Evidence for an additional source of atmospheric N2O

Kim and Craig [1993] have shown that N2O in the stratosphere is enriched in both O-18 and N-15 relative to tropospheric N2O. It is difficult to account for this result in light of the measurements of Selwyn and Johnston [1981] and Johnston et al. [1995] indicating that isotopic fractionation associated with the known stratospheric sinks, photolysis and reaction with O(D-1), is minimal. It is suggested that the puzzle could be resolved by invoking an additional source of isotopically heavy N2O formed perhaps by reaction of N with CO3 produced by recombination of O(D-1) and CO2. This source would require a significant reduction in estimates for net, mainly biological, emission of N2O at the surface.