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.