Atmos. Chem. Phys. Discuss., 2, 1735-1763, 2002
www.atmos-chem-phys-discuss.net/2/1735/2002/
doi:10.5194/acpd-2-1735-2002
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Wavelength dependence of isotope fractionation in N2O photolysis
J. Kaiser1, T. Röckmann2, C. A. M. Brenninkmeijer1, and P. J. Crutzen1,3
1Abteilung Chemie der Atmosphäre, Max-Planck-Institut für Chemie, Mainz, Germany
2Bereich Atmosphärenphysik, Max-Planck-Institut für Kernphysik, Heidelberg, Germany
3Center for Atmospheric Sciences, Scripps Institution of Oceanography, University of California, San Diego, USA

Abstract. 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(15N14N16O) 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(15N14N16O)/s(14N216O) - 1 = (3.7 ± 0.2)%o
18e = s(14N218O)/s(14N216O) - 1 = (4.5 ± 0.2)%o  and  
15e2 = s(\chem{14N15N16O)/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 is created.


Citation: Kaiser, J., Röckmann, T., Brenninkmeijer, C. A. M., and Crutzen, P. J.: Wavelength dependence of isotope fractionation in N2O photolysis, Atmos. Chem. Phys. Discuss., 2, 1735-1763, doi:10.5194/acpd-2-1735-2002, 2002.
 
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