Constraining N 2 O emissions since 1940 using firn air isotope measurements in both hemispheres

• Published in: Atmospheric Chemistry and Physics
• Main Authors: M. Prokopiou, P. Martinerie, C. J. Sapart, E. Witrant, G. Monteil, K. Ishijima, S. Bernard, J. Kaiser, I. Levin, T. Blunier, D. Etheridge, E. Dlugokencky, R. S. W. van de Wal, T. Röckmann
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2017
• Subjects: Physics; QC1-999; Chemistry; QD1-999; Greenland; Antarc*; Antarctica
• Online Access: https://doi.org/10.5194/acp-17-4539-2017; https://doaj.org/article/4741aab3e6af4dad90f89681c4118740

N 2 O 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 N 2 O 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 N 2 O mole fraction increased from (290 ± 1) nmol mol −1 in 1940 to (322 ± 1) nmol mol −1 in 2008, the isotopic composition of atmospheric N 2 O decreased by (−2.2 ± 0.2) ‰ for δ 15 N av , (−1.0 ± 0.3) ‰ for δ 18 O, (−1.3 ± 0.6) ‰ for δ 15 N α , and (−2.8 ± 0.6) ‰ for δ 15 N 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 N 2 O lifetime, which we choose to fix at 123 years. The average isotopic composition over the investigated period is δ 15 N av = (−7.6 ± 0.8) ‰ (vs. air-N 2 ), δ 18 O = (32.2 ± 0.2) ‰ (vs. Vienna Standard Mean Ocean Water – VSMOW) for δ 18 O, δ 15 N α = (−3.0 ± 1.9) ‰ and δ 15 N β = (−11.7 ± 2.3) ‰. δ 15 N av , and δ 15 N 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 15 N are discussed. The 15 N site preference ( = δ 15 N α − δ 15 N β ) provides evidence of a shift in emissions from denitrification to nitrification, although the uncertainty envelopes are large.