(2) The injection of water vapor and NOX which are involved in the complex sequence of chemical reactions governing the abundance of ozone in the region from 20 to 35 km where ozone is most abundant. Increases in either constituent are believed to result in lowering of the mean abundance of ozone, but there is great uncertainty regarding the roles of each and little agreement between critics. The ozone abundance is a very important factor in determining the amount and wavelength of potential damaging UV radiation which reaches the surface of the earth. The possible effects of NO and H2 O injected into the stratosphere by the shuttle vehicles can be identified by comparing their emissions with those estimated for the SST fleets. The actual effects of any given rate of injection of either component are difficult to determine because of (1) uncertainties regarding the vertical and horizontal movements in the stratosphere which govern the rate at which the injected material is distributed within the stratosphere, and ultimately removed from it, (2) lack of reliable experimental observations of the composition of the stratosphere as a function of altitude, season, and location of the surface of the globe, and (3) great uncertainties regarding the nature of the chemical and photochemical reactions which determine the abundances of chemical species involved in the ozone equilibrium. Vertical mixing in the stratosphere is very slow and declines with increasing altitude. Consequently, gases injected into the stratosphere will accumulate, and even a low annual rate of injection will yield a large equilibrium value at very high altitudes. One way of evaluating the effects of injection of constituents is to employ available knowledge of the mean lifetime at the altitude in question to compute an equilibrium value and compare it with estimates of the “natural” abundance of the gas in question. This approach is limited by the uncertainties regarding the composition of the stratosphere. It can be used for the main engine water vapor exhaust, but uncertainties regarding the abundance of NO at any level in the stratosphere and of the importance of NO in the ozone equilibrium suggest that other criteria should be employed for this constituent. b. Water Vapor Pollution The main engines of the launch vehicle bum for 8 minutes and consume 1.6xl06 pounds of fuel in the form of hydrogen and oxygen which combine to produce a like quantity of water. After 30 seconds, the vehicle reaches an altitude of 5,000 feet and in another 30 seconds it reaches the tropopause, which is assumed for the purposes of this calculation to be 10 km. During the remaining 7 minutes it climbs an additional 100 km. 1.4xl05 pounds of H2O are assumed to be emitted in each 10 km depth of the atmosphere above the tropopause. 360 launches are assumed per year. Residence times in the stratosphere are not known with certainty: Martell (55) estimates residence time to be one month at the tropopause, about one to two years at the 20 km level, and 4 to 20 years at 50 km (the stratopause). Table 30 shows the calculated increase of water vapor to be expected in each 10-km slice of the stratosphere, assuming the range of residence times listed. These are compared with the best available estimates of the natural abundance of water vapor at these altitudes, namely 2x10~6 kg/kg of air up to 30 km and 5xl0-6 kg/kg of air above this level. The water vapor increment due to main engine shuttle vehicle exhaust is seen to be insignificant in the altitude region where ozone is most abundant. Thus, the increment corresponds to between 0.02
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