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Discussion papers
https://doi.org/10.5194/acp-2019-829
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-829
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 01 Oct 2019

Submitted as: research article | 01 Oct 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

The isotopic composition of atmospheric nitrous oxide observed at the high-altitude research station Jungfraujoch, Switzerland

Longfei Yu1, Eliza Harris1,a, Stephan Henne1, Sarah Eggleston1, Martin Steinbacher1, Lukas Emmenegger1, Christoph Zellweger1, and Joachim Mohn1 Longfei Yu et al.
  • 1Laboratory for Air Pollution & Environmental Technology, Empa, Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstr. 129, CH-8600 Duebendorf, Switzerland
  • acurrent address: Institute of Ecology, University of Innsbruck, Sternwartestrasse 15, A-6020 Innsbruck, Austria

Abstract. Atmospheric nitrous oxide (N2O) levels have been continuously growing since preindustrial times. Mitigation requires information about sources and sinks on the regional and global scales. Isotopic composition of N2O in the atmosphere could contribute valuable constraints. However, isotopic records of N2O in the unpolluted atmosphere remain too scarce for large-scale N2O models. Here, we report the results of discrete air samples collected weekly to bi-weekly over a five-year period at the high-altitude research station Jungfraujoch, located in central Switzerland. High-precision N2O isotopic measurements were made using a recently developed preconcentration-laser spectroscopy technique. The measurements of discrete samples were accompanied by in situ continuous measurements of N2O mixing ratios. Our results indicate a pronounced seasonal pattern with minimum N2O mixing ratios in late summer, associated with a maximum in δ15Nbulk and a minimum in intramolecular 15N site preference (δ15NSP). This pattern is most likely due to stratosphere-troposphere exchange (STE), which delivers N2O-depleted but 15N-enriched air from the stratosphere into the troposphere. Variability in δ15NSP induced by changes in STE may be masked by biogeochemical N2O production processes in late summer, which are possibly dominated by a low-δ15NSP pathway of N2O production (denitrification), providing an explanation for the observed seasonality of δ15NSP. Footprint analyses and atmospheric transport simulations of N2O for Jungfraujoch suggest that regional emissions from the planetary boundary layer contribute to seasonal variations of atmospheric N2O isotopic composition at Jungfraujoch, albeit more clearly for δ15NSP and δ18O than for δ15Nbulk. With the time-series of five years, we obtained a significant interannual trend for δ15Nbulk after deseasonalization (−0.052±0.012 ‰ a−1), indicating that the atmospheric N2O increase is due to isotopically depleted N2O sources. We estimated the average isotopic signature of anthropogenic N2O sources with a two-box model to be −8.6±0.6 ‰ for δ15Nbulk, 34.8±3 ‰ for δ18O and 10.7±4 ‰ for δ15NSP. Our study demonstrates that seasonal variation of N2O isotopic composition in the background atmosphere is important when determining interannual trends. More frequent, high-precision and inter-laboratory compatible measurements of atmospheric N2O isotopocules, especially for δ15NSP, are needed to better constrain anthropogenic N2O sources, and thus the contribution of biogeochemical processes to N2O growth on the global scale.

Longfei Yu et al.
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Short summary
We observed the isotopic composition of nitrous oxide in the unpolluted air at Jungfraujoch for five years. Our results indicate a clear season pattern in the isotopic composition, corresponding with that in atmospheric nitrous oxide levels. This is most likely due to temporal variations in both emission processes and air mass sources for Jungfraujoch. Our findings are of importance to global nitrous oxide modelling and to better understanding of long-term trends in atmospheric nitrous oxide.
We observed the isotopic composition of nitrous oxide in the unpolluted air at Jungfraujoch for...
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