Photolytic fractionation of seven singly and doubly substituted nitrous oxide isotopocules measured by quantum cascade laser absorption spectroscopy

Abstract

There is strong interest in using isotopic analysis to better constrain the budget of atmospheric nitrous oxide (N2O). This interest is supported by emerging instruments that allow analysis of multiply substituted species. We have studied fractionation during UV photolysis of singly and doubly isotopically substituted molecules (isotopocules) of N2O. N2O was photolyzed in an electropolished stainless-steel reactor using a broadband laser-driven light source with bandpass filters. Isotopocule ratios were quantified at different stages of photolysis using a quantum cascade laser absorption spectroscopy (QCLAS) system. Wavelength-dependent fractionation constants were determined using the Rayleigh distillation model. The fractionation constants for photolysis with 200 nm and 214 nm bandpass filters, respectively, for the seven most abundant isotopocules of N2O (after 14N14N16O) are: 14N15N16O (456): (−48.9 ± 7.4) ‰ /(−82.4 ± 22.3) ‰, 15N14N16O (546): (−22.2 ± 5.3) ‰ /(−36.1 ± 19.6) ‰, 14N14N17O (447): (−12.7 ± 4.5) ‰ /(−21.9 ± 15.7) ‰, 14N14N18O (448): (−33.5 ± 12.0) ‰ /(−44.1 ± 29.8) ‰, 14N15N18O (458): (−80.9 ± 6.5) ‰ /(−120.9 ± 23.7) ‰, 15N14N18O (548): (−52.7 ± 10.8) ‰ /(−79.1 ± 28.5) ‰, 15N15N16O (556): (−66.9 ± 9.8) ‰ /(−110.9 ± 27.5) ‰. The fractionation constants determined here for isotopocules 456, 546, 447, 448, and 556 are in agreement with previous theoretical models employed in this study and previous experiments. For 458 and 548, the fractionation constants were determined for the first time, confirming the prediction of more negative fractionation for 15N substitution in the central position. The effect of stratospheric photolysis on the clumped isotope Δ values of tropospheric N2O was found to be modest with Δ458 = (4.0 ± 1.0) ‰, Δ548 = (−4.0 ± 1.0) ‰, and Δ556 = (−1.5 ± 1.0) ‰ at 9% photolysis. Therefore, atmospheric variations of doubly substituted N2O isotopocules will likely be dominated by the characteristics of the N2O sources, which strongly supports their value for source attribution and quantification.