oxide, carbon dioxide, carbon monoxide, sulfur dioxide) and created during reentry (nitric oxide). In order to simulate the long-term effects, the 1-D and 2-D photochemical models were run for 10 years of continuous HLLV operations at the rate of 400 launches per year with the contaminants averaged over different horizontal extents (Northern Hemisphere, the globe, etc.). The 1-D model simulations appeared to have reached a steady state. The 2-D model simulations had closely approached a steady state in the mesosphere, but had not quite attained steady state in the stratosphere. The 1-D model directly simulated water vapor deposition at all altitudes up to 120 km (75 mi). Water vapor deposition at higher altitudes was simulated by inserting it into the model at 120 km altitude from where it could be transported downward or converted photochemically to H2 and flow upward as well as downward. The hydrogen could then escape as discussed below. In the 2-D model, which did not compute H2, all water deposited above 90 km (56 mi) altitude was assumed to be injected at 90 km; the upper boundary condition was constructed such that water vapor could move across the boundary, seeking diffusive equilibrium of high-altitude water vapor (i.e., above 90 km) with the water vapor below. The short-term injection effects are treated with the plume (dispersion) model discussed in Sec. 2. Rocket contrails are treated in a companion paper (30). In the present section we discuss the widespread accumulation of ejecta under quasi-steady-state conditions and the dispersion reentry nitric oxide plumes for single events. Water Vapor The predicted increases in the water vapor abundances obtained with the 1-D and
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