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 β-hydroxy alkoxy radicals produced from the dissociation
reaction of nitrite is a prompt dissociation, whereas δ-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∞(T) = (3.49±0.46)x10-11exp(366±40)/T molecule-1 cm3 s-1 and k∞(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 δ-hydroxy alkoxy radicals in agreement
with theoretical predictions.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/1343 |
| Date | 17 February 2005 |
| Creators | Park, Ji Ho |
| Contributors | North, Simon W. |
| Publisher | Texas A&M University |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | 1481199 bytes, electronic, application/pdf, born digital |
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