Experimantal and theoretical studies of isoprene oxidation initiated by hydroxyl radical

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Title: Experimantal and theoretical studies of isoprene oxidation initiated by hydroxyl radical
Author: Park, Ji Ho
Abstract: Isoprene (2 -methyl -1 ,3 -butadiene ) is the most abundant non -methane hydrocarbon mostly emitted from the trees and its oxidation by hydroxyl radical contributes significantly to the tropospheric ozone production . We investigate the development of a detailed predictive mechanism for isoprene oxidation using both theory and experiment . We have identified a novel cyclization pathway for the radicals formed by hydroxy radical (OH ) addition to the inner carbons of isoprene . The pathway predicted that C5 carbonyl compounds are produced , and it may also provide information on the preference of sites for OH addition . The nitrite /nitrate isomerization is directly related to the competition between ozone production and radical termination and was investigated using variational RRKM theory coupled with the master equation . We find that the dominant fate of the & #946 ; -hydroxy alkoxy radicals produced from the dissociation reaction of nitrite is a prompt dissociation , whereas & #948 ; -hydroxy radicals isomerize to form dihydroxy radicals . We have performed experiments using laser photolysis (LP ) / laserinduced fluorescence (LIF ) spectroscopy to study the initial addition reaction of the hydroxyl radical to isoprene . The overall reaction rates were estimated from experiments conducted at various pressures and temperatures . The determined Arrhenius rates are k & #8734 ; (T ) = (3 .49 ?0 .46 )x10 -11exp (366 ?40 ) /T molecule -1 cm3 s -1 and k & #8734 ; (T ) = (3 .58 ?0 .18 )x10 - 11exp (356 ?18 ) /T molecule -1 cm3 s -1 , for the OH and OD addition reactions , respectively . Isoprene oxidation in the presence of O2 and NO was studied and , based on simulations to OH cycling curves , we determined a value of (9 .0 ?3 .0 )x10 -12 molecule -1 cm3 s -1 for the overall reaction rate constant of hydroxy peroxy radical with NO at 298 K . We report a rate constant for O2 addition to the hydroxy alkyl radical of (2 .3 ?2 .0 )x10 -12 molecule -1 cm3 s -1 at 298 K . We find little generation of OH from the OD initiated oxidation of isoprene , and no significant differences in OH and OD cycling , which suggests that the H -shift isomerization is the major pathway for & #948 ; -hydroxy alkoxy radicals in agreement with theoretical predictions .
URI: http : / /hdl .handle .net /1969 .1 /1343
Date: 2005-02-17

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Experimantal and theoretical studies of isoprene oxidation initiated by hydroxyl radical. Available electronically from http : / /hdl .handle .net /1969 .1 /1343 .

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