Impact of aircraft NOx emissions.: Part 2:: Effects of lowering the flight altitude

Aircraft emissions of NOx amount to a small proportion of total emissions of NOx from man-made and natural sources. However, NOx from subsonic aircraft are directly emitted in the upper troposphere and lower stratosphere and have a relatively strong effect on the production of ozone (O-3), a greenhouse gas, in that region. Furthermore air traffic is expected to increase significantly in the next decades. Possibilities for reducing the environmental impacts of air traffic by operational, technological and economic measures are currently under discussion. One potential option is to reduce flight altitudes. As a first step we investigate the effect of lowering cruise altitudes by 1 km on the chemical composition of the atmosphere for a subsonic fleet in 2015. Other parameters are deliberately kept constant, e.g., fuel consumption and NOx emissions. The simulations with the coupled climate-chemistry model E39/C clearly show that this fleet leads to a significantly smaller ozone increase compared to a fleet with a standard cruise altitude. In the northern hemisphere aircraft cause an ozone increase of 12.5 Tg for a 2015 simulation. The reduction in the ozone increase of 1.5 Tg is greater than the estimated additional ozone increase of 0.3 Tg due to decreased aerodynamic efficiency and therefore higher fuel consumption and NOx emissions. This value (1.2 Tg) is in the order of 10% relative to the simulated aircraft induced ozone increase of 12.5 Tg.