The present-day and future impact of NOx emissions from subsonic aircraft on the atmosphere in relation to the impact of NOx surface sources

The e€ect of present-day and future NOx emissions from aircraft on the NOx and ozone concentrations in the atmosphere and the corresponding radiative forcing were studied using a three-dimensional chemistry transport model (CTM) and a radiative model. The e€ects of the aircraft emissions were compared with the e€ects of the three most important anthropogenic NOx surface sources: road trac, electricity generation and industrial combustion. From the model results, NOx emissions from aircraft are seen to cause an increase in the NOx and ozone concentrations in the upper troposphere and lower stratosphere, and a positive radiative forcing. For the reference year 1990, the aircraft emissions result in an increase in the NOx concentration at 250 hPa of about 20 ppt in January and 50 ppt in July over the eastern USA, the North Atlantic Flight Corridor and Western Europe, corresponding to a relative increase of about 50%. The maximum increase in the ozone concentrations due to the aircraft emissions is about 3±4 ppb in July over the northern mid-latitudes, corresponding to a relative increase of about 3±4%. The aircraft-induced ozone changes cause a global average radiative forcing of 0.025 W/m in July. According to the ANCAT projection for the year 2015, the aircraft NOx emissions in that year will be 90% higher than in the year 1990. As a consequence of this, the calculated NOx perturbation by aircraft emissions increases by about 90% between 1990 and 2015, and the ozone perturbation by about 50±70%. The global average radiative forcing due to the aircraftinduced ozone changes increases by about 50% between 1990 and 2015. In the year 2015, the e€ects of the aircraft emissions on the ozone burden and radiative forcing are clearly larger than the individual e€ects of the NOx surface sources. Taking chemical conversion in the aircraft plume into account in the CTM explicitly, by means of modi®ed aircraft NOx emissions, a signi®cant reduction of the aircraft-induced NOx and ozone perturbations is realised. The NOx perturbation decreases by about 40% and the ozone perturbation by about 30% in July over Western Europe, the eastern USA and the North Atlantic Flight Corridor.