Recently, atomic nitrogen has been measured in the upper thermosphere with a mass spectrometer carried on the Atmosphere Explorer satellites. Only a small fraction of N atoms are directly measured by the mass spectrometer, however. The majority appear as NO, formed within the ion source. Occasionally a relatively large NO2 signal has also been observed, showing a strong dependence on the ion source surface temperature. Comparison of a numerical simulation of the reactions leading to NO2 with data obtained in circular orbits on AE-C provides insight into the pertinent chemical reactions. It demonstrates that the techniques used in open source neutral mass spectrometer operation-warmup and outgassing of the source before measurements-were successful in minimizing the effects of this contaminant.