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Description:
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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 . |