Concentration and Isotopic Composition of Atmospheric N2O Over the Last Century

Abstract

AbstractTemporal changes in the magnitude and geographic distribution of different sources of nitrous oxide (N2O) are not well constrained. To better understand the dynamics of N2O in the atmosphere over the last century, we have reconstructed the mole fraction, δ15Nbulk, δ18O, and δ15NSP values of N2O from ice cores, firn air archives, and modern atmospheric samples. We have provided new firn air records from the Styx Glacier, Antarctica, and the North Greenland Eemian Ice drilling Project, and updated the firn air transport modeling of the published records. The composite reconstruction shows that the N2O growth rates were 0.26 ± 0.05, 0.15 ± 0.05 and 0.75 ± 0.01 ppb yr−1 during 1850–1930 (P1), 1931–1965 (P2) and 1966–2021 CE (P3), respectively. The temporal slope found in a linear least squares fit in δ15Nbulk and δ18O were −0.010 ± 0.025 and −0.004 ± 0.031‰ yr−1, −0.014 ± 0.013 and −0.009 ± 0.017‰ yr−1, and −0.040 ± 0.013 and −0.022 ± 0.005‰ yr−1 during P1, P2 and P3 phases, respectively. Overall, a significant long‐term trend was not observed in δ15NSP data. Two‐box model calculations using N2O mole fraction suggest that the total N2O flux (FT) at 2015 CE was 17.5 ± 1.1 TgN yr−1, where flux from the natural (FN) and anthropogenic (FA) sources were ∼60% and 40% of FT, respectively, and the contribution of FA was ∼30% of FT at 1900 CE. Estimated FA and δ15Nbulk of atmospheric N2O suggest that the anthropogenic emissions from continental regions were 12%, 25% and 76% of FA during P1, P2 and P3 phases, respectively.