Measurements of N2O isotopologues in the stratosphere: Influence of transport on the apparent enrichment factors and the isotopologue fluxes to the troposphere

Abstract

Stratospheric N2O is known to be enriched in the heavy isotopes 15N and 18O relative to tropospheric N2O, primarily because of the preferential photolysis of light isotopologues. We present measurements of δ15N, δ18O, and site‐specific δ15N on N2O from 32 stratospheric whole air samples collected by the NASA ER‐2 aircraft between 1997 and 2000 from 62°N to 89°N with N2O mixing ratios ranging from 51 to 313 ppbv. The relationships between the isotopic compositions and N2O mixing ratios show significant differences between aircraft deployments and with previous measurements for N2O < 200 ppbv. The differences between ER‐2 deployments at low N2O are significant at the 3σ level and are due to the effects of transport and mixing. The ratios of enrichment factors for the different isotopologues, however, are the same to within their 1σ uncertainties for N2O > 200 ppbv and N2O < 200 ppbv. The observed isotope:N2O relationships are also used to estimate the fluxes of the N2O isotopologues from the stratosphere to the troposphere given independent estimates of the N2O loss rate. On the basis of the robustness of isotope:N2O relationships for N2O > 200 ppbv we conclude that the fluxes to the troposphere estimated from these relationships and, therefore, the influence of stratosphere‐to‐troposphere transport on the isotopic compositions of N2O in the free troposphere are now relatively well quantified, leaving the isotopic compositions of the N2O sources as the remaining largest uncertainties in the global N2O isotope budget.