LAUSR

Nitrous oxide (N2O) is a strong greenhouse gas whose mole fraction in the atmosphere has increased over the industrial period. We present a new set of isotope measurements of N2O in air extracted from ice cores covering the last 3,000 years. For the preindustrial (PI) atmosphere, we find an average N2O mole fraction of (267 ± 1) nmol/mol and average tropospheric N2O isotopic values of δN PI = (9.5 ± 0.1)‰, δ OPI = (47.1 ± 0.2)‰, δ NPI = (17.8 ± 0.4)‰, and δ ΝPI = (1.2 ± 0.4)‰. From PI to modern times all isotope signatures decreased with a total change of δN = ( 2.7 ± 0.2)‰, δO = ( 2.5 ± 0.4)‰, δN = ( 2.0 ± 0.7)‰, and δΝ ( 3.5 ± 0.7)‰. Interestingly, the temporal evolution is not the same for δN and δO. δO trends are relatively larger during the early part, and δN trends are larger during the late part of the industrial period, implying a decoupling of sources over the industrial period. Using a mass balance model, we determined the isotopic composition of the total average N2O source. Assuming that the total present source is the sum of a constant natural source and an increasing anthropogenic source, this anthropogenic source has an isotopic signature of δN source,anthrop = ( 15.0 ± 2.6)‰, δOsource,anthrop = (30.0 ± 2.6)‰, δ Nsource,anthrop = ( 4.5 ± 1.7)‰, and δNsource,anthrop = ( 24.0 ± 8.4)‰. The N site preference of the source has increased since PI times, which is indicative of a relative shift from denitrification to nitrification sources, consistent with agricultural emissions playing a major role in the N2O increase.