Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 6 2005 10.5194/acpd-5-12255-2005 http://www.atmos-chem-phys-discuss.net/5/12255/2005/ http://www.atmos-chem-phys-discuss.net/5/12255/2005/acpd-5-12255-2005.html http://www.atmos-chem-phys-discuss.net/5/12255/2005/acpd-5-12255-2005.pdf 12255 12311 2005-11-25 Impact of aircraft NO_x emissions on the atmosphere – tradeoffs to reduce the impact M. Gauss michael.gauss@geo.uio.no I. S. A. Isaksen D. S. Lee O. A. Søvde Department of Geosciences, University of Oslo, Oslo, Norway Centre for Air Transport and the Environment, Manchester Metropolitan University, Manchester, UK Within the EU-project TRADEOFF, the impact of NO_x (=NO+NO₂) emissions from subsonic aviation upon the chemical composition of the atmosphere has been calculated with focus on changes in reactive nitrogen, ozone, and the chemical lifetime of methane. We apply a 3-D chemical transport model that includes comprehensive chemistry for both the troposphere and the stratosphere and uses various aircraft emission scenarios developed during TRADEOFF for the year 2000. The environmental effects of enhanced air traffic along polar routes and of possible changes in cruising altitude are investigated. <br><br> In the reference case the model predicts aircraft-induced maximum increases of zonal-mean NO_y (=total reactive nitrogen) between 156 pptv (August) and 322 pptv (May) in the tropopause region of the Northern Hemisphere. Resulting maximum increases in zonal-mean ozone vary between 3.1 ppbv in September and 7.7 ppbv in June. The lifetime of methane is calculated to decrease by 0.71%, inducing a radiative forcing of &minus;6.4 mW/m². <br><br> Enhanced use of polar routes implies significantly larger zonal-mean ozone increases in high Northern latitudes during summer, while the effect is negligible in winter. <br><br> Lowering the flight altitude leads to smaller ozone increase in the lower stratosphere and upper troposphere, and to larger ozone increase at lower altitudes. Regarding total ozone change, the degree of cancellation between these two effects depends on latitude and season, but annually and globally averaged the stratospheric decrease dominates, mainly due to washout of NO_y in the troposphere, which weakens the tropospheric increase. <br><br> Raising flight altitudes increases the ozone burden both in the troposphere and the lower stratosphere, primarily due to a more efficient accumulation of pollutants in the stratosphere.