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

Summary, in English

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)nmolmol^-1 in 1940 to (322±1)nmolmol^-1 in 2008, the isotopic composition 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 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 δ¹⁵Nav Combining double low line (-7.6±0.8)% (vs. air-N2), δ¹⁸O Combining double low line (32.2±0.2)% (vs. Vienna Standard Mean Ocean Water-VSMOW) for δ¹⁸O, δ¹⁵Nα Combining double low line (-3.0±1.9)% and δ¹⁵Nβ Combining double low line (-11.7±2.3)%. δ¹⁵Nav, and δ¹⁵Nβ 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 (Combining double low line δ¹⁵Nα-δ¹⁵Nβ) provides evidence of a shift in emissions from denitrification to nitrification, although the uncertainty envelopes are large.

• Author: Markella Prokopiou; Patricia Martinerie; Célia J. Sapart; Emmanuel Witrant; Guillaume Monteil; Kentaro Ishijima; Sophie Bernard; Jan Kaiser; Ingeborg Levin; Thomas Blunier; David Etheridge; Ed J. Dlugokencky; Roderik S. W. van de Wal; Thomas Röckmann
• Department/s: Dept of Physical Geography and Ecosystem Science; MERGE: ModElling the Regional and Global Earth system
• Publishing year: 2017-04-05
• Language: English
• Pages: 4539-4564
• Publication/Series: Atmospheric Chemistry and Physics
• Volume: 17
• Issue: 7
• Links: http://dx.doi.org/10.5194/acp-17-4539-2017
• Document type: Journal article
• Publisher: Copernicus GmbH
• Topic: Climate Research
• Status: Published
• ISBN/ISSN/Other: ISSN: 1680-7316