logical conditions typical of the Cape Kennedy area, ground-level NO2 concentrations in the vicinity of the cloud on the order of 0.10 ppmV may be produced following a launch. Thus, the launch by itself is unlikely to cause a violation of a 0.25 ppmV standard, but may be sufficient to cause a violation if ambient concentrations are already close to the standard. Under more adverse meteorological conditions, the contribution of the ground cloud may be even higher and further work is required to adequately investigate this possibility. Figure 1 shows the results of one calculation of the NO2 concentration at ground level as a function of time after launch and of the radial distance from the point just below the ground cloud center. The distance has been normalized by the standard deviation a of the (horizontal) Ganssian distribution assumed for the ground cloud contribution to the N0x concentration. The estimated initial value of cr was 2200 meters, and 3220 meters after one hour. The cloud stabilization height was 1300 m. Ambient concentrations were: ozone = 0.12 ppmV, N0x = 0.25 ppmV (NO2 = 0.23, NO = 0.02). Contour values are given in ppmV. A region in which the NO2 concentration is greater than 0.25 ppmV clearly exists and persists for approximately two hours. The second effect that must be considered is a possible enhancement of acid rain in the vicinity of the launch site. When dissolved in water, NO2 forms a mixture of nitric and nitrous acids. Rainfall through the ground cloud (or rising from within) will therefore be more acidic than would otherwise be the case. By making the unphysical assumptions that dissolved NO2 is instantly and completely converted to acid and ignoring the fact that cloud NO2 will be depleted by the washout process, an estimate of the pH of rain (rainfall rate= 10 mm/hour) of 3.5 is obtained. This should be regarded as an unrealistically low value; a more realistic estimate would lie in the range 4.0 - 5.5. This is not considered excessively acidic, and no significant effects are expected especially when the highly localized and transient nature of the precipitation is taken into account.
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