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 NOy-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 NOy-O-3 relationship in air masses, upon which aircraft emissions were considered to have little impact. For this purpose, the integrated input of NO, from aircraft into an air mass along a 10-day back trajectory (Delta NOy) was calculated based on the Abatement of Nuisance Caused by Air Traffic/European Commission (ANCAT/EC2) emission inventory. The excess NOy (dNO(y)) was calculated from the observed NO, and the reference NOy-O-3 relationship. As a result, a weak positive correlation was found between the dNO(y) and Delta NOy and dNO(y) and NOx/NOy values, while no positive correlation between the dNO(y) and CO values was found suggesting that dNO(y) values can be used as a measure of the NOx input from aircraft emissions. The excess NOy values calculated from another NOy-O-3 reference relationship made using in situ condensation nuclei data also agreed with these dNO(y) values, within the uncertainties. At the NAFC region (45 degrees N-60 degrees N) the median value of dNO(y) in the troposphere increased with altitude above 9 km and reached 70 parts per trillion by volume (pptv) (20% of NOy) at 11 km. The excess NOx was estimated to be about half of the dNO(y) values, corresponding to 30% of the observed NOx level. Higher dNO(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 dNO(y) in the stratosphere at the NAFC region at 8.5-11.5 km was about 120 pptv. The higher dNO(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 dNO(y) values were also obtained in air masses sampled over the U.S. continent.