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| Titel |
Branching ratios between the abstraction and addition channels in the reactions of OH radicals with monoterpenes |
| VerfasserIn |
C. Rio, J. C. Loison, F. Caralp, P. M. Flaud, E. Villenave |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250024144
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| Zusammenfassung |
| Secondary Organic Aerosol (SOA) formation in the atmosphere is described as a mass
transfer of volatile organic compound oxidation products with low vapour pressures in
particular phases. Among the different aerosol components, the SOA represent an important
fraction, but, the fundamental processes governing their physics and chemistry in the
atmosphere are poorly understood. So it is important to characterize and understand the
mechanisms of their formation.
It is well-known that atmospheric oxidation of monoterpenes is an important process in
tropospheric SOA formation. Consequently, the identification and quantification of reaction
products from the oxidation of monoterpenes in the gas phase have been receiving great
attention over the past years.
However, the atmospheric degradation leads to the formation of a plethora of reaction
products and proceeds through a very complex mechanism that is still not fully
characterised.
In our study, we have focused on SOA formation from OH + monoterpene reactions and
more precisely on the primary oxidation steps of γ-terpinene and d-limonene by OH radicals.
Indeed, the primary reaction of monoterpenes with hydroxyl radicals can in principle
occur by two reaction pathways: OH-addition and H-abstraction. In this work, we
have determined branching ratios of these reactions. Although there seems to be a
consensus in the literature that OH-monoterpene reactions proceed almost exclusively by
addition, several measurements have shown that in some case H-abstraction can
represent up to 30% of the total reaction rate constant. Therefore it is necessary to
determine this branching ratio in order to know, in particular, the main peroxy radicals
formed and propose a mechanism for the gas phase oxidation of terpene by hydroxyl
radicals.
(γ-terpinene + OH) and (d-limonene + OH) reactions have been studied i) at atmospheric
pressure, using laser photolysis coupled with UV absorption radical detection, and ii) at low
pressure, using discharge-flow reactor coupled to mass spectrometry. Therefore, it has been
possible to measure the branching ratios between the abstraction and addition channels at
different pressures and to discuss on H-abstraction importance for all reactions of
monoterpenes with hydroxyl radicals.
This work has shown that, contrary to the results of the available literature, H-abstraction
is a significant reaction pathway for the reaction of monoterpenes with hydroxyl radical.
Therefore, oxidation products resulting from the H-abstraction should not be neglected in
mechanisms describing the reaction of monoterpene + OH and SOA formation. |
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