Study On The Pollution Characteristics Of Typical Atmospheric Volatile Organic Compounds In A Coastal Site At Qingdao And Their Oxidation Processes

Volatile organic compounds（VOC_S）play an important role in atmospheric complex pollution,and are important precursors of ozone（O₃）and secondary organic aerosol（SOA）,which have a significant impact on the formation of photochemical smog,human health and ecological environment.The contribution of VOCs oxidation to SOA generation has become a research hotspot in the field of atmospheric environment.In this study,the VOCs pollution characteristics,sources and impacts on the atmospheric environment were investigated by field observation campaign of VOCs in Mt.Lao,Qingdao from April 18 to May 19,2021.Furthermore,taking the alkenes with higher reactivity in the atmospheric environment as the research object,α-pinene,the most common monoterpene in nature,was selected and its ozone oxidation experiment was carried out using large smoke chamber simulation to study the mechanism ofα-pinene ozonolysis and the generation of SOA under different conditions.The concentration of total volatile organic compounds（TVOC）in the atmosphere at Mt.Lao station was 18.40 ppbv,including alkanes,carbonyl compounds（OVOCs）,alkenes,aromatic hydrocarbons and alkynes.The average concentration of alkanes was 11.17 ppbv,which dominated the five categories of VOCs（60.45%）.Propane,formaldehyde,acetone,butenal,pentane,butane,isobutane,propylene,acetaldehyde and ethylene were the top ten VOCs with the highest concentrations during the observation period.The diurnal variation characteristics of different VOC species are bimodal,and the peak values appear at 12:00-14:00and around 18:00,which are mainly influenced by human activities,vehicle exhaust emissions and VOCs photolysis.Incomplete combustion of cooking fuel in commercial catering industry in scenic spots of Mt.Lao leads to the formation of diurnal peak values of propane,butane and isobutane.The results of regional transport analysis showed that the sampling sites were mainly influenced by the southwest air mass,which was consistent with the dominant wind direction.Although the marine air mass accounted for a high proportion,it contributed less pollutants.Shandong Province（mainly Jinan city,Rizhao city）,the southwest direction of the Provinces of Jiangsu,Anhui and other places,the northwest direction of the North China Plain contributed to higher weights of VOCs.Based on Positive Matrix Factorization（PMF）source analysis,the contribution ratios of five major emission sources were obtained,which were liquefied petroleum gas（LPG）usage（52.2%）,OVOCs-related sources（26.5%）,combustion sources（14.0%）,gasoline vehicle exhaust emissions（5.4%）and solvent use（1.9%）.Through the analysis of VOCs reactivity at Mt.Lao,it was found that OVOCs and alkenes not only contributed greatly to OH radical activity,but also were the dominant species in ozone formation potential（OFP）.For single VOCs species,formaldehyde contributed the most to OFP,accounting for 27.42%of the total OFP of 65 VOCs sampled.In addition,acetaldehyde,propylene,n-butyraldehyde,propane and 1-butene are important OFP-contributing species.For controlling atmospheric photochemical pollution in Mt.Lao,OVOCs and alkenes are the priority control species.Aromatic hydrocarbons are the most important contributors to SOA,accounting for 89%of total SOA generation potential（SOA_P）.Toluene and m/p-xylene contribute the most,accounting for 25.22%and 18.29%of total SOA_P,respectively,with 5.68μg/m~3and 4.12μg/m~3,respectively.By analyzing the atmospheric photochemical reaction mechanism,it is found that OH radical plays a major role in atmospheric oxidation,and its diurnal concentration exhibited a single peak with higher concentration during daytime and lower concentration during nighttime,and the maximum concentration could reach 2.3×10~6 molecule cm^-3,while NO₃ is vice versa.HO₂+NO reaction is the main way of OH radical generation,up to 2.70 ppb/h,and OH+NO₂reaction and OH+CO reaction contribute the most to its consumption.NO₂+O₃ reaction is the main way of NO₃ radical generation,followed by N₂O₅decomposition reaction,NO₃ radical and NOx reaction is the main path of NO₃ consumption,among which NO+NO₃ reaction accounts for 62.9%,NO₂+NO₃ reaction accounts for 32.8%.The secondary formation mechanism of carbonyl compounds showed that RO+O₂ reactions and O₃+VOCs reaction mainly control the secondary formation of formaldehyde and acetaldehyde at Mt.Lao,among which the oxidation reactinos of CH₃O+O₂ plays an important role in the secondary formation of formaldehyde,and the oxidation of alkenes made an important contribution to the secondary formation of acetaldehyde.The autophotolysis of formaldehyde and acetaldehyde plays an important role in its consumption,and the reaction with OH radicals contributes greatly to acetaldehyde consumption.PTR-TOF-MS was used to qualitatively analyze 13 gaseous products ofα-pinene ozonylis reaction,such as acetone,norpinonaldehyde,norpinalic acid,pinonaldehyde,pinalic acid,norpinonic acid,pinalic-3-acid,norpinic acid,pinonic acid,hydroxyl-pinonaldehyde,pinic acid,hydroxy-pinonic acid and peroxo-pinic acid.Then the reaction mechanism is analyzed:O₃reacts with the C=C double bond ofα-pinene by addition reaction.POZs（Primary Ozonides）are rapidly decomposed into two kinds of Criegee intermediates due to instability.On the one hand,Criegee intermediates can react quickly with H₂O and eventually produce pinonaldehyde and pinonic acid.On the other hand,due to the rearrangement of H atoms,three different kinds of high-energy hydroperoxides are formed.Further,through a series of reactions,formaldehyde,acetone,norpinonaldehyde,norpinalic acid,pinalic acid,norpinonic acid,pinalic-3-acid,norpinic acid,hydroxyl-pinonaldehyde,pinic acid,hydroxy-pinonic acid and peroxo-pinic acid and other oxidation products are generated.In the ozonolysis experiment ofα-pinene,the particle size distribution of SOA during the reaction process was analyzed by scanning mobility particle sizer（SMPS）,and it was found that under the conditions of the same concentration ofα-pinene,higher concentration of O₃significantly promoted the formation of SOA.However,higher relative humidity（RH）and ammonium sulfate seed aerosol significantly reduced the generation rate of SOA,which requires further experimental verification and is also the future research plan of our laboratory.