Unimolecular Reactions Of Peroxy Radicals In Atmospheric Chemistry

The atmospheric oxidation mechanism of volatile organic compounds?VOCs?,being the foundation in modeling and predicting the formation of the secondary pollutants?ozone and SOA?,is one of the most important parts in atmosphere chemistry,of which the gas phase kinetics is of particular importance.In the atmosphere,simple VOCs such as small alkanes and alkenes are oxidized and degradated almost exclusively via the gas phase reactions,and the reaction kinetics in gas phase is crucial to the understanding of ozone formation from these compounds.As VOCs become more complicate,reactions in gas phase and multiphase together contribute to the kinetics are the basis of atmospheric chemistry,and are the most challenging tasks in the simulation and prediction of the entire atmospheric reaction process.However,the current mechanism model has its limitations and defects.For example,the current model significantly underestimates the concentration of OH radicals,and there are still missing links between the oxidation mechanisms and SOA formation.Further development of the oxidation mechanisms of the main VOCs and the exploration of their reaction pathways under different atmospheric conditions are crucial for us to understand the entire atmospheric chemical process,of which the core is free radical chemistry.Peroxy radicals are important intermediates in the atmospheric oxidation of VOCs.It is usually assumed that the fate of RO₂ radicals in the atmosphere is to react bimolecularly with other trace radicals such as NO,NO₂,HO₂,and other peroxy radicals.However,recent theoretical calculations and experimental studies have found that the unimolecular hydrogen shift may occur in a few important peroxy radicals.Without the involvement of NO_x,this process acts as an auto-oxidation and self-cleaning mechanism of degradating VOCs in the atmosphere.Furthermore,the products formed after unimolecular hydrogen shifts are usually characterized with high O:C ratio and contain multifunctional groups,such as,–OH,–OOH and>C=O,suggesting their high polarity and?extremely?low vapour pressures and therefore highly likely significant contributions to SOA formation.This dissertation focuses on three types of anthropogenic VOCs,i.e.,ethers,aldehydes and ketones,benzenes,and one important biogenic VOC,i.e.,isoprene.These VOCs exist widely in the atmosphere.Peroxy radicals are formed in their atmospheric oxidation initiated by OH radicals.We found that the H-shifts in all these peroxy radicals could be fast enough to compete with their possible bimolecular reactions in the atmosphere.?1?Ethers and Carbonyls:In the peroxy radicals formed in OH-initiated oxidation of dimethyl,diethyl and diisopropyl ethers,the rates for intramolecular H-shifts were obtained as0.074,1.6 and 1.1 s^–1,respectively,at 298 K and 760 Torr.For the acyl peroxy radicals?RC?O?O₂?derived from aldehydes and ketones,the rates of the H-shifts were obtained as 7.3×10^–4,0.067,and 18.0 s^–1 for R=C₂H₅,n-C₃H₇,and i-C₄H₉,respectively,at 298 K and 760Torr.These rates of H-shifts are comparable to the effective bimolecular rates of 0.01¹00s^–1 for their reactions with NO/HO₂ under typical atmospheric conditions.Thus,these intramolecular H-shifts will compete with the bimolecular reaction,affecting the final products and their branching ratios.The hydrogen shift reactions would be particulaly important when the atmospheric NO/HO₂ concentrations are low.?2?Substituted Benzenes:We have predicted theoretically and confirmed experimentally the occurrence of intramolecular hydrogen shifts in BPRs?bicyclic peroxy radicals?formed in the atmospheric oxidation of toluene,ethylbenzene and isopropylbenzene and the subsequent formation of highly oxidized multifunctional products in gas phase.The overall rates of the first intramolecular hydrogen shift in BPRs formed from toluene,ethylbenzene and isopropylbenzene are 0.026,7.0 and?8.8¹4?s^–1,respectively.And the newly formed peroxy radicals can also undergo a second or even a third hydrogen shift reactions to form highly oxidized species.The unimolecular isomerization reaction provides a possible explanation for a relatively high yield of SOA under the conditions of low NO_x in previous chamber studies.The theoretical findings are experimentally supported by flow tube studies using chemical ionization mass spectrometry to detect the highly oxidized peroxy radical intermediates and closed-shell products.?3?Isoprene:It is generally assumed that isoprene related SOA precursors are formed by second-or multi-generation reactions with OH radicals,while the primary oxidation products do not contribute to SOA formation due to their high volatility.However,we have predicted theoretically the primary formation of highly oxidized peroxy radicals,such as C₅H₉O₇ and C₅H₉O₉,in the OH-initiated oxidation of isoprene via intramolecular hydrogen shift of Z-?-ISOPO₂ and subsequent reactions,and the primary formation of the closed shell products including C₅H₁₀O₇,C₅H₁₀O₈,C₅H₈O₆,C₄H₈O₅,etc.These species are very likely important precursors of isoprene-derived aerosols.In the experiments carried out in the gas-phase flow reactor,in the absence of NO,we successfully identified these highly oxygenated peroxy radical intermediates and their corresponding closed shell compounds for the first time.In addition,we also found that the alkoxy radical Z-?-ISOPO could have similar reaction pathways.With these,we estimated the primary yields of HOMs in the OH-initiated oxidation of isoprene.With the bimolecular reaction lifetime of peroxy radicals of about 100 s?i.e.,the concentration of NO is about 50 ppt?,which is the typical atmospheric condition in forest areas,the total branching ratio of HOM formation could reach¹1%.