This analysis is intended only to be suggestive of the effects that may result. More refined estimates will be necessary before reliable conclusions can be drawn. The photolysis of water vapor in the upper stratosphere and mesosphere, together with attack by excited oxygen atoms produced by ozone photolysis, may be expected, for example, to very significantly decrease the residence time and with it the perturbation ratio. At altitudes of 70-80 km, for example, the water residence time may be reduced to a small fraction of a year due to such chemical and photolytic processes. If the residence time is reduced to 0.1 year, the perturbation ratio would be correspondingly reduced to 0.12. The consequences, however, would be greater potential involvement in the complex set of chemical reactions which control the ozone concentration at these altitudes. Even the direction of the effect is not predictable without a much closer examination. The effects of vertical and horizontal transport must also be included. Finally, it should be pointed out that at the present time, the budget of water vapor in the stratosphere and above is only poorly understood. Considerably more information will be needed in this regard before reliable quantitative predictions of the effects of water from rocket exhaust can be made. If similar calculations are made for carbon dioxide as were just discussed for water, the perturbation ratio for a one-year residence time is computed to be approximately 5.9 X 10"“^ at 16 km, 1.2 X 10~5 at 35 km, and 1.0 X 10“^ at 50 km. Even allowing for more realistic residence times, it does not appear likely that the carbon dioxide emissions would have even a detectable effect. For example, using the residence times given in Table 3.2, the predicted change in the carbon dioxide concentration is 0.0001 percent at 16 km, 0.01 percent at 35 km, and 0.09 percent at 50 km. Of course, since carbon dioxide is emitted only by the first stage of the HLLV, emissions are zero above about 50 km. Nitric oxide is formed from ambient nitrogen and oxygen in the hot rocket exhaust plume. Estimates of the amount produced in the exhaust of the space shuttle booster, as given in Refs. 3.36 and 3.37, amount to only 3-4 kg/km/flight at an altitude of 30 km. If the same estimate holds for the HLLV, the NO-injection rate at 30 km would be 1.2-1.6 metric tons/km/yr. The HLLV is expected to be significantly larger, however, and more nitric oxide is expected to be generated. Based upon more recent calculations of Gomberg
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