Study On The Chemical Mechanism Of Fine Particles And Ozone Formation From Photooxidation Of Typical Aromatic VOCs

Atmospheric Volatile Organic compounds(VOCs)is various kind pollutants from complicated sources of anthropogenic sources as well as indusrial sources.The typical VOCs including aromatic hydrocarbon,alkanes,alkenes,aldehydes and so on.As one of the important categories in atmospheric VOCs,aromatic hydrocarbons are mainly from anthropogenic source such as motor vehicle emissions and plant solvent,which plays an important role in the formation of ozone(O3)and secondary organic aerosols(SOAs)in atmospheric photochemical oxidation.Ozone plays an important role in protecting human beings and the environment in the stratosphere.However,in the troposphere,the rise of ozone concentration will cause a series of adverse effects on human health and ecological environment.The organic constituents of atmospheric particles are mainly divided into primary organic aerosols(POA)and SOA,and SOA attract more attention due to their large proportion in aerosols and great impact on atmospheric visibility.Therefore,the study of photochemical reactions of aromatic hydrocarbons in the atmosphere is of great significance to the control of air pollution.In this study,the photochemical behavior of aromatic hydrocarbons in the atmosphere was studied in smog chamber,and the feasibility of aerosol formation of aromatic hydrocarbons in smog chamber simulation was emphatically analyzed,finally the mechanism was explained in different reaction systems.Accurate understanding of VOCs concentration in modern urban and rural atmospheric environments is of great significance for studying the photochemical reaction of VOCs in the atmosphere.In this study,the proton transfer reaction time-of-flight mass spectrometry was used to monitor ambient concentration of VOCs online in real time in winter near three sampling sites in Guangzhou Economic and Technological Development District(GETDD)(industrial source),Guangzhou Higher Education Mega Center(HEMC)(urban background source)and Pingyuan City in Dezhou(PYC)(rural source),Shandong province.The results show that in different sampling sites,the total VOCs concentration of industrial sources is higher than that of urban background sources and that of rural sources.Among the VOCs measured in GETDD in guangzhou,the proportion of nitrogen-containing volatile organic compiunds(NVOCs)was the highest,followed by oxygenated volatile organic compiunds(OVOCs).Comparatively,OVOCs accounted for the highest proportion of TVOCs in HEMC and PYC.Through the assessment of ozone formation potential(OFP),aliphatic hydrocarbons(AlHs)are the constituents with the largest contribution in industrial areas(GETDD),while aromatics hydrocarbons(AHs)in urban areas(HEMC).These dominant volatile organic compounds(AHs and AlHs)should be given priority in controlling volatile organic compound emissions.Based on the results of field observation,this study selected aromatic hydrocarbons as the research object and used smog chamber to simulate photochemical homogeneous reaction.First of all,a new dual-reactor smog chamber system was designed and constructed to characterize the performance,proving that the new dual-reactor smog chamber system can effectively compare the simulated process of VOCs photochemical transformation from gas phase to particle phase in the atmosphere.Then,the photochemical homogeneous reaction process of monocyclic aromatic hydrocarbons was successfully compared and simulated in this smog chamber system,and the feasibility of the formation of ozone and particulate matter in the homogeneous reaction process was analyzed.The results of this study show that:1.In the absence of NOx,photooxidation of monocyclic aromatic hydrocarbons can produce ozone at a concentration of about 16-28 ppb.This is caused by the oxidation of aromatic hydrocarbons promoted by a small number of free radicals generated by oxygen in the reactor.However,the formation of aerosol was not observed,and its possible mechanism was that the concentration of intermediate products after photooxidative degradation of aromatic hydrocarbons was low,so no further nuclear reaction occurred.2.Compared with free-NOx free conditions,when NOx exists,the peak concentration of ozone generation significantly increases,reaching to 230-440 ppb.Meanwhile,the number and mass concentrations of particulate matter generated by photooxidation of monocyclic aromatic hydrocarbons both increase significantly.According to the concentration and yield of photooxidation intermediates of aromatic hydrocarbons,the intermediates obtained from the degradation of aromatic hydrocarbons have the potential to generate particulate matter,and the nucleation of gaseous intermediates eventually leads to the significant increase of the observed particle phase.Therefore,the existence of NO2 promotes the generation of aerosol and is also conducive to the generation and accumulation of ozone.3.The results at different VOCs/NOx ratios showed that under the control of VOC,at low VOC/NOx(VOC/NOx<15 ppbc:ppb),the peak concentration of ozone produced was stable in a fixed range for the same single cyclic aromatic hydrocarbon.Increasing the VOC/NOx ratio is conducive to the advance of the peak ozone time and the increasing of the yield of VOC intermediates obtained by photooxidation of aromatic hydrocarbons,which will further promote the production of particle phase products and ultimately improve the yield of SO A.In addition,the results show that the amount of methyl substituents on the benzene ring reduces the oxidation of aromatic hydrocarbons,and the reduced degree of oxidation promotes the formation and accumulation of ozone.The ortho methyl group on the aromatic hydrocarbon will lead to higher yield of the generated particles in the photooxidation process.The results are helpful to understand the photochemical oxidation process of aromatic hydrocarbons in real atmosphere,and also provide experimental basis for studying the photochemical oxidation of aromatic hydrocarbons to form aerosol nanoparticles in atmospheric homogeneous process.The heterogeneous reaction process in the atmosphere is also an important path for the generation of ozone and SO A.In this study,the heterogeneous reactions of benzene and 1,2,3-trimethylbenzene with sodium chloride(NaCl)seeds were studied by simulating the photochemical reactions of benzene and 1,2,3-trimethylbenzene in a single reactor smog chamber.The results show that on the premise of the existence of NaCl particulate seeds,the liquid water in the particle phase is conducive to the transformation of N2O5 in heterogeneous phase,which promotes the formation of components of secondary organic aerosols,and it is also a key factor for O3 decline.As LWC0 increased from 0 to 349.8 μg/m3,the yield of SOA in benzene increased from 5.2%to 10.5%.Although NO2 is the controlling factor in the photooxidation experiment of benzene-NO2,the increase of NO2 has little effect on the formation of SOA in heterogeneous systems.When LWC0 increased from 0 to 85.0 μg/m3,the SOA yield increased from 1,2,3-trimethylbenzene from 3.0%to 14.7%.The results of this study indicate that liquid water in heterogeneous processes is a key factor in the photooxidation of benzene and 1,2,3-trimethylbenzene,which leading to the decrease of peak ozone and the generation of SOA.