资源环境科技发展态势分析平台

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 3,000years. For the preindustrial (PI) atmosphere, we find an average N2O mole fraction of (2671) nmol/mol and average tropospheric N2O isotopic values of N-15(PI)av=(9.50.1), O-18(PI)=(47.10.2)parts per thousand, N-15(PI)=(17.8 +/- 0.4)parts per thousand, and (15)(PI)=(1.2 +/- 0.4)parts per thousand. From PI to modern times all isotope signatures decreased with a total change of N-15(av)=(2.7 +/- 0.2)parts per thousand, O-18=(2.5 +/- 0.4)parts per thousand, N-15=(2.0 +/- 0.7)parts per thousand, and (15) (3.5 +/- 0.7)parts per thousand. Interestingly, the temporal evolution is not the same for N-15(av) and O-18. O-18 trends are relatively larger during the early part, and N-15(av) 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 N-15(source,anthrop)av=(15.0 +/- 2.6)parts per thousand, O-18(source,anthrop)=(30.0 +/- 2.6)parts per thousand, N-15(source,anthrop)=(4.5 +/- 1.7)parts per thousand, and N-15(source,anthrop)=(24.0 +/- 8.4)parts per thousand. The N-15 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.