(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 on the surface of the globe, and (3) great uncertainties regarding the nature and rates 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” abunaance 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 Shuttle vehicle burn for 8 minutes and consume 1.6xl06 pounds of hydrogen and oxygen, producing a like quantity of water. Seven minutes of the operation are performed above the tropopause (est. 10km) as burning continues to orbital altitude. In the 10-20km altitude band, about 35 metric tons of water are emitted per launch and the quantity per km drops steadily as speed increases. Only 15 tons are placed in the 40-50km band. 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 20km level, and 4 to 20 years at 50km (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 and 360 Shuttle launches per year. These are compared with the best available estimates of the natural abundance of water vapor at these altitudes, namely 2xl0-6 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
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