2-D photochemical models for 10 years of HLLV operations are shown in Fig. 7. It is apparent from the results of the 2-D model that a significant "corridor effect” (enhanced accumulation in the vicinity of the launch latitude) is likely only at altitudes above 75 km (45 mi). At an altitude of 85 km (the highest altitude at which the 2-D model yields reliable results) the computed corridor enhancement is —50%. Corridor effects will be discussed in detail elsewhere. The globally averaged increase in H2O obtained with the 1-D model is shown by the broken curve in Fig. 7. In the stratosphere [i.e., at altitudes below 50 km (31 mi)] the two models predict nearly the same increase in water vapor. However, between 50 and 75 km (31-47 mi) (lower mesosphere), the increase obtained with the 2-D model is about double that obtained with the 1-D model. The difference is a consequence of the vertical “eddy diffusion” coefficient in the 2-D model being somewhat smaller than that in the 1-D model. On the other hand, the actual value of that coefficient is uncertain and our results give an indication of the effects of this parameter on the H2O prediction. Many meteorologists believe that the influx of water vapor to the stratosphere is controlled by the low-temperature “cold trap” of the tropical tropopause (16,19,31,32). The mechanisms that remove water vapor from the stratosphere are still unknown, but are probably dominated by subsidence at midlatitudes and high latitudes; we have made such an assumption in our model calculations. Another but less important loss mechanism for water vapor is photochemical conversion to relatively inert molecular hydrogen in the mesosphere, mainly through the reaction sequence some of the H2 diffuses to the troposphere, and some is dissociated at high altitude, contributing to the hydrogen escape flux to space (for discussion of the escape mechanisms, see Ref. 33). According to recent measurements the hydrogen escape rate is —108 atoms cm-2 sec-1 (34). An escape flux of this magnitude was imposed as an upper boundary condition on the hydrogen in our 1-D model. Molecular hydrogen may also be converted back into water vapor by reactions such as However, in the mesosphere, H2 is produced at the expense of H2O, and diffuses into the stratosphere and eventually the troposphere. Our results are in substantial agreement with those of Forbes (35). The effect of the deposition of water vapor and hydrogen at high altitudes [up to 120 km (75 mi)] is a substantial buildup of thermospheric molecular and atomic hydrogen. The H2 dominates below about 120 km and the H above 120 km (e.g., see Refs. 34 and 36). The computed increase in total hydrogen above 100 km (62 mi) (globally averaged) is about a factor of 2 (i.e., a doubling of thermospheric H and H2).
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