Measurements of nitrogen oxides from aircraft in the northeast Atlantic flight corridor

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 NO y 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 NO y measurements. These plumes were up to 1.5 km wide (along our flight paths) and showed NO y mixing ratios of up to 10 ppb above the background levels. The measurements showed that on larger scales the composition of NO y in the background air masses was very heterogeneous. On top of this natural variability of the NO y , 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 NO χ to NO y in the relatively fresh plumes. The measured NO/NO y ratios were near the NO/NO χ 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/NO y and NC/NO χ ratios were almost equal, and these were consistent with the measured NO/NO y , ratios. These calculations showed that oxidation of NO χ to higher oxides played only a negligible role in our measured plumes of ages between 14 and 90 min.