Atmospheric Oxidation Mechanism And Kinetics Study Of Isoprene Initiated By Chlorine Radicals

Atmospheric chemistry of volatile organic compounds?VOCs?has broad implications for regional air quality and global climate.Isoprene,as the largest biogenic volatile organic compounds?BVOCs?,has the global emissions quantity of 600 Tg C yr^-1.It has high reactivity toward atmospheric oxidants such as hydroxyl radicals??OH?,ozone?O₃?,nitrogen peroxide?NO₃?and chlorine radicals??Cl?.The contribution of?Cl to isoprene was considered to mainly be in coastal areas historically.In recent years,with the new findings of the continental source of?Cl,?Cl would play a more important role in the transformation of isoprene.However,the atmospheric reaction mechanism of isoprene initiated by?Cl has not been fully understood and revealed.Therefore,this study investigated the initiation reaction of?Cl with isoprene,the subsequent reactions and fate of important intermediates produced in the initiation reaction by using a combination of quantum chemistry calculations and kinetic modeling.The results are as follows:?1??Cl addition to terminal C-atoms of two C=C bonds of isoprene,forming C-center radicals IM_1-1 and IM_1-4 are the main reaction pathways?82.6%?and direct H-abstraction,forming allylic radicals??C₅H₇?is the minor reaction pathway?17.4%?.It is worth noting that this is the first time to confirm that direct H-abstraction is the main H-abstraction pathway by comparing the experimental reaction rates constants of H-abstraction with calculated ones for the direct H-abstraction and indirect H-abstraction.?2?The further reactions of the forming addition intermediates IM_1-1 ends after one-time O₂ addition to form methyl vinyl ketone MVK?C₄H₆O?,methacrolein MACR?C₄H₆O?and1-chloro-3-methyl-3-buten-2-one CMBO?C₅H₇ClO?finally.IM_1-4 ends after two times O₂addition and one-time H-shift?auto-oxidation mechanism?to form highly oxidized multi-functional molecules?HOMs?,aldehydes?C₅H₇ClO₃?and?OH.More importantly,the estimated contribution of?Cl on HOMs formation from isoprene is lower than that of?OH in addition pathways,implying overall HOMs yield from isoprene oxidation could be overestimated in addition pathways if the role of?Cl in transforming isoprene is ignored.?3?The forming H-abstraction active intermediates?C₅H₇ can barrierlessly combine with O₂ to form peroxy radicals IM_6-2.On the condition of low nitric oxide?NO?concentration,IM_6-2 mainly undergoes two times successive cyclization and O₂ addition to finally form2-alkoxy-acetaldehyde?C₂H₃O₂?and 3-hydroperoxy-2-oxopropanal?C₃H₄O₄?,clarifying a novel cyclization-driven auto-oxidation mechanism.On the condition of high concentration of NO,NO can mediate the cyclization-driven auto-oxidation process to form various products such as alkoxy radicals?C₅H₇NO₃??and organonitrate?C₅H₇NO₇?which partially belongs to HOMs.In summary,this study reveals the atmospheric oxidation mechanism and kinetics of isoprene initiated by?Cl.The studies shown that the reaction of?Cl with isoprene can form HOMs through two different auto-oxidation mechanisms.These results are of great significance for understanding the atmospheric chemistry of isoprene and the chemistry of?Cl.In order to evaluate the contribution of?Cl chemistry to HOMs further,more attention should be paid to the atmospheric oxidation of organic compounds by?Cl.