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| Titel |
Hydroxyl radicals in the tropical troposphere over the Suriname rainforest: comparison of measurements with the box model MECCA |
| VerfasserIn |
D. Kubistin, H. Harder, M. Martinez, M. Rudolf, R. Sander, H. Bozem, G. Eerdekens, H. Fischer, C. Gurk, T. Klüpfel, R. Königstedt, U. Parchatka, C. L. Schiller, A. Stickler, D. Taraborrelli, J. Williams, J. Lelieveld  |
| 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 ; 10, no. 19 ; Nr. 10, no. 19 (2010-10-15), S.9705-9728 |
| Datensatznummer |
250008829
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| Publikation (Nr.) |
 copernicus.org/acp-10-9705-2010.pdf |
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| Zusammenfassung |
As a major source region of the hydroxyl radical OH, the Tropics
largely control the oxidation capacity of the atmosphere on a global scale.
However, emissions of hydrocarbons from the tropical rainforest that react
rapidly with OH can potentially deplete the amount of OH and thereby reduce
the oxidation capacity. The airborne GABRIEL field campaign in equatorial
South America (Suriname) in October 2005 investigated the influence of the
tropical rainforest on the HOx budget (HOx = OH +
HO2). The first observations of OH and HO2 over a tropical
rainforest are compared to steady state concentrations calculated with the
atmospheric chemistry box model MECCA. The important precursors and sinks for
HOx chemistry, measured during the campaign, are used as
constraining parameters for the simulation of OH and HO2. Significant
underestimations of HOx are found by the model over land during the
afternoon, with mean ratios of observation to model of 12.2 ± 3.5 and
4.1 ± 1.4 for OH and HO2, respectively. The discrepancy between
measurements and simulation results is correlated to the abundance of
isoprene. While for low isoprene mixing ratios (above ocean or at altitudes
>3 km), observation and simulation agree fairly well, for mixing ratios
>200 pptV (<3 km over the rainforest) the model tends to underestimate
the HOx observations as a function of isoprene.
Box model simulations have been performed with the condensed chemical
mechanism of MECCA and with the detailed isoprene reaction scheme of MCM,
resulting in similar results for HOx concentrations. Simulations
with constrained HO2 concentrations show that the conversion from HO2
to OH in the model is too low. However, by neglecting the isoprene chemistry
in the model, observations and simulations agree much better. An OH source
similar to the strength of the OH sink via isoprene chemistry is needed in
the model to resolve the discrepancy. A possible explanation is that the
oxidation of isoprene by OH not only dominates the removal of OH but also
produces it in a similar amount. Several additional reactions which directly
produce OH have been implemented into the box model, suggesting that upper
limits in producing OH are still not able to reproduce the observations
(improvement by factors of ≈2.4 and ≈2 for OH and HO2,
respectively). We determine that OH has to be recycled to 94% instead of
the simulated 38% to match the observations, which is most likely to
happen in the isoprene degradation process, otherwise additional sources are
required. |
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