Clumped isotope perturbation in tropospheric nitrous oxide from stratospheric photolysis

Abstract

Nitrous oxide (N2O) is potent greenhouse gas and source of ozone depleting NO to the stratosphere. Recent advances in mass spectrometry allow accurate measurement of multiply substituted ("clumped") N2O isotopocules, providing new constraints on the N2O source budget. However, this requires a quantification of the "clumped" N2O fractionation from stratospheric photolysis (main sink). We use time-dependent quantum dynamics and a 1-D atmospheric model to determine the effect of stratospheric photolysis on the abundances of multisubstituted N2O isotopocules in the atmosphere. The ultraviolet absorption cross sections of 557 (i.e., 15N15N17O), 458, 548, 457, and 547 are presented for the first time and used to derive altitude-dependent photolysis rates and fractionation constants. We find that photolysis alters the N2O isotopic composition with multisubstituted mass 47 isotopocules being less abundant than expected from stochastics (Δ47=-1.7‰ in the troposphere and down to -12‰ in the upper stratosphere). Key Points Stratospheric photochemistry will have a large impact on tropospheric N2O This is the first determination of the N2O isotopocule cross sections Multiple isotopes are the key to reducing uncertainties in N2O emissions budget