| Summary: | We describe a manual extraction and purification method for mass spectrometric isotope analyses of tropospheric N 2 O. A theoretical framework to correct for (hydro)fluorocarbon and CO 2 interferences is developed and verified experimentally. The standard deviation for analysis of one sample on a single day is 0.05‰ for δ 15 N and δ 18 O and 0.2‰ for the relative enrichment of the terminal ( 1 δ 15 N) and central ( 2 δ 15 N) nitrogen atoms. The isotopic composition of N 2 O in tropospheric background air could thus be measured with unprecedented precision on samples from six locations. We obtained overall average values of δ 15 N = (6.72 ± 0.12)‰ versus air N 2 and δ 18 O = (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 2 δ 15 N calibration of our working standard versus air N 2 , we find large differences in the position-dependent 15 N/ 14 N ratios: The mean for all samples is 1 δ 15 N = (-15.8 ± 0.6)‰ and 2 δ 15 N = (29.2 ± 0.6)‰ versus air N 2 . In light of a new definition for oxygen isotope anomalies, we reevaluate the existing measurements and derive a 17 O excess of Δ 17 O = (0.9 ± 0.1)‰.
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