Constraining N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O emissions since 1940 using firn air isotope measurements in both hemispheres

Markella Prokopiou,Jan Kaiser,Patricia Martinerie,Ed Dlugokencky,Roderik S W Van De Wal,Sophie Bernard,Thomas Röckmann,Thomas Blunier,Kentaro Ishijima,Guillaume Monteil,Ingeborg Levin,David Etheridge,Célia J Sapart,Emmanuel Witrant

doi:10.5194/acp-17-4539-2017

Abstract

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 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 = (−7.6 ± 0.8) ‰ (vs. air-N2), δ18O = (32.2 ± 0.2) ‰ (vs. Vienna Standard Mean Ocean Water – VSMOW) for δ18O, δ15Nα = (−3.0 ± 1.9) ‰ and δ15Nβ = (−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 ( = δ15Nα − δ15Nβ) provides evidence of a shift in emissions from denitrification to nitrification, although the uncertainty envelopes are large.

Highlights

The rise in nitrous oxide (N2O) since pre-industrial times has contributed significantly to radiative forcing (Forster et al, 2007)
The destruction of N2O in the stratosphere provides an important source of nitrogen oxides (NOx), which contribute to stratospheric ozone depletion (Ravishankara et al, 2009; Crutzen, 1979; McElroy et al, 1971)
New firn air samples added in this study to the total dataset were collected in 2008 and 2009 during the firn campaign (Buizert et al, 2012) as part of the North Eemian Ice Drilling programme (NEEM) in Greenland (77.45◦ N, 51.06◦ W)

Summary

Introduction

The rise in nitrous oxide (N2O) since pre-industrial times has contributed significantly to radiative forcing (Forster et al, 2007). Over the past 4 decades, the N2O mole fraction has increased by 0.25 % per year, reaching 324 nmol mol−1 in 2011 (Ciais et al, 2013). M. Prokopiou et al.: Constraining N2O emissions since 1940 the biogeochemical cycle of N2O is important for a reliable assessment of future climate change. The destruction of N2O in the stratosphere provides an important source of nitrogen oxides (NOx), which contribute to stratospheric ozone depletion (Ravishankara et al, 2009; Crutzen, 1979; McElroy et al, 1971)

Methods

Results

Discussion

Conclusion