Photo-induced isotopic fractionation of stratospheric N2O

Context Abstract: N2O has been identi®ed 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 wellde®ned 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 di€erent 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 signi®cant 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 e€ects (PHIFE), explain the distinct fractionation signatures found for N/N and O/O ratios in both laboratory and remote sensing measurements. Furthermore, PHIFE predicts substantially di€erent isotopic fractionations in the stratosphere for the isotopomers NNO and NNO, which have identical molecular weights but di€erent isotopic substitution sites. Modeling results based on this theory suggest that there is no demonstrable reason to invoke a signi®cant chemical source of N2O in the middle atmosphere and that N2O multi-isotope correlations should prove a useful measure of stratospheric air parcel history. Ó 2000 Elsevier Science Ltd. All rights reserved.