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

We investigated the size and composition of exhaust plumes of commercial ai rliners in the northeast Atlantic flight corridor off the shores of Ireland and Scotland, an area with high air traffic density in the tropopause regi on. The primary objective was to measure the contribution of the NO and NOy , emitted from aircraft to the nitrogen oxide background. We have made meas urements in aircraft flight corridors by flying a research aircraft perpend icular to the routes of commercial transatlantic air traffic. Although prev ious studies had succeeded in identifying a few plumes, this study was the first systematic investigation of over 60 aircraft plumes through in situ N O and NOy measurements. These plumes were up to 1.5 km wide (along our flig ht 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 N Oy in the background air masses was very heterogeneous. On top of this natu ral variability of the NOy, we could not identify any influence of air traf fic exhaust on air chemistry on scales larger than a few kilometers from ou r data. A secondary objective was to estimate the importance of the oxidati on of NOx to NOy in the relatively fresh plumes. The measured NO/NOy, ratio s were near the NO/NOx ratios calculated from a simple photostationary stat e assumption. This result was also consistent with calculations made with a n expanding box model that included gas-phase chemistry for the measured pl ume conditions. The model calculated NO/NOy, and NO/NOx ratios were almost equal, and these were consistent with the measured NO/NOy ratios. These cal culations showed that oxidation of NOx to higher oxides played only a negli gible role in our measured plumes of ages between 14 and 90 min.