Complete and accurate mass spectrometric isotope analysis of tropospheric nitrous oxide -: art. no. 4476

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 0.05parts per thousand for delta(15)N and delta(18)O and 0.2parts per thousand for the relative enrichment of the terminal ((1)delta(5)N) and central ((2)delta(5)N) 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 delta(15)N = (6.72 +/- 0.12)parts per thousand versus air N-2 and delta(18)O = (44.62 +/- 0.21)parts per thousand versus Vienna Standard Mean Ocean Water. Neither a clear spatial pattern from 28degreesN to 79degreesN, 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 0.04parts per thousand/a derived from analyses of Antarctic firn air and isotopic budget calculations. Using an independent (2)delta(15)N calibration of our working standard versus air N-2, we find large differences in the position-dependent N-15/N-14 ratios: The mean for all samples is (1)delta(15)N = (-15.8 +/- 0.6)parts per thousand and (2)delta(15)N = (29.2 +/- 0.6)parts per thousand versus air N-2. In light of a new definition for oxygen isotope anomalies, we reevaluate the existing measurements and derive a O-17 excess of Delta O-17 = (0.9 +/- 0.1)parts per thousand.