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| Titel |
HOx radical regeneration in isoprene oxidation via peroxy radical isomerisations: secondary OH formation following hydroperoxy-methyl-butenal photolysis |
| VerfasserIn |
Jozef Peeters, Thanh Lam Nguyen, Trissevgeni Stavrakou, Jean-Francois Müller |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250052357
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| Zusammenfassung |
| To explain the unexpectedly high OH and HO2 radical concentrations recently observed over
several isoprene-rich areas, we proposed and theoretically quantified a novel isoprene
oxidation mechanism for low NOx conditions.1-3 The major features in the Leuven Isoprene
Mechanism (LIM) are (a) fast interconversion of the thermally labile β-OH-peroxy,
Z-δ-OH-peroxy and E-δ-OH-peroxy isomers of the allyl-stabilized hydroxy-isoprene
adducts; (b) fast, allyl-assisted 1,6-H-shift isomerisations of the Z-δ-OH-peroxys carrying
most of the peroxy reaction flux and yielding HO2 + 4-hydroperoxy-2/3-methyl-but-2-enals
(HPALDs); and (c) rapid photolysis of the labile HPALDs to regenerate OH. Evidence
for our new mechanism could be construed from recent literature,2 including the
observation of HPALDs in a photoreactor study at levels consistent with LIM-based
expectations.4 Also, modelling studies have demonstrated the major potential impact of the
new chemistry on the OH-level in isoprene-rich areas and have shown that LIM
can closely reproduce the unusually high observed [OH] and [HO2] in specific
areas.5,6
In the present theoretical study, employing high levels of both ab initio and statistical rate
theories, we address the detailed follow-up chemistry of the OCH-C(CH3)=CH-CH2O• and
OCH-CH=C(CH3)-CH2O• radicals co-produced with OH from HPALD photolysis. We find
that – different from the traditional-reactions scheme adopted by Archibald et al.5 – the acyl
radicals that result from the prompt 1,5-H shift of the formyl-H to the oxy-O•, follow
competing pathways governed by unimolecular peroxy radical reactions far faster
than the traditional atmospheric reactions with NOx and HO2, and yielding in part
multi-chromophore hydroperoxide intermediates as fast photolytic OH-sources. Of prime
importance for the issues at hand, it could be concluded that the co-product radicals of
HPALD-photolysis yield quasi-directly 1 to 2 additional OH radicals, bringing the total
to 2 to 3 hydroxyl radicals regenerated per HPALD photolyzed, in line with our
earlier suggestions2,6 and underscoring the potentially dominant role of HPALD
formation.
References
1. J. Peeters, T. L. Nguyen, L. Vereecken, Phys. Chem. Chem. Phys., 2009, 11,
5935
2. J. Peeters, J.-F. Müller, Phys. Chem. Chem. Phys., 2010, 12, 14227
3. T. L. Nguyen, L. Vereecken, J. Peeters, Chem. Phys. Chem., 2010, 11, 3996
4. F. Paulot, J. D. Crounse, H. G. Kjaergaard, A. Kurten, J. M. St. Clair, J. H. Seinfeld,
P. O. Wennberg, Science, 2009, 325, 730
5. A. T. Archibald, M. C. Cooke, S. R. Utembe, D. E. Shallcross, R. G. Derwent, M. E.
Jenkin, Atmos. Chem. Phys., 2010, 10, 8097
6. T. Stavrakou, J. Peeters, J.-F. Müller, Atmos. Chem. Phys., 2010, 10, 9863 |
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