Impact of aircraft emissions on tropospheric and stratospheric ozone. Part I: chemistry and 2-D model results

The impact of air-traffic-induced NOx and water vapor emissions on the chemical composition of the global troposphere and stratosphere is investigated for current conditions (1991) and a future scenario (2015). The NOx dependence of ozone chemistry is studied using photochemical steady-state calculations for a typical upper tropospheric chemical composition. These calculations demonstrate that above a critical NOx mixing ratio of about 0.3 ppbv, additional NOx emitted by aircraft can actually decrease the net ozone production, whereas below this value there is the commonly accepted increase in ozone production. Subsequently, we assess the impact of aircraft emissions on photochemical ozone production using the Mainz two-dimensional photochemical model including effects of heterogeneous chemistry in the lower stratosphere. Based on not well-represented convection, 2-D models generally underestimate background values of NOx in the free troposphere, hence overestimate the ozone increase caused by subsonic aircraft. In particular, convection might shift the NOx mixing ratio above the critical 0.3 ppbv level. To correctly reproduce the impact of this non-linear relation on ozone, a 3-D model calculation is essential, especially for mid-latitude summer, where significant convection take place. For northern winters, where due to weak convection the 2-D calculations are most appropriate, current aircraft emissions are calculated to yield a tropospheric ozone increase of about 3% and little effect on stratospheric ozone. For the case of installation of 500 commercial supersonic transport in the year 2015 (flight altitude 18–21 km, cruise speed Mach 2.4, emission index 15 g NO2 kg-1 fuel), ozone decreases of 3% in the lower polar stratosphere are predicted leading to decreases in ozone columns by up to 1.5%.