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| Titel |
Water independent SO2 oxidation by Stabilised Criegee Intermediates from Biogenic Alkenes |
| VerfasserIn |
Mike Newland, Andrew Rickard, Luc Vereecken, Mat Evans, Amalia Muñoz, Milagros Ródenas, William Bloss |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250101604
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| Publikation (Nr.) |
 EGU/EGU2015-782.pdf |
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| Zusammenfassung |
| Biogenic VOCs account for about 90% of global VOC emissions and these are dominated by
the unsaturated hydrocarbons: isoprene (600 Tg yr-1) and monoterpenes (100 Tg yr-1).
Stabilized Criegee Intermediates (SCI) are thought to be formed in the atmosphere mainly
from reactions of unsaturated hydrocarbons with ozone. SCI have been shown in laboratory
experiments to rapidly oxidise SO2 (k > 2x10-11 cm3 s-1) and NO2 (k = 7x10-12 cm3 s-1),
providing a potentially important gas phase oxidation route for these species in the
atmosphere.
The importance of the SCI reaction with traces gases has been shown in modelling work to be
critically dependent on the ratio of the rate constants for the reaction of the SCI with these
trace gases and with H2O. Such modelling work has suggested that the SCI + SO2 reaction is
only likely to be important in regions with high alkene emissions, e.g. forests, and that
elsewhere SCI are likely to be almost entirely quenched by reaction with water, thus
negating their importance as trace gas oxidants. However, it has been shown in
laboratory experiments with small SCI that the reaction rate of SCI with water is
structure dependent, with anti-CH3CHOO reacting fast with H2O (k > 1x10-14 cm3
s-1), and syn-CH3CHOO reacting orders of magnitude slower (k < 2x10-16 cm3
s-1).
Here we present results from a series of ozonolysis experiments performed at the
EUPHORE atmospheric simulation chamber in Valencia. These experiments measure
the loss of SO2, in the presence of various biogenic alkenes (isoprene and three
monoterpenes: α-pinene, β-pinene and limonene), as a function of water vapour.
The SO2 loss shows a dependence on relative humidity for all systems studied,
decreasing with increasing relative humidity. However, for all species, there also
appears to be a fraction of the SO2 loss that shows a much lower sensitivity to relative
humidity.
We quantify the relative rates of reaction of the SCI produced in the ozonolysis of these
biogenics with water and SO2, and their decomposition rates. The results suggest that the
alkenes studied produce a mixture of SCIs with widely varying reactivity towards H2O under
atmospheric conditions. This behaviour is likely dependent on structure, in agreement with
direct observations of the small SCI CH3CHOO, and suggests that different SCIs have
different fates in the atmosphere.
The impact of these observations for the identity, abundance and behaviour of SCIs expected
to predominate in regions dominated by biogenic emissions, and their scope to act as
atmospheric oxidants for other trace gases, is discussed. |
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