The two-dimensional numerical model predictions for NO deposition are shown in Fig. 4. A prominent corridor effect is predicted for altitudes from 70 through 85 km. Figure 4 differs from Fig. 3 in that the peak abundance at a given altitude (above 50 km) shifts toward lower latitudes for increasing altitude. This effect is caused by the vehicle entering the atmosphere in a south-to-north trajectory. Although the numerical model predictions shown in Fig. 4 agree with the scaling arguments results shown in Fig. 2, in that both predict a corridor effect for altitudes above 60 km, the numerical model predicts much lower values of 17 above 75 km. The difference between the two calculations is caused by the effects of chemical reactions that become increasingly important with altitude. In particular, the chemical lifetime of NO above 60 km is much shorter than the transport time. Consequently, the previous analysis must be modified to include the effects of chemistry on the production and loss of NO. Figure 5 shows the lifetimes of H2O, H2, and NO versus altitude at latitude 30° N. Also plotted in Fig. 5 is the characteristic time for meridional diffusion. Because the chemical lifetimes of H2 and H2O are much longer than their transport time, the previous analysis is satisfactory for these compounds. EFFECTS OF CHEMICAL REACTIONS When chemical lifetimes are comparable or short compared with characteristic times for transport, the effects of chemical reactions cannot be ignored. Chemical reactions cause the transformation of the injected species into other species and cause changes in the abundance of other species. An equation similar to Eq. 5 can be
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