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| Titel |
OH reactivity in a South East Asian tropical rainforest during the Oxidant and Particle Photochemical Processes (OP3) project |
| VerfasserIn |
P. M. Edwards, M. J. Evans, K. L. Furneaux, J. Hopkins, T. Ingham, C. Jones, J. D. Lee, A. C. Lewis, S. J. Moller, D. Stone, L. K. Whalley, D. E. Heard |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 13, no. 18 ; Nr. 13, no. 18 (2013-09-26), S.9497-9514 |
| Datensatznummer |
250085713
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| Publikation (Nr.) |
 copernicus.org/acp-13-9497-2013.pdf |
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| Zusammenfassung |
| OH (hydroxyl radical) reactivity, the inverse of the chemical lifetime of the hydroxyl radical,
was measured for 12 days in April 2008 within a tropical rainforest on Borneo
as part of the OP3 (Oxidant and Particle Photochemical Processes) project. The maximum observed value was 83.8 ±
26.0 s−1 with the campaign averaged noontime maximum being 29.1 ±
8.5 s−1. The maximum OH reactivity calculated using the diurnally
averaged concentrations of observed sinks was ~ 18 s−1,
significantly less than the observations, consistent with other studies in
similar environments. OH reactivity was dominated by reaction with isoprene
(~ 30%). Numerical simulations of isoprene oxidation using the
Master Chemical Mechanism (v3.2) in a highly simplified physical and chemical
environment show that the steady state OH reactivity is a linear function of
the OH reactivity due to isoprene alone, with a maximum multiplier, to
account for the OH reactivity of the isoprene oxidation products, being equal
to the number of isoprene OH attackable bonds (10). Thus the emission of
isoprene constitutes a significantly larger emission of reactivity than is
offered by the primary reaction with isoprene alone, with significant scope
for the secondary oxidation products of isoprene to constitute the observed
missing OH reactivity. A physically and chemically more sophisticated
simulation (including physical loss, photolysis, and other oxidants) showed
that the calculated OH reactivity is reduced by the removal of the OH
attackable bonds by other oxidants and photolysis, and by physical loss
(mixing and deposition). The calculated OH reactivity is increased by
peroxide cycling, and by the OH concentration itself. Notable in these
calculations is that the accumulated OH reactivity from isoprene, defined as
the total OH reactivity of an emitted isoprene molecule and all of its
oxidation products, is significantly larger than the reactivity due to
isoprene itself and critically depends on the chemical and physical lifetimes
of intermediate species. When constrained to the observed diurnally averaged
concentrations of primary VOCs (volatile organic compounds), O3, NOx and other
parameters, the model underestimated the observed diurnal mean OH reactivity
by 30%. However, it was found that (1) the short lifetimes of isoprene
and OH, compared to those of the isoprene oxidation products, lead to a large
variability in their concentrations and so significant variation in the
calculated OH reactivity; (2) uncertainties in the OH chemistry in these high
isoprene environments can lead to an underestimate of the OH reactivity;
(3) the physical loss of species that react with OH plays a significant role
in the calculated OH reactivity; and (4) a missing primary source of reactive
carbon would have to be emitted at a rate equivalent to 50% that of
isoprene to account for the missing OH sink. Although the presence of
unmeasured primary emitted VOCs contributing to the measured OH reactivity is
likely, evidence that these primary species account for a significant
fraction of the unmeasured reactivity is not found. Thus the development of
techniques for the measurement of secondary multifunctional carbon compounds
is needed to close the OH reactivity budget. |
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