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| Titel |
Quantifying the magnitude of a missing hydroxyl radical source in a tropical rainforest |
| VerfasserIn |
L. K. Whalley, P. M. Edwards, K. L. Furneaux, A. Goddard, T. Ingham, M. J. Evans, D. Stone, J. R. Hopkins, C. E. Jones, A. Karunaharan, J. D. Lee, A. C. Lewis, P. S. Monks, S. J. Moller, 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 ; 11, no. 14 ; Nr. 11, no. 14 (2011-07-22), S.7223-7233 |
| Datensatznummer |
250009942
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| Publikation (Nr.) |
 copernicus.org/acp-11-7223-2011.pdf |
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| Zusammenfassung |
| The lifetime of methane is controlled to a very large extent by the
abundance of the OH radical. The tropics are a key region for methane
removal, with oxidation in the lower tropical troposphere dominating the
global methane removal budget (Bloss et al., 2005). In tropical
forested environments where biogenic VOC emissions are high and NOx
concentrations are low, OH concentrations are assumed to be low due to rapid
reactions with sink species such as isoprene. New, simultaneous measurements
of OH concentrations and OH reactivity, k'OH, in a Borneo rainforest are
reported and show much higher OH than predicted, with mean peak
concentrations of ~2.5×106 molecule cm−3 (10 min
average) observed around solar noon. Whilst j(O1D) and humidity were
high, low O3 concentrations limited the OH production from O3
photolysis. Measured OH reactivity was very high, peaking at a diurnal
average of 29.1±8.5 s−1, corresponding to an OH lifetime of only
34 ms. To maintain the observed OH concentration given the measured OH
reactivity requires a rate of OH production approximately 10 times greater
than calculated using all measured OH sources. A test of our current
understanding of the chemistry within a tropical rainforest was made using a
detailed zero-dimensional model to compare with measurements. The model
over-predicted the observed HO2 concentrations and significantly
under-predicted OH concentrations. Inclusion of an additional OH source
formed as a recycled product of OH initiated isoprene oxidation improved the
modelled OH agreement but only served to worsen the HO2
model/measurement agreement. To replicate levels of both OH and HO2, a
process that recycles HO2 to OH is required; equivalent to the OH
recycling effect of 0.74 ppbv of NO. This recycling step increases OH
concentrations by 88 % at noon and has wide implications, leading to much
higher predicted OH over tropical forests, with a concomitant reduction in
the CH4 lifetime and increase in the rate of VOC degradation. |
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