Complete and accurate mass spectrometric isotope analysis of tropospheric nitrous oxide

We describe a manual extraction and purification method for mass spectrometric isotope analyses of tropospheric N2O. A theoretical framework to correct for (hydro)fluorocarbon and CO2 interferences is developed and verified experimentally. The standard deviation for analysis of one sample on a single day is 0.05‰ for d15N and d18O and 0.2‰ for the relative enrichment of the terminal (1d5N) and central (2d5N) nitrogen atoms. The isotopic composition of N2O in tropospheric background air could thus be measured with unprecedented precision on samples from six locations. We obtained overall average values of d15N = (6.72 ± 0.12)‰ versus air N2 and d18O = (44.62 ± 0.21)‰ versus Vienna Standard Mean Ocean Water. Neither a clear spatial pattern from 28°N to 79°N, nor a temporal trend over the course of 2 years was found. Within the experimental uncertainties, this is in line with small trends of 0.02 to 0.04‰/a derived from analyses of Antarctic firn air and isotopic budget calculations. Using an independent 2d15N calibration of our working standard versus air N2, we find large differences in the position-dependent 15N/14N ratios: The mean for all samples is 1d15N = (-15.8 ± 0.6)‰ and 2d15N = (29.2 ± 0.6)‰ versus air N2. In light of a new definition for oxygen isotope anomalies, we reevaluate the existing measurements and derive a 17O excess of ?17O = (0.9 ± 0.1)‰.