tions of both H and H2. At 120 km (75 mi) both H and H2 have upward fluxes. Therefore, we specify the upper boundary fluxes as The constant h is determined from simultaneous estimates of the escape flux and nH, and the constant n is estimated from the flux of H2 at 120 km (75 mi). This H2 is decomposed into H above 120 km. In Fig. 2, 1-D-model predictions for the ambient atmosphere are compared with observations. The agreement between the mesospheric prediction of atomic oxygen, O(3P), and its observation is reasonable, considering the large zenith angle of observation; the computed concentrations are perhaps too large by a factor of 2 between 90 and 100 km (56-62 mi). Computed ozone concentrations are generally within the error bars of the Krueger-Minzner empirical model (18), although the high-altitude [above 45 km (28 mi)] predicted concentrations are somewhat too small. Predictions for OH and water vapor appear to lie within the range of observed values; however, it should be remarked that the range of observed water vapor is quite broad, extending from as low as 2 parts per billion by volume (ppbv) to as high as 7 ppbv (20). Also, the OH observations were made at large solar zenith angles whereas the computed values are for average midlatitude daytime conditions; to correspond to the same conditions, the computed values would have to be reduced by a factor of ~2. The predicted nitric oxide abundance in the stratosphere lies well within the range of measurements. However, in the upper mesosphere and lower thermosphere, there is disagreement, which may be due in part to the large zenith angle of the observations and in part to adjustments needed in the mesospheric NO photochemical source [production of NO from N(2D) may have been overestimated] and sink and in the diffusion rates at these heights. The 2-D model extends from the surface to an altitude of 90 km (56 mi) in 2.5-km (1.6-mi) steps and from latitude 80 °N to latitude 80 °S in 5° steps; the time steps are fixed at 1 day. End boundaries are taken at latitude 80 °S and latitude 80. °N because meridional fluxes are expected to be small at those latitudes. Hence, the end boundary conditions are taken to be zero flux of all constituents across the vertical boundaries. The upper boundary conditions are given by setting the fluxes equal to zero for all species except H, NO, and O(3P). We have checked the upper boundary conditions [at 90 km (56 mi)] for the various species against results from a one-dimensional model that extended up to 120 km (75 mi). It was found that the effect of choosing a mixing equilibrium condition at the upper boundary had very little effect on any of the constituents at altitudes below 50 km (31 mi). The lower boundary condition used for all species, except N2O, CH,, HNO:S, NO2, O3, HC1, H2O2, and the halocarbons, is chemical equilibrium because of their short lifetimes against chemical loss. Because HNO:1, HC1, and H2O2 are water soluble, their number densities are set equal to zero at the lower boundary. The number densities of CH.(, NO2, and N2O are fixed at
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