Constraining N₂O emissions since 1940 using firn air isotope measurements in both hemispheres

Abstract. N₂O is currently the third most important anthropogenic greenhouse gas in terms of radiative forcing and its atmospheric mole fraction is rising steadily. To quantify the growth rate and its causes over the past decades, we performed a multi-site reconstruction of the atmospheric N₂O mole fraction and isotopic composition using new and previously published firn air data collected from Greenland and Antarctica in combination with a firn diffusion and densification model. The multi-site reconstruction showed that while the global mean N₂O mole fraction increased from (290 ± 1) nmol mol⁻¹ in 1940 to (322 ± 1) nmol mol⁻¹ in 2008, the isotopic composition of atmospheric N₂O decreased by (−2.2 ± 0.2) ‰ for δ¹⁵N^av, (−1.0 ± 0.3) ‰ for δ¹⁸O, (−1.3 ± 0.6) ‰ for δ¹⁵N^α, and (−2.8 ± 0.6) ‰ for δ¹⁵N^β over the same period. The detailed temporal evolution of the mole fraction and isotopic composition derived from the firn air model was then used in a two-box atmospheric model (comprising a stratospheric box and a tropospheric box) to infer changes in the isotopic source signature over time. The precise value of the source strength depends on the choice of the N₂O lifetime, which we choose to fix at 123 years. The average isotopic composition over the investigated period is δ¹⁵N^av =  (−7.6 ± 0.8) ‰ (vs. air-N₂), δ¹⁸O  =  (32.2 ± 0.2) ‰ (vs. Vienna Standard Mean Ocean Water – VSMOW) for δ¹⁸O, δ¹⁵N^α =  (−3.0 ± 1.9) ‰ and δ¹⁵N^β =  (−11.7 ± 2.3) ‰. δ¹⁵N^av, and δ¹⁵N^β show some temporal variability, while for the other signatures the error bars of the reconstruction are too large to retrieve reliable temporal changes. Possible processes that may explain trends in ¹⁵N are discussed. The ¹⁵N site preference ( = δ¹⁵N^α − δ¹⁵N^β) provides evidence of a shift in emissions from denitrification to nitrification, although the uncertainty envelopes are large.