Abstract
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 δ15N and δ18O and 0.2‰ for the relative enrichment of the terminal (1δ5N) and central (2δ5N) 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 δ15N = (6.72 ± 0.12)‰ versus air N2 and δ18O = (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 2δ15N 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 1δ15N = (−15.8 ± 0.6)‰ and 2δ15N = (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 Δ17O = (0.9 ± 0.1)‰.
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