Photo-induced isotopic fractionation of stratospheric N_2O

Abstract

This paper shows that N_2O isotopic fractionation in the stratosphere may be understood within the limits of the standard photochemical models if mass-dependent photodissociation rates for the various N_2O isotopomers are incorporated. Thus, we conclude that there is no demonstrable reason to invoke a significant chemical source of N_2O in the middle atmosphere. This paper presents a general theory for isotopomer dependent photodissociation rates that accounts for the isotopic fractionation observed in stratospheric N_2O and how photodissociations appear to be both a source and a sink of N_2O in the middle atmosphere. Photo-induced isotopic fractionation effects (PHIFE), explain the distinct fractionation signatures found for ^(15)N/^(14)N and ^(18)O/^(16)O ratios in both laboratory and remote sensing measurements. Furthermore, PHIFE predicts substantially different isotopic fractionations in the stratosphere for the isotopomers ^(15)N^(14)N^(16)O and ^(14)N^(15)N^(16)O, which have identical molecular weights but different isotopic substitution sites. Modeling results based on this theory suggest that there is no demonstrable reason to invoke a significant chemical source of N_2O in the middle atmosphere and that N_2O multi-isotope correlations should prove a useful measure of stratospheric air parcel history.

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