Characteristics And Photochemistry Of Volatile Organic Compounds At A Regional Background Site In Central China

Atmospheric volatile organic compounds（VOCs）samples were collected from August 2018 to April 2019 at Jinsha,a regional atmospheric background site in central China,and measured by chromatography-mass spectrometry/flame ionization detector（GCMS/FID）.Combined with the HYSPLIT model and an observation-based photochemical box model,the pollution characteristics of VOCs,the impact of air masses transport on VOCs and the photochemical reactivity of VOCs in the background area in central China were analyzed.The study is expected to provide data support and scientific basis for atmospheric VOCs and ozone pollution prevention in central China.The results showed that the annual concentration of TVOCs at Jinsha was13944.2±6943.0 pptv,with the highest concentration of alkanes,followed by alkenes,halogenated hydrocarbons,aromatic hydrocarbons and acetylene.C₂-C₅ alkanes,ethene,ethyne,isoprene,methyl chloride and so on were the main VOCs species at Jinsha.Except for isoprene,the concentrations of TVOCs,alkanes,alkenes and aromatic hydrocarbons in winter were considerably higher than those in summer,which might be due to the weaker photochemical reactions and meteorological factors were not conducive to the removal or diffusion of pollutants in winter.Air masses transport also made an impact.The concentrations of some VOC species in summer were higher than those in spring or autumn,indicating that they had stronger sources in summer.The concentrations of alkanes,aromatic and halogenated hydrocarbons were lower at noon,higher in the morning and at dusk.As the rate of isoprene emissions from plants accelerated with increasing temperature,the level of isoprene in summer was considerably higher than that in winter,and the concentration of isoprene at noon was higher than that in the morning and at dusk.Using the HYSPLIT model to study the impact of air masses transport on VOCs in the atmosphere in central China,it was found that short-distance air masses transported from the north of Jinsha made an important contribution to VOCs in the atmosphere at Jinsha.The aging process of the short-distance air masses transported from the north was the lowest in winter,reflecting that the nearby area to the north of Jinsha might have more local emissions in winter.An observation-based photochemical box model was used to simulate the photochemical reactions of HO_X（HO_X=OH+HO₂）radicals and O₃ in the atmosphere at Jinsha in summer and autumn.It was found that the concentrations of OH radical and HO₂ radical in summer were obvious higher than those in autumn,showing the stronger atmospheric reactivity in summer.In summer and autumn,the production of OH radical was dominated by the reaction between HO₂ radical and NO,the reaction between OH radical and VOCs were the main reactions for the destruction of OH radical.The reaction between OH radical and CO was also important for the loss of OH radical in autumn.The main reactions of HO₂ radical formation were RO₂+NO reactions and OH+CO reaction in summer and autumn,respectively.The HO₂ radical was mainly consumed by reaction with NO.The net O₃ production rate in summer(4.9±1.7 ppbv h^-1)was significantly higher than that in autumn(2.4±1.3 ppbv h^-1),and due to the higher concentration of NO in autumn than that in summer,the formation of O₃ was controlled by VOCs and NO_X in summer and changed to the VOCs-limited regime in autumn.The production of O₃ was dominated by the reaction between HO₂ radical and NO in summer and autumn,while the destruction of O₃ was dominated by the photolysis of O₃ in summer and the reaction between OH radical and NO₂ in autumn.