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| Titel |
NOx cycle and the tropospheric ozone isotope anomaly: an experimental investigation |
| VerfasserIn |
G. Michalski, S. K. Bhattacharya, G. Girsch |
| 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 ; 14, no. 10 ; Nr. 14, no. 10 (2014-05-21), S.4935-4953 |
| Datensatznummer |
250118725
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| Publikation (Nr.) |
 copernicus.org/acp-14-4935-2014.pdf |
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| Zusammenfassung |
| The oxygen isotope composition of nitrogen oxides (NOx) in the
atmosphere is a useful tool for understanding the oxidation of NOx
into nitric acid / nitrate in the atmosphere. A set of experiments was
conducted to examine change in isotopic composition of NOx due to
NOx–O2–O3 photochemical cycling. At
low NOx / O2 mixing ratios, NOx became progressively and nearly
equally enriched in 17O and 18O over time until it reached a steady
state with Δ17O values of 39.3 ± 1.9‰ and
δ18O values of 84.2 ± 4‰, relative to the isotopic
composition of the initial O2 gas. As the mixing ratios were increased, the
isotopic enrichments were suppressed by isotopic exchange between O atoms,
O2, and NOx. A kinetic model was developed to simulate the
observed data and it showed that the isotope effects occurring during O3
formation play a dominant role in controlling NOx isotopes and, in
addition, secondary kinetic isotope effects or isotope exchange reactions are
also important during NOx cycling. The data and model were
consistent with previous studies which showed that the NO + O3 reactions
occur mainly via the transfer of the terminal atoms of O3. The model
predicts that under tropospheric concentrations of NOx and O3,
the timescale of NOx–O3 isotopic equilibrium ranges from hours
(for ppbv NOx / O2 mixing ratios) to days (for pptv mixing
ratios) and yields steady state Δ17O and δ18O values of
45‰ and 117‰ respectively (relative to Vienna Standard Mean Ocean Water (VSMOW)) in both cases.
Under atmospheric conditions when O3 has high concentrations, the
equilibrium between NOx and O3 should occur rapidly (h) but
this equilibrium cannot be reached during polar winters and/or nights if the
NOx conversion to HNO3 is faster. The experimentally derived
rate coefficients can be used to model the major NOx–O3
isotopologue reactions at various pressures and in isotope modeling of
tropospheric nitrate. |
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