The Study On The Oxidation Degradation Mechanism Of OH Initiated Typical Isoprene Derivatives In The Atmosphere

As an important component of PM2.5,secondary organic aerosol(SOA)has an important impact on global ecology and human health.Formation and transformation of SOA in the atmosphere are important research contents in the treatment of air pollution.Isoprene emitted from plants accounts for more than half of SOA biological source.Isoprene can be triggered by various oxides to generate different substances in the atmospheric environment.Different reactions in the water phase and gas phase contribute to SOA formation.It has done a lot of research and exploration on the characteristics and regional conditions of isoprene derivatives in the environment.But reaction mechanism is still not comprehensiveIn this paper,the typical compounds 3,4-bihydroxy-2-butanone(DHBO),2,3-bihydroxy-2-methylpropanal(DHMP)and 3-methyltetraolsulfate(3-MTS)was used as the research object.With OH radical as the main initiator,the functional reaction in the water phase and the fragmentation reaction in the gas phase are in the presence of oxygen.It was studied to explore the conversion mechanism of IEPOX in the atmosphere.Density functional theory(DFT)was utilized in the mechanistic computation at the M06-2X/6-311++G(d,p)levels and 6-311++G(3df,3 dp)levels And Transition State Theory was applied to calculate the initial reaction rate constants.The following conclusions are obtained1.The reaction of DHBO and DHMP in the atmospheric aqueous phase has a significant contribution to the formation of aqSOADHBO and DHMP can react with common atmospheric free radicals OH,SO4·-,and NO3 in the liquid phase.The reaction rate decreases as the temperature increases At a temperature range of 225-375 K and 1 atm,k(OH)is 211-1116 times as fast as k(NO3)and 110-165 times as fast as k(SO4·-).The abstraction reaction barrier of OH+DHMP is slightly lower than that of DHBO.DHMP can also undergo an addition reaction with H2O with a high of 39.06 kcal mol-1.The products of the initial reaction can continue to react with OH radical and O2.The main product is acid produced by the conversion of oxygen-containing functional groups,which lifts C/O ratios of organic compounds and can enhance their ability to aggregate to form oligomers.Acetic acid and methylglyoxal generated in aqueous phase have was verified as SOA precursors.The HO2,RO2 radicals produced during the reaction promotes the tropospheric NO2/NO cycle,which increasing the ozone yield.The addition of explicit water molecules shows its excellent facilitation for tautomerization and decarboxylation because that the presence of water increases the size of the cyclic transition state and reduces the ring tension.And two water molecules may change the reaction path2.Theoretical study on the conversion of 3-MTS to inorganic sulfate in the atmospheric gas phaseThe 8 different hydrogen atoms contained in 3-MTS can be extracted by OH radical.The barriers do not exceed 20 kcal mol-1 and the minimum is only 3.07 kcal mol-1.At a temperature of 298.15 K and an atmospheric pressure of 1 atm,the total reaction rate constant is 3.05x10-9 cm3 molecule-1s-1.It was judged that its atmospheric life is only 5.63 minutes,and it is easier to degrade at low temperatures.The intermediates of 3-MTS extracted from hydroxyl can continue to undergo oxidation reaction in the presence of oxygen.The cleavages of free radicals can occur with low energy barriers,the highest being only 7.49 kcal mol-1.However,both pathways for eliminate of sulfate radicals from 1-IM3 and 2-IM6 require higher barrier over 25.39 kcal mol-1.Russell mechanism reactions generate production with a carbonyl or carboxylic acid group.Formic acid,acetic acid and formaldehyde can be generated,which can contribute to the formation of SOA.