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 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)‰.