Measurements of nitrous oxide isotopologues ((15)N(14)NO, (14)N(15)NO, and N(2)(18)O): Insights into global biogeochemical cycling and atmospheric chemistry and transport

Measurements of the nitrogen and oxygen isotopic compositions of nitrous oxide (N2O) provide additional constraints on the magnitudes of the sources and sinks of atmospheric N2O beyond those provided by concentration measurements alone since the various biogeochemical production and destruction processes produce distinct 15N/14N and/or 18O/16O ratios. New measurements of the 18O and bulk 15N isotopic compositions, as well as the site-specific 15N isotopic compositions (i.e. , the 15N isotopic composition of N2O for either the terminal or central nitrogen atom) are presented here for (1) stratospheric N2O from whole air samples collected from the NASA ER-2 aircraft, (2) N2O produced by microbial activity in tropical rain forest soils, and (3) free tropospheric N2O from whole air samples archived between 1978 and 2004. The measurements were made by continuous-flow isotope ratio mass spectrometry using a preconcentrator and gas chromatograph, which allowed the direct analysis of N2O from whole air samples without manual purification or conversion to N2 and O2. A novel mass spectrometric means to place the site-specific nitrogen isotopic compositions on the international air-N2 isotopic scale is also described.; For the stratosphere, the observed variations in isotopic compositions as a function of N2O mixing ratio are well-explained by transport and mixing and are not due to changes in the underlying isotope effects nor the relative proportion of N2O destruction by photolysis versus reaction with O(1D). These observations also allowed empirical estimates of the isotope fluxes between the stratosphere and troposphere to be made, thus reducing one of major uncertainties in the global N2O isotope budget. For N2O produced in the tropical soils, both under natural and artificial conditions, measurements of the isotopic compositions are used to estimate isotopic fractionation occurring during nitrification and denitrification, which are shown to be distinct. Correlations among the isotopic compositions are also used to characterize the extent of N2 O consumption in the soils. For the free tropospheric whole air samples dating back to 1978, the N2O isotopic compositions show significant decreasing trends and are consistent with the prediction that the increase in the atmospheric N2O burden is a result of agricultural activities that produce isotopically light N2O.