Isotope information about N2O source changes since pre-industrial times inferred from Antarctic firn air and stratospheric measurements

Nitrous oxide (N₂O) is an important atmospheric trace gas for two main reasons: It is an efficient greenhouse gas and it represents the major source of stratospheric NO_x, which catalytically destroys ozone in the stratosphere. Therefore, the observed increase of N₂O in the atmosphere is of concern and it is clearly important to understand its global atmospheric cycle. However, despite its importance, the N₂O budget is still not well quantified today. Further information about its global cycle may be gained from measurements of its isotopic composition, because they provide additional independent observables. Unfortunately, attempts to determine globally averaged N₂O isotopic source signatures using isotope measurements on N₂O sources are subject to large uncertainties, because of the spatial and temporal heterogeneity of the emissions. Using recent measurements of the isotope fractionation in the stratospheric N₂O sink, which is now much better constrained that the sources, a top-down approach yields a much more precise estimate of the global N₂O isotopic source signatures. Isotope measurements on N₂O from Antarctic firn air samples show that the observed N₂O increase in the atmosphere since pre-industrial times is accompanied by a significant isotopic depletion in ¹⁵N at both positions in the molecule and in ¹⁸O. Top-down budget calculations, performed for the present and the pre-industrial atmospheres, reveal that the observed isotopic depletions are not simply due to an increase in the source strength of the isotopically depleted surface sources. In addition, the global average N₂O source signature has significantly decreased since pre-industrial times. This demonstrates that the isotope signature anthropogenic of anthropogenic N₂O is depleted relative to the global average natural emissions, strongly supporting recent indications from soil emission measurements.