Reactivity Contribution Of Volatile Organic Compounds(VOCs) In Guangzhou Urban Region

Volatile organic compounds（VOC）play an important role in the tropospheric atmosphere and are important precursors of ozone（O₃）and secondary organic aerosol（SOA）.Understanding the concentration and composition of VOCs in the urban atmosphere,emission ratio and contribution to atmospheric reactivity will help to better understand the atmospheric reaction process and propose effective measures to prevent atmospheric pollutants.In this study,VOCs were measured at an urban site in Guangzhou,one of the mega-cities in Pearl River Delta（PRD）using a gas chromatograph mass spectrometer/flame ionization detection（GC-MS/FID）and a proton transfer reaction time-of-flight mass spectrometer（PTR-ToF-MS）.OH reactivity and other parameters were continuously measured in this campaign.Based on this,the VOC concentration level,composition,chemical conversion,and contribution to atmospheric reaction activity in Guangzhou were discussed.Diurnal profile analyses show that stronger chemical removal by OH radicals for more reactive hydrocarbons during the daytime,which is used to estimate the daytime average OH radical concentration.In comparison,diurnal profiles of OVOCs indicate evidence of contributions from secondary formation.During the campaign,around 1700 ions were detected in PTR-ToF-MS mass spectra,among of which 462 ions with noticeable concentrations.VOCs signals from these ions are quantified based on sensitivities of available VOCs species.OVOC-related ions dominated PTR-ToF-MS mass spectra with an average contribution of77.2%,which mainly contained one oxygen（54.1%）and two oxygen（22.0%）compounds.Combining measurements from PTR-ToF-MS and GC-MS/FID,OVOCs contribute 57.4%to the total concentration of VOCs,which indicates that OVOCs is an important category of VOCs in the ambient atmosphere of the Pearl River Delta urban area and their concentration levels contribute significantly to the total VOCs concentration.Reactions with OH radical were the most important removal pathway for most VOC species.A photochemical age-based parameterization method was used to calculate the emission ratio of NMHCs and quantify the contributions of primary anthropogenic emissions,secondary anthropogenic formation,biogenic emissions and background to concentrations of various OVOCs.The calculated value of the concentration was basically consistent with the measured concentration.We find that secondary formation is generally important for aldehydes,while primary emissions from anthropogenic sources dominated the concentrations of ketones.Alcohols have significant fractions from primary anthropogenic emission with no observed anthropogenic secondary formation.Overall,both primary emission and secondary generation contribute to OVOCs,and as the number of oxygen atoms increases,the contribution of secondary generation also increases significantly.The OH reaction activity was used to quantify the contribution of VOCs to atmospheric chemical activity.OVOCs measured by PTR-ToF-MS contribute greatly to the OH reactivity（19.3%）,of which 8.2%comes from some common OVOC,and other measured new OVOCs account for 11.1%.In comparison,hydrocarbons account for 20.0%of OH reactivity,among which NMHCs measured by GC-MS/FID accounted for 15.1%,and NMHCs measured by PTR-ToF-MS accounted for 4.9%.Adding up the contributions from inorganic gases（47.9%）,～12%of the OH reactivity remains as“missing”.According to the diurnal variation and correlation analysis,the missing of reactivity may be mainly derived from species that have not been measured from anthropogenic sources.