A Simulation Study Of Secondary Organic Aerosol Formation Process From The Photooxidation Of Isoprene By Using An Outdoor Photochemical Smog Chamber

Secondary organic aerosol(SOA)formation mechanism has become a frontier of atmospheric chemistry because of its greater harm to environment,climate and human health.Recent studies indicated that the contribution of isoprene to the formation of SOA in the atmosphere could not be ignored.However,the process and mechanism of SOA formation from isoprene were not yet clear.In this study,SOA formation mechanism of isoprene was studied using CARES outdoor photochemical smog chamber under ambient air conditions.Based on the simulation experiments,the SOA yield,SOA composition characteristics,the atmospheric photochemical reaction mechanism of isoprene and the influence factor of SOA yield were investigated.Before carrying out the smog chamber simulation experiment,the characteristics,mixing property and wall effects of the chamber were characterized.The results showed that the smog chamber had good sealing performance,and the mixing time in the chamber was less than 3.5 min.The decay rate of O3 was 7×10-6s-1,the decay rate of NO2 was 7×10-6s-1,isoprene wall loss rate constant was 6×10-6 s-1,the wall loss of the number secondary organic aerosol concentrations was 6.6×10-5 s-1,its life cycle is 4.2 h.The results showed that the simulation system of large outdoor photochemical smog chamber meet the require of isoprene photochemical experiments.In the isoprene photochemical reaction,the initial concentration ratio of[isoprene]/[NOx]was varied from 0.01 to 0.22,the experimental results showed that the increase of ozone in the light irradiation phase is relatively rapid,the particles number concentration is high and the particle size increases rapidly,and the SOA production also higher at high[isoprene]/[NOx]ratio stations.The gaseous products were mainly formaldehyde,methacrolein,glyoxal and methylglyoxal,the concentration maximum range of formaldehyde was 46.28-257.60 ?g/m3,methacrolein was 108.62-155.30?g/m3,glyoxal was 0.56-78.75?g/m3,methyl methylglyoxal was 0.75-66.41 ?g/m3.The concentration of these carbon compounds began to increase gradually after light irradiation,and the maximum time was basically consistent with the maximum time of O3 concentration.The main oligomer products were 2-methyltetrols and 2-methylglyceric acid,which occupy about 24.9%-56.6%and 23.15%-44.24%of the mass fraction of the detected components respectively.Studies have found that high NOx conditions are more conducive to the formation of 2-MG.In this study,the mechanism of photochemical reaction of isoprene was deduced.Under the high concentration of NOx,the SOA produced by isoprene mainly came from the important first-generation methacrolein,and the OH radical mainly react with methacrolein to produce methacryloylperoxynitrate(MPAN)by the addition reaction,and after a series of reactions will produce 2-MG oligomers,mono-nitrate oligomers and mono-acetate oligomers.In the isoprene ozonation reaction,the initial concentration ratio of[isoprene]/[O3]was varied from 0.10 to 0.26.The experimental results showed that the reaction would be promoted when the initial ratio of[isoprene]/[O3]was moderate,too low or too high would inhibit the progress of isoprene oxidation.The gaseous products were mainly formaldehyde and methacrolein,the concentration maximum range of formaldehyde was3o2.94-641.39 ?g/m3,methacrolein was 259.81-459.29 ?/m3.The concentration of carbon compounds reached the maximum value shortly after the start of reaction,and remained unchanged after that.The main oligomer products was 2-methylglyceric acid,which occupy about 48%-49.8%of the detected components,and the high concentration of O3 would inhibit the production of particulate oligomers to a certain extent.We found that methacrolein was also an important intermediate product in the process of isoprene oxidation.The results showed that the factors affecting the atmospheric chemical reaction of isoprene include[isoprene]/[NOx]and[isoprene]/[O3]initial ratios,temperature,and UV irradiation.The initial ratio of reactant concentrations affected the chemical reaction of isoprene and affects the yield of SOA.The higher temperature is conducive to improve the SOA yield,but the lower humidity is conducive to the growth of particle size and number concentration.Strong UV radiation can increase the photochemical reaction rate of isoprene and promote the generation of SOA concentration.The above results indicate that isoprene is an important source of SOA during the day and night.