We investigated the size and composition of exhaust plumes of commercial airliners in the northeast Atlantic flight corridor off the shores of Ireland and Scotland, an area with high air traffic density in the tropopause region. The primary objective was to measure the contribution of the NO and NOy emitted from aircraft to the nitrogen oxide background. We have made measurements in aircraft flight corridors by flying a research aircraft perpendicular to the routes of commercial transatlantic air traffic. Although previous studies had succeeded in identifying a few plumes, this study was the first systematic investigation of over 60 aircraft plumes through in situ NO and NOy measurements. These plumes were up to 1.5 km wide (along our flight paths) and showed NOy mixing ratios of up to 10 ppb above the background levels. The measurements showed that on larger scales the composition of NOy in the background air masses was very heterogeneous. On top of this natural variability of the NOy, we could not identify any influence of air traffic exhaust on air chemistry on scales larger than a few kilometers from our data. A secondary objective was to estimate the importance of the oxidation of NOx to NOy in the relatively fresh plumes. The measured NO/NOy ratios were near the NO/NOx ratios calculated from a simple photo-stationary state assumption. This result was also consistent with calculations made with an expanding box model that included gas-phase chemistry for the measured plume conditions. The model calculated NO/NOy and NO/NOx ratios were almost equal, and these were consistent with the measured NO/NOy ratios. These calculations showed that oxidation of NOx to higher oxides played only a negligible role in our measured plumes of ages between 14 and 90 min. ¿ 1998 American Geophysical Union