Heterogeneous Oxidation Of Toluene SOA Tracers By Hydroxyl Radical

Secondary organic aerosol（SOA）is an important part of PM2.5 in China.To understand the contribution of volatiles organic compounds（VOCs）,corresponding prevention and control of haze pollution can be taken at the sou rce.At present,tracer based method has been widely used to analyze the contribution of specific VOCs to SOA in the atmosphere.However,it is unclear that whether the tracer r e-acts with the oxidant.Therefore,it is important to investigate the influence of het-erogeneous oxidation process of tracers on their stability.In this study,the tracer of toluene SOA,2,3-Dihydroxy-4-oxopentanoic Acid（DHOPA）,was the aim of this experiment.The heterogeneous oxidation exper i-ments of hydroxyl radical of DHOPA aerosol and DHOPA in toluene SOA under different conditions were carried out in a flow tube system.In this experiment,the flow tube experimental system was designed and built to simulate atmospheric photooxidation.Hydroxyl radical was produced by phot o-degradation of hydrogen peroxide aqueous solution by UV-H₂O₂.The concentration of hydroxyl radical was estimated by measuring the attenuation concentration of gaseous toluene flowing into the flow tube and combining with the known kinetic data.The concentration of hydroxyl radical increased with the increase of the mass fraction of hydrogen peroxide.The range of hydroxyl radical concentration was17.62-25.482 mg/m³ in hydrogen peroxide aqueous solution with mass fraction of9.67%,15.78%,24.49%and 30%.In the heterogeneous hydroxyl oxidation experiment of DHOPA aerosol,with the increasing of hydroxyl radical concentration,the amount of hydroxyl radical oxidation of DHOPA aerosol increased gradually.The maximum of the effective rate constant for the heterogeneous oxidation of hydroxyl in the aerosol of DHOPA is 1.801×10^-17 cm³·molecule^-1·s^-1,and the minimum of the effective lifetime is 0.17h.The quasi first order rate constant and uptake coefficient of DHOPA particles d e-creased with the increasing of time.At the same time,the quasi first order rate con-stant and uptake coefficient increased with the increasing of hydroxyl radical co n-centration.The hydroxyl oxidation rate of DHOPA aerosol was higher than that of DHOPA particles,but kept at the same order of magnitude.The results showed that seed aerosol promoted the heterogeneous oxidation of hydroxyl of DHOPA in SOA,and the order was ammonium bisulfate sulfuric acid mixture>ammonium sulfate.The effective rate constant of heterogeneous oxid a-tion of DHOPA hydroxyl increased significantly when NO concentration increased from 10 ppm to 20 ppm.By comparing the effective rate constants of pure aerosol DHOPA and toluene photooxidation to DHOPA in SOA,it is clear that the coexis t-ing components of toluene SOA can inhibit the heterogeneous oxidation of hydroxyl of DHOPA,so that the effective rate constant of DHOPA in toluene SOA is0.35-0.64 times of that of pure material.Assuming that the oxidation mechanism of DHOPA in the environment of high hydroxyl radical concentration is consistent with that in the environment of low hydroxyl radical concentration,the e-fold life of DHOPA is about 5 h.After 12 h,the concentration of DHOPA decays to 9.52%.