Constraining N<sub>2</sub>O emissions since 1940 using firn air isotope measurements in both hemispheres

Abstract

Abstract. 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.

Revised records of atmospheric trace gases CO<sub>2</sub>, CH<sub>4</sub>, N<sub>2</sub>O, and <i>δ</i><sup>13</sup>C-CO<sub>2</sub> over the last 2000 years from Law Dome, Antarctica

“…To the best of our knowledge, there are only two attempts to use δ 15 N-N 2 O and δ 18 O-N 2 O records to better constrain the land vs. ocean sources of N 2 O over the last century (Park et al, 2012;Prokopiou et al, 2017), and these used the larger volumes of air available in firn. Only one study (Prokopiou et al, 2018) has extended the investigation to the last millennia, but there is an analysis covering the last deglaciation by Schilt et al (2014). As already mentioned, the pre-industrial inter-hemispheric N 2 O difference is also poorly constrained.…”

Sources and sinks for N2O, can microbiologist help to mitigate N2O emissions?

“…It is beyond the scope of this editorial to review this vast literature. However, it is interesting to note that bottom up estimates of global emission based on measured emissions show reasonable agreement with top down estimates based on observed trends in atmospheric N 2 O concentrations (Syakila and Kroeze, ), as well as estimates based on measured concentrations and isotope signatures in firn air (air entrapped in compacted snow) and in the atmosphere (Prokopiou et al ., ), albeit with wide uncertainty for both estimates. The emission studies have also provided crucial information for establishing IPCC's emission factors, identification of soils and ecosystems with high (and low) emissions, hot spots (sites with high emissions) and hot moments (periods/seasons with high emissions) within individual fields (Oktarita et al ., and Repo et al ., are two of many examples).…”

The isotopic composition of atmospheric nitrous oxide observed at the high-altitude research station Jungfraujoch, Switzerland

“…A number of studies have analyzed temporal trends in N 2 O isotopic composition in the modern atmosphere (Kaiser et al, 2003;Park et al, 2012;Röckmann and Levin, 2005;Toyoda et al, 2013) and in the past from firn and ice cores (Bernard et al, 2006;Ishijima et al, 2007;Prokopiou et al, 2018;Sowers et al, 2002). These isotopic measurements have shown a decrease in both δ 15 N bulk -N 2 O and δ 18 O-N 2 O associated with an increasing trend in atmospheric N 2 O mixing ratios since preindustrial times, indicating that the recent increase of atmospheric N 2 O may be due to agricultural emissions ( 15 N and 18 O depleted).…”