point. The possible local effects have not been considered, since there is little reliable information regarding the rate at which they will be horizontally mixed. c. NO Pollution of the Stratosphere Booster engines will emit approximately 40 metric tons of NO into the stratosphere in the region from 10 to 24 km per launch, or a total of 2.4x10' ' grams per year. These emissions are on the order of those to be expected from a 500-unit SST fleet, and are in approximately the same altitude region. Some authors have argued that this is a modest increase when compared with the probable “natural” concentrations of NO at these levels. However, there are few direct measurements of NO or NO2 in the stratosphere, and all of the estimates of abundance are derived from models which are recognized as being simplifications of the real situation. The strongest critic of the SST program is Harold Johnston (56, 57), who argues that the significance of the SST NO injection should be determined by comparing it with the natural flux of NO from the upper atmosphere, where it is produced by photolytic decomposition of N2 O. Ozone is less abundant in the region from 10 to about 35 km than calculations of the equilibrium concentration that would occur in a stagnant atmosphere if there were no sinks due to the presence of water, methane, and oxides of nitrogen. Johnston argues that this decrease of ozone is due to the NO carried down from the stratopause. Others suggest that some of the discrepancies must be due to mixing in the stratosphere or to other effects. The best (e.g., most conservative) approach to evaluating the effects of booster engine NO pollution in the stratosphere is to employ Johnston's arguments and see whether the shuttle vehicle NO emissions are large compared to the natural fluxes he and others have calculated. Table 31 shows these comparisons. Several “SST fleets” have been employed in the literature. The earliest controversy concerned a 500-unit fleet of SST's of the U.S. type, which consume 66 tons of fuel per hour while cruising in the stratosphere and emit from 13.2 (latest estimate) to 42 lb of NO per 1000 pounds of fuel. The Concorde is reported as consuming 33,000 (SCEP (58)) to 18,000 pounds of fuel per hour, and to emit 13.2 to 15 pounds of NO per hour. All SST's are assumed to average 7 hours per day in the stratosphere at an altitude of about 20 km. Table 31 shows that the NO emissions from the booster are large and within the range of SST emissions that have caused concern. Thus NO emissions may constitute a pollution problem in the stratosphere if Johnston's approach is the correct one. Consequently, the SST controversy should be followed closely, and the localization of these emissions should be evaluated if possible. Other Booster Emissions. —The remaining 2.6xl06 pounds of pollutants emitted by the booster should be identified and their effects on the stratosphere and troposhere should be evaluated.
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