c. NO Pollution of the Stratosphere The booster engines emit a very small percentage of NOX (est. 0.01%), but an undetermined amount is generated by the mixing of the hot plume with air. Studies are in progress to define the amount formed. It is certainly less than 10% of the exhaust plume mass, but if we were to use that amount for an upper bound calculation, 25 metric tons of NOX would be deposited in the 10-24 km band per.launch. At a launch rate of 360 per year, this would be 9000 metric tons per year. Even this exaggerated amount is only 0.5 to 5% of the “natural” NO flux into the stratosphere and even a smaller percentage of the anticipated SST inputs. 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 SSI' 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 calculated values 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 stratospause. Others suggest that some ot the discrepancies must be due to mixing in the stratosphere or to other effects. Table 31 compares the NO flux estimates for the booster with those for the natural and SST systems. 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 pounds 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 io emit 13.2 to 15 pounds of NO per 1000 pounds of fuel. All SST's are assumed to average 7 hours per day in the stratosphere at an altitude of about 20 km. TABLE 31 NATURAL AND ARTIFICIAL FLUX OF NO INTO THE STRATOSPHERE
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