Photo-induced isotopic fractionation of stratospheric N2O

Abstract

Context Abstract: N2O has been identified in the Kyoto Protocol as one of the six greenhouse gases for which anthropogenic emissions should be regulated, however, regulation procedures may not be implemented until a well-defined N2O budget has been established. The measurement of N2O isotopic fractionation provides a potential means for constraining the global budget since biological and anthropogenic sources have distinctly different isotopic signatures.Main Abstract: This paper shows that N2O isotopic fractionation in the stratosphere may be understood within the limits of the standard photochemical models if mass-dependent photodissociation rates for the various N2O isotopomers are incorporated. Thus, we conclude that there is no demonstrable reason to invoke a significant chemical source of N2O in the middle atmosphere. This paper presents a general theory for isotopomer dependent photodissociation rates that accounts for the isotopic fractionation observed in stratospheric N2O and how photodissociations appear to be both a source and a sink of N2O in the middle atmosphere. Photo-induced isotopic fractionation effects (PHIFE), explain the distinct fractionation signatures found for 15N/14N and 18O/16O ratios in both laboratory and remote sensing measurements. Furthermore, PHIFE predicts substantially different isotopic fractionations in the stratosphere for the isotopomers 15N14N16O and 14N15N16O, 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 N2O in the middle atmosphere and that N2O multi-isotope correlations should prove a useful measure of stratospheric air parcel history.