cations to the composition and perhaps the temperature structure of the stratosphere and mesosphere (middle atmosphere) might be expected. The purpose of the study reported here is to assess possible effects on the middle atmosphere of continued HLLV activity for 10 years or more. The principal emissions of the liquid-fueled HLLV launch motors that might modify atmospheric composition are water vapor, hydrogen, and carbon dioxide (Table 1); copious amounts of nitric oxide are also formed during reentry. In addition, small amounts of nitric oxide and sulfur dioxide are expected to be formed in the combustion process and to be deposited during launch. Figure 1 presents water vapor and hydrogen deposition profiles for a single launch in units of molecules per centimeter; note the discontinuity at an altitude of 56 km (35 mi), where the first stage, which uses methane as fuel, cuts off and the second stage, which used hydrogen as fuel, ignites. Also note the large increase in the deposition rate as the vehicle approaches an altitude of 120 km (75 mi); this is a result of nearly horizontal flight near the end of the trajectory and burnout at 120 km. Carbon dioxide, a product of methane combustion, is also emitted up to first-stage burnout; its deposition rate, by number, has been estimated to be 0.4 of the water vapor rate. The emission rate of nitric oxide during the launch phase is also shown in Fig. 1. The emission rate of sulfur dioxide is estimated by assuming that the first-stage fuel contains 0.05% sulfur (K. L. Brubaker, Argonne National Laboratory, private communication, 1980). Because the production of nitric oxide during reentry is the subject of a rather lengthy calculation, we defer its discussion to a later section. The complexity of the physical processes occurring in the middle atmosphere requires mathematical models to obtain quantitative estimates of the effects of HLLV operations. General circulation models that consider the interactions of radiation, chemistry, and dynamics are beyond current capabilities. Hence, a range of more limited models must be used to describe each area of concern. In the following sections, we discuss the atmospheric photochemical models used in this assessment, including a model for the calculation of nitric oxide production during HLLV reentry. We then apply the models to the assessment of HLLV effects on the middle atmosphere, evaluating possible global effects and short-term local effects. Specialized topics (i.e., “corridor” effects and contrail and cloud formation) are treated in accompanying papers. Finally, we summarize our conclusions and identify those effects that could be significant and thus require further study.
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