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| Titel |
The influence of small-scale variations in isoprene concentrations on atmospheric chemistry over a tropical rainforest |
| VerfasserIn |
T. A. M. Pugh, A. R. MacKenzie, B. Langford, E. Nemitz, P. K. Misztal, C. N. Hewitt |
| 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. 9 ; Nr. 11, no. 9 (2011-05-04), S.4121-4134 |
| Datensatznummer |
250009693
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| Publikation (Nr.) |
 copernicus.org/acp-11-4121-2011.pdf |
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| Zusammenfassung |
| Biogenic volatile organic compounds (BVOCs) such as isoprene constitute a
large proportion of the global atmospheric oxidant sink. Their reactions in
the atmosphere contribute to processes such as ozone production and secondary
organic aerosol formation. However, over the tropical rainforest, where
50 % of the global emissions of BVOCs are believed to occur, atmospheric
chemistry models have been unable to simulate concurrently the measured
daytime concentration of isoprene and that of its principal oxidant, hydroxyl
(OH). One reason for this model-measurement discrepancy may be incomplete
mixing of isoprene within the convective boundary layer, leading to
patchiness or segregation in isoprene and OH mixing ratios and average
concentrations that appear to be incompatible with each other. One way of
capturing this effect in models of atmospheric chemistry is to use a reduced
effective rate constant for their reaction. Recent studies comparing
atmospheric chemistry global/box models with field measurements have
suggested that this effective rate reduction may be as large as 50 %;
which is at the upper limit of that calculated using large eddy simulation
models. To date there has only been one field campaign worldwide that has
reported co-located measurements of isoprene and OH at the necessary temporal
resolution to calculate the segregation of these compounds. However many
campaigns have recorded sufficiently high resolution isoprene measurements to
capture the small-scale fluctuations in its concentration. Assuming uniform
distributions of other OH production and loss processes, we use a box model
of atmospheric chemistry, constrained by the spectrum of isoprene
concentrations measured, as a virtual instrument, to estimate the variability
in OH at a point and hence, to estimate the segregation intensity of isoprene
and OH from high-frequency isoprene time series. The method successfully
reproduces the only directly observed segregation, using measurements made in
a deciduous forest in Germany. The effective rate constant reduction for the
reaction of isoprene and OH over a South-East Asian rainforest is calculated
to be typically <15 %. Although there are many unconstrained
uncertainties, the likely nature of those processes suggests that this value
represents an upper limit. The estimate is not sensitive to heterogeneities
in NO at this remote site, unless they are correlated with those of isoprene,
or to OH-recycling schemes in the isoprene oxidation mechanism, unless the
recycling happens in the first reaction step. Segregation alone is therefore
unlikely to be the sole cause of model-measurement discrepancies for isoprene
and OH above a rainforest. |
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