Impact of aircraft emissions on reactive nitrogen over the North Atlantic Flight Corridor region

The impact of aircraft emissions on reactive nitrogen in the upper troposphere (UT) and lowermost stratosphere (LS) was estimated using the NO y ‐O 3 correlation obtained during the Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX) carried out over the U.S. continent and North Atlantic Flight Corridor (NAFC) region in October and November 1997. To evaluate the large‐scale impact, we made a reference NO y ‐O 3 relationship in air masses, upon which aircraft emissions were considered to have little impact. For this purpose, the integrated input of NO x from aircraft into an air mass along a 10‐day back trajectory (ΔNO y ) was calculated based on the Abatement of Nuisance Caused by Air Traffic/European Commission (ANCAT/EC2) emission inventory. The excess NO y ( d NO y ) was calculated from the observed NO y and the reference NO y ‐O 3 relationship. As a result, a weak positive correlation was found between the d NO y and ΔNO y , and d NO y and NO x /NO y values, while no positive correlation between the d NO y and CO values was found, suggesting that d NO y values can be used as a measure of the NO x input from aircraft emissions. The excess NO y values calculated from another NO y ‐O 3 reference relationship made using in situ condensation nuclei data also agreed with these d NO y values, within the uncertainties. At the NAFC region (45°N–60°N) the median value of d NO y in the troposphere increased with altitude above 9 km and reached 70 parts per trillion by volume (pptv) (20% of NO y ) at 11 km. The excess NO x was estimated to be about half of the d NO y values, corresponding to 30% of the observed NO x level. Higher d NO y values were generally found in air masses where O 3 = 75–125 ppbv, suggesting a more pronounced effect around the tropopause. The median value of d NO y in the stratosphere at the NAFC region at 8.5–11.5 km was about 120 pptv. The higher d NO y values in the LS were probably due to the accumulated effect of aircraft emissions, given the long residence time of affected air in the LS. Similar d NO y values were also obtained in air masses sampled over the U.S. continent.