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| Titel |
Wavelength dependence of isotope fractionation in N2O photolysis |
| VerfasserIn |
J. Kaiser, T. Röckmann, C. A. M. Brenninkmeijer, P. J. Crutzen  |
| 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 ; 3, no. 2 ; Nr. 3, no. 2 (2003-03-21), S.303-313 |
| Datensatznummer |
250000869
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| Publikation (Nr.) |
 copernicus.org/acp-3-303-2003.pdf |
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| Zusammenfassung |
In previous reports on isotopic fractionation in the ultraviolet photolysis of nitrous oxide
(N2O) only enrichments of heavy isotopes in the remaining N2O
fraction have been found. However, most direct photolysis experiments have been performed at wavelengths far from
the absorption maximum at 182 nm. Here we present high-precision measurements of the
15N and 18O fractionation constants (e) in photolysis at
185 nm. Small, but statistically robust depletions of heavy isotopes for the terminal atoms in the linear
N2O molecule are found. This means that the absorption cross sections
s(15N
14N 16O) and s(14N218O) are larger than
s(14N216O) at this specific wavelength. In contrast, the central N atom becomes enriched in
15N. The corresponding fractionation constants (±1 standard deviation) are
15e1 =
s(15N
14N
16O)/s(14N2
16O) - 1 =
(3.7±0.2)
%o
18e =
s(\14N218O)/s(14N216O) - 1 =
(4.5±0.2) %o
and
15e2 =
s(14N
15N 16O)/s(14N216O)
- 1 = (-18.6±0.5) %o
To our knowledge, this is the first documented case of such a heavy isotope depletion in the
photolysis of N2O which supports theoretical models and pioneering vacuum ultraviolet
spectroscopic measurements of 15N substituted N2O species that predict fluctuations of
e
around zero in this spectral region (Selwyn and Johnston, 1981). Such a variability in isotopic
fractionation could have consequences for atmospheric models of N2O
isotopes since actinic flux varies also strongly over narrow wavelength regions between 175 and
200 nm due to the Schumann-Runge bands of oxygen. However, the spacing between maxima and minima of
the fractionation constants and of the actinic flux differ by two orders of magnitude in the
wavelength domain. The wavelength dependence of fractionation constants in
N2O photolysis can thus be approximated by a linear fit with negligible consequences on the actual value of
the spectrally averaged fractionation constant. In order to establish this linear fit, additional
measurements at wavelengths other than 185 nm were made using broadband light sources,
namely D2, Hg/Xe and Sb lamps. The latter lamp was used in conjunction with various
interference filters to shift the peak photolysis rate to longer wavelengths. From these
experiments and existing data in the literature, a comprehensive picture of the wavelength
dependence of N2O photolysis near room-temperature is created. |
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