The 17O Excess of Stratospheric Nitrous Oxide in Mid-latitude Air

Abstract:

Tropospheric nitrous oxide (N₂O) has a ¹⁷O excess of (0.9±0.1) ‰ relative to Vienna Standard Mean Ocean Water (VSMOW). The origin of this ¹⁷O excess is under debate: tropospheric and stratospheric in-situ sources as well as isotope fractionation and isotope exchange during biological N₂O production are all considered to make a contribution, as might the stratospheric photolysis sink. To constrain the relative contributions of the different processes and to improve our understanding of the underlying atmospheric chemical and microbial processes, more measurements are required.

We have measured the ¹⁷O excess of stratospheric samples from mid-latitudes, from altitudes between 8 and 26 km. N₂O was extracted cryogenically, separated from CO₂ and CHF₃ by a PoraPlotQ pre-column and then thermally decomposed in a gold furnace at 900 ºC. The precision for the ¹⁷O excess of a single 5 nmol N₂O aliquot was ±0.3 ‰.

This dataset significantly enhances the limited range of oxygen triple isotope measurements in mostly lower stratospheric samples reported by Cliff et al. (1999). The average ¹⁷O excess of the stratospheric samples analysed was (-0.19 ±0.46) ‰ relative to tropospheric N₂O.

Since the ¹⁷O excess of the first measurements of stratospheric air is not significantly different to that in tropospheric air, this data suggests that the ¹⁷O excess is not of stratospheric origin. This confirms the idea that the origin of the ¹⁷O excess is not due to either stratospheric photolysis or reaction with electronically excited oxygen atoms. It suggests that the origin of the ¹⁷O excess may therefore be related to tropospheric in situ sources, e.g. NH₂+NO₂ as proposed by Röckmann et al., 2001, or to microbial nitrogen conversion reactions as suggested by Kaiser and Röckmann, 2005.