The effect of aircraft flight altitude on atmospheric ozone has been studied by using NOx emitted from hypothetical fleets operating individually in an altitude range between 10.8 and 18 km. Three classes of aircraft fleets were chosen so as to cover the present subsonic types operating at a typical altitude of 10.8 km, advanced subsonics at either 12.7 or 14.5 km, and present SST's (Concorde and Tupolev) at about 18 km. The study is based on the use of an empirical two-dimensional (2-D) photochemical model of the troposphere and stratosphere. An important characteristic of this model is the incorporation of the methane oxidation reactions, which produce ozone in the troposphere and lowest stratosphere. The chemical system otherwise includes the mechanisms of stratospheric ozone destruction by NOx and HOx. The model did not include chlorine reactions. The 2-D model reproduces several characteristics of the latitudinal and seasonal variations in ozone and other species. Other characteristics of the model are (1) the numerical simulation of water vapor data in the natural troposphere and stratosphere and (2) the inclusion of Rayleigh scattering for wavelengths longer than 300 nm in the determination of photodissociation rates. Results are presented in terms of ozone column changes as a function of latitude and season for the assumed (not forecast) NOx injection rates for each fleet. In general, the results show (1) a small (less than 1%) enhancement in the ozone column due to the methane oxidation reactions in regions of heavy traffic for injections at 10.8 km,(2) almost no effect on the ozone column for injections at 14.5 km, and (3) a decrease in the ozone column due to the NOx catalytic cycle for SST injections at 18 km. These results for the present and advanced subsonics contradict earlier Climatic Impact Assessment Program results showing an ozone decrease due to subsonic aircraft. The conclusions from this 2-D model are still of a preliminary nature owing to the uncertainties in the knowledge of the atmospheric chemistry and dynamics. Furthermore, this study could not incorporate the effect of aircraft water emissions on the ozone balance, which may well be a significant factor to consider.
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