Prokopiou, M., Martinerie, P., Sapart, C.J., Witrant, E., Monteil, G.A., Ishijima, K., Bernard, S., Kaiser, J., Levin, I., Sowers, T., Blunier, T., Etheridge, D., Dlugokencky, E., van de Wal, R.S.W., Röckmann, T., (2016):
Constraining N2O emissions since 1940 using firn air isotope measurements in both hemispheres.
Atmos. Chem. Phys. Discuss. 2016, 1–50. doi:10.5194/acp-2016-487
N2O is currently the 3rd 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, we performed a multi-site reconstruction of the atmospheric N2O mole fraction and isotopic composition using 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 delta [values] of atmospheric N2O decreased by (−2.2 ± 0.2) ‰ for δ15Nav, (−1.0 ± 0.3) ‰ for δ18O, (−1.3 ± 0.6) ‰ for δ15Nα, and (−2.8 ± 0.6) ‰ for δ15Nβ 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 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 be 123−19+29 a. Adopting this lifetime results in total average source isotopic signatures of (−7.6 ± 0.8) ‰ (vs. Air-N2) for δ15Nav, (32.2 ± 0.2) ‰ (vs. VSMOW) for δ18O, (−3.0 ± 1.9) ‰ (vs. Air-N2) for δ15Nα, and (−11.7 ± 2.3) ‰ (vs. Air-N2) for δ15Nβ over the investigated period. δ15Nav and δ15Nβ show some temporal variability while the other source isotopic signatures remain unchanged. The 15N site-preference (= δ15Nα – δ15Nβ) can be used to reveal further information on the source emission origins. Based on the changes in the isotopes we conclude that the main contribution to N2O changes in the atmosphere since 1940 is from soils, with agricultural soils being the principal anthropogenic component which is in line with previous studies.
Received: 08 Jun 2016 – Accepted for review: 10 Jun 2016 – Discussion started: 21 Jun 2016