Temperature dependence of isotope fractionation in N2O photolysis

Kaiser, J, Röckmann, T and Brenninkmeijer, CAM (2002) Temperature dependence of isotope fractionation in N2O photolysis. Physical Chemistry Chemical Physics, 4 (18). pp. 4420-4430. ISSN 1463-9084

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Abstract

Stratospheric ultraviolet (UV) photolysis is the dominant sink reaction and main origin of isotopic enrichment for atmospheric nitrous oxide (N2O). To a large extent, the flux of isotopically heavy N2O from the stratosphere is responsible for the enrichment of tropospheric N2O relative to its sources at the Earth's surface. In order to simulate the stratospheric enrichments quantitatively in atmospheric models and to examine the global N2O cycle using isotope measurements, knowledge of the fractionation constants is required. However, to date, all experimental studies of isotopic enrichment in N2O photolysis have been performed at room temperature only. Here we report the first temperature-dependent (193N218O>15N14NO, similar to the absolute values. If temperature was the only parameter of influence, not only the fractionation constants themselves, but also the ratio of fractionation constants at the central to terminal nitrogen sites, ?=15e2/15e1, should decrease along the vertical stratospheric temperature gradient. These temperature effects do not help to explain the lower ? values observed in the lower stratosphere, but they are nevertheless essential ingredients for models of atmospheric isotope chemistry. We also investigate a hitherto unexplained artefact in laboratory measurements of N2O photolysis: At high degrees of conversion, N2O loss by the reaction with O(1D) becomes important, presumably due to the photochemical production and subsequent photolysis of NO2 in the reaction cell. The effect gains importance with increasing concentration and in the present study, it caused decreases in the measured fractionation constants requiring correction for initial N2O mixing ratios of 4 mmol mol-1.

Item Type: Article
Faculty \ School: Faculty of Science > School of Environmental Sciences
University of East Anglia > Faculty of Science > Research Groups > Marine and Atmospheric Sciences
University of East Anglia > Faculty of Science > Research Groups > Atmospheric Chemistry
University of East Anglia > Faculty of Science > Research Groups > Climate, Ocean and Atmospheric Sciences
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Depositing User: Rosie Cullington
Date Deposited: 15 Jun 2011 11:46
Last Modified: 30 Aug 2018 16:32
URI: https://ueaeprints.uea.ac.uk/id/eprint/32602
DOI: 10.1039/b204837j

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