Airborne measurements of NOx, total reactive nitrogen (NOy), O3, and condensation nuclei (CN) were made within air traffic corridors over the U.S. and North Atlantic regions (35–60 °N) in the fall of 1997. NOx and NOy data obtained in the lowermost stratosphere (LS) were examined using the calculated increase in NOy (ΔNOy) along five-day back trajectories as a parameter to identify possible effects of aircraft on reactive nitrogen. It is very likely that aircraft emissions had a significant impact on the NOx levels in the LS inasmuch as the NOx mixing ratios at 8.5–12 km were significantly correlated with the independent parameters of aircraft emissions, i.e., ΔNOy levels and CN values. In order to estimate quantitatively the impact of aircraft emissions on NOx and CN, the background levels of CN and NOx at O3=100–200 ppbv were derived from the correlations of these quantities with ΔNOy. On average, the aircraft emissions are estimated to have increased the NOx and CN values by 130 pptv and 400 STP cm−3, respectively, which corresponds to 70±30% and 30±20% of the observed median values.