Clumped isotope perturbation in tropospheric nitrous oxide from stratospheric photolysis

Nitrous oxide (N 2 O) 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”) N 2 O isotopocules, providing new constraints on the N 2 O source budget. However, this requires a quantification of the “clumped” N 2 O 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 N 2 O isotopocules in the atmosphere. The ultraviolet absorption cross sections of 557 (i.e., 15 N 15 N 17 O), 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 N 2 O isotopic composition with multisubstituted mass 47 isotopocules being less abundant than expected from stochastics (Δ 47  = −1.7‰ in the troposphere and down to −12‰ in the upper stratosphere).