 |
| Titel |
Impacts of mechanistic changes on HOx formation and recycling in the oxidation of isoprene |
| VerfasserIn |
A. T. Archibald, M. C. Cooke, S. R. Utembe, D. E. Shallcross, R. G. Derwent, M. E. Jenkin |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1680-7316
|
| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 10, no. 17 ; Nr. 10, no. 17 (2010-09-01), S.8097-8118 |
| Datensatznummer |
250008739
|
| Publikation (Nr.) |
 copernicus.org/acp-10-8097-2010.pdf |
|
|
|
|
|
| Zusammenfassung |
Recently reported model-measurement discrepancies for the concentrations of
the HOx radical species (OH and HO2) in locations characterized by
high emission rates of isoprene have indicated possible deficiencies in the
representation of OH recycling and formation in isoprene mechanisms
currently employed in numerical models; particularly at low levels of
NOx. Using version 3.1 of the Master Chemical Mechanism (MCM v3.1) as a
base mechanism, the sensitivity of the system to a number of detailed
mechanistic changes is examined for a wide range of NOx levels, using a
simple box model. The studies consider sensitivity tests in relation to
three general areas for which experimental and/or theoretical evidence has
been reported in the peer-reviewed literature, as follows: (1)
implementation of propagating channels for the reactions of HO2 with
acyl and β-oxo peroxy radicals with HO2, with support
from a number of studies; (2) implementation of the OH-catalysed conversion
of isoprene-derived hydroperoxides to isomeric epoxydiols, as characterised
by Paulot et al.~(2009a); and (3) implementation of a mechanism involving
respective 1,5 and 1,6 H atom shift isomerisation reactions of the β-hydroxyalkenyl
and cis-δ-hydroxyalkenyl peroxy radical isomers,
formed from the sequential addition of OH and O2 to isoprene, based on
the theoretical study of Peeters et al. (2009). All the considered
mechanistic changes lead to simulated increases in the concentrations of OH,
with (1) and (2) resulting in respective increases of up to about 7% and
16%, depending on the level of NOx. (3) is found to have potentially
much greater impacts, with enhancements in OH concentrations of up to a
factor of about 3.3, depending on the level of NOx, provided the
(crucial) rapid photolysis of the hydroperoxy-methyl-butenal products of the
cis-δ-hydroxyalkenyl peroxy radical isomerisation reactions is
represented, as also postulated by Peeters et al.~(2009). Additional tests
suggest that the mechanism with the reported parameters cannot be fully
reconciled with atmospheric observations and existing laboratory data
without some degree of parameter refinement and optimisation which would
probably include a reduction in the peroxy radical isomerisation rates and a
consequent reduction in the OH enhancement propensity. However, an order of
magntitude reduction in the isomerisation rates is still found to yield
notable enhancements in OH concentrations of up to a factor of about 2, with
the maximum impact at the low end of the considered NOx range.
A parameterized representation of the mechanistic changes is optimized and
implemented into a reduced variant of the Common Representative
Intermediates mechanism (CRI v2-R5), for use in the STOCHEM global
chemistry-transport model. The impacts of the modified chemistry in the
global model are shown to be consistent with those observed in the box model
sensitivity studies, and the results are illustrated and discussed with a
particular focus on the tropical forested regions of the Amazon and Borneo
where unexpectedly elevated concentrations of OH have recently been
reported. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|