis as well as deposition/surface absorption.OVOCs also affect the oxidative capacity of the atmosphere.Several studies have found that carbonyl radicals are one of the most important sources of free radicals,especially in polluted urban environments.BVOCs are mainly composed of isoprene,terpenes,alkanes,olefins,alcohols,esters,carbon esters,acids,etc.Since both BVOCs and OVOCs have an ozone-producing potential worldwide,a quantitative understanding of the sources of VOCs plays an important role in combating urban air pollution,while the role of OVOCs in the southwest remains largely unexplored,thus reliable observational studies are lacking.In this study,proton transfer reaction mass spectrometry（PTR-MS）was used for continuous online observation,（PTR-MS）which has been widely used to study the emission,distribution and chemical evolution of volatile organic compounds in the atmosphere.The application of PTR-MS has greatly contributed to the understanding of VOCs sources and their role in air quality problems.Therefore,in order to understand the source of VOCs in the atmosphere in southwest China,two typical areas in southwest China,Kunming（Yunnan-Guizhou Plateau）and Chengdu（Sichuan Basin）,PTR-MS,were selected for 10-40 days of continuous online observation.It mainly measured 18 VOCs in Spring on the Yunnan-Guizhou Plateau and winter in the Sichuan Basin,including eight OVOCs（acetaldehyde,acrolein,acetone,MEK,MVK+MACR,MIK,pentanone,ethyl acetate）,two BVOCs（isoprene,α-pinene）,seven aromatic hydrocarbons（benzene,toluene,styrene,C8,C9,dichlorobenzene,naphthalene）and acetonitrile.During the observation period,the concentration level and diurnal variation characteristics were analyzed,the ozone generation potential was calculated,the key components were identified,and the source of pollutants was analyzed by photochemical age parameter method,in order to deepen the scientific understanding of the causes of near-ground-level ozone.The total concentration of VOCs observed in the 18 main observed VOCs on the Yunnan-Guizhou Plateau was（10.97±5.21）×10^-9,and the total concentration of eight OVOCs was（7.49±3.10）×10^-9.The concentration of the two BVOCs was（1.32±0.79）×10^-9;The total concentration of the six aromatic hydrocarbons was（1.50±1.14）×10^-9.The ozone production potential of isoprene,acetaldehyde and 2-acrolein is among the top three.The main sources of the three OVOCs species（acetaldehyde,acetone and MEK）were local biological sources and anthropogenic primary sources,and the background sources of acetaldehyde and acetone accounted for a relatively high proportion,indicating that the pollution in this area was significantly affected by regional transport.The total concentrations of VOCs observed in 18 mains observed in Chengdu were（29.96±16.15）×10^-9,of which the total concentrations of eight OVOCs were（17.93±8.89）×10^-9,followed by aromatic hydrocarbons（8.08±5.14）×10^-9,BVOCs were（2.91±1.64）×10^-9,and the smallest concentrations were acetonitrile（1.04±0.47）×10^-9.The photochemical age parameter method was used to analyze the source of VOCs,and the main sources of pollutants in Chengdu were primary emission sources and secondary generation sources,and their cumulative contribution rate was as high as64%-82%.Since the importance of biological sources for formaldehyde formation is almost comparable to man-made olefins during the day.Anthropogenic olefins and alkanes dominate the photochemical production of other organic volatile organic compounds such as acetaldehyde,acetone and MEK.The relative variation of formaldehyde,acetaldehyde,methyl vinyl ketone,and methacrylenone（MVK+MACR）before and between the pollution control period reflects that current mechanisms can largely explain the secondary production of these species in urban conditions.