Concentration and Isotopic Composition of Atmospheric N2O Over the Last Century

Temporal 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, delta N-15(bulk), delta O-18, and delta N-15(SP) 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 delta N-15(bulk) and delta O-18 were -0.010 +/- 0.025 and -0.004 +/- 0.031 parts per thousand yr(-1), -0.014 +/- 0.013 and -0.009 +/- 0.017 parts per thousand 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 delta N-15(SP) data. Two-box model calculations using N2O mole fraction suggest that the total N2O flux (F-T) at 2015 CE was 17.5 +/- 1.1 TgN yr(-1), where flux from the natural (FN) and anthropogenic (FA) sources were similar to 60% and 40% of F-T, respectively, and the contribution of F-A was similar to 30% of F-T at 1900 CE. Estimated FA and delta N-15(bulk) of atmospheric N2O suggest that the anthropogenic emissions from continental regions were 12%, 25% and 76% of F-A during P1, P2 and P3 phases, respectively. Plain Language Summary Our composite data suggest a three-phased growth of N2O mole fraction in the atmosphere over the last century, where the growth rate was significantly higher after 1965 CE. delta N-15(bulk) and delta O-18 values of N2O show a decreasing trend during the last century, whereas a significant trend in delta N-15(SP) values has not been observed. The increasing growth rate and simultaneous lowering trend in isotope values suggest that contribution from anthropogenic sources has been increasing over the last century. The two-box model estimation also confirms that the continental region has been the hot spot of anthropogenic emissions over the last four decades. Invariant delta N-15(SP) values suggest that the global balance between nitrification and denitrification process driven N2O emission has not been significantly altered by anthropogenic impacts.