FIGURE 12.9 Calculated percent change in total column ozone during March as a function of latitude due to a Mach 2.4 HSCT fleet from the three models for which results were shown in Fig. 12.7, assuming NOA. emission radiation of 5 g of N02/kg of fuel. These are the predicted changes due to the projected HSCT fleet compared to a projected solely subsonic fleet (adapted from Stolarski et al, 1995).

Modeling studies have concluded that the longer term, global-scale, perturbation on the stratosphere due to the HC1 emissions is likely to be small (Prather et al., 1990; Denison et al., 1994; Jackman et al., 1996, 1998). For example, Jackman et al. (1998) predicted that the annually averaged loss of global total ozone by 1997 due to the previous launches of the space shuttle and Titan III and IV rockets was only 0.025%; two-thirds of this was attributed to HC1 reactions and one-third to reactions on alumina (vide infra). Even on the local scale along the flight path, the perturbation is predicted to be quite small if most of the chlorine is in the form of HC1. The conversion of HC1 to atomic chlorine by reaction with OH, and to a lesser extent by reaction with O or direct photolysis, is relatively slow. However, if even a small percentage of the HC1 is converted in the exhaust into more active forms such as CI 2, or if CI 2 is emitted directly, the predicted ozone depletion could be significant in the plume itself, via chemistry similar to that occurring over Antarctica in winter (see Sections C.4 and C.5).

For example, modeling studies in which 1% of the HC1 is assumed to be in the form of Cl2 in the exhaust predict that ozone depletions of 2-70% can occur inside the exhaust plume, increasing from 2% at about fO-km altitude to 70% at -40 km (Danilin, 1993). Observing and measuring significant depletion of ozone in the exhaust plume using satellites such as TOMS (Total Ozone Mapping Spectrometer) are not straightforward since the spatial resolution covers a much larger area than that over which ozone loss is anticipated (Syage and Ross, 1996; Ross, 1996). However, in one study in which stratospheric ozone was followed in the wake of two Titan IV rockets, 03 concentrations were observed to fall to near zero within a half hour of the launch and then to recover in the next half hour (Ross et al., 1997). Similar effects have been observed in the plume of a Delta rocket (D. W. Toohey, personal communication). This occurred over a fairly limited geographical region, ~4-8 km wide. Thus, such launches are not believed to be important in ozone destruction on a global scale.

The impact on the generation of active forms of chlorine by heterogeneous reactions on the A1203 particles is somewhat controversial (Prather et al., 1990; Danilin, 1993; Denison et al., 1994; Jackman et al., 1996). For example, the reaction of C10N02 with HC1 to generate Cl2, for example, proceeds relatively rapidly (reaction probability of ~0.02) at 208-223 K on hy-droxylated a-alumina (Molina et al., 1997). However, it appears that this is not unique to the A1203 surface but rather is due to water layers adsorbed on the solid.

A potential direct effect is the destruction of 03 directly on the particles, which appears, based on labo ratory studies, not to be very important (Hanning-Lee et al., 1996). It has also been proposed that these A1203 particles could act as seed particles for nucleation of other aerosol particles and clouds (Turco et al., 1982).

Robinson and co-workers (Dai et al., 1997; Robinson et al., 1997) have observed that y-Al203 that has been preheated to 1000 K under vacuum to dehydroxylate the surface and generate active sites for reaction can dissociate halomethanes such as CF3C1, CF2C12, and CC14. In these experiments, the gases were adsorbed on the preheated solid at low temperatures (100 K) and subsequently warmed. HC1 and CFxCly fragments des-orbed at ~ 150 K, and C02 between 240 and 320 K. Infrared as well as X-ray photoelectron spectroscopy (XPS) and temperature-programmed reaction (TPR) studies showed the formation of carbonate, bicarbonate, and/or formate and inorganic as well as organic forms of fluorine on the surface at f50-200 K. However, while such reactions may have some impact in the rocket plume itself, the low mass accommodation coefficient (~10"4-f0-5) combined with surface saturation effects will preclude significant effects on a global scale.

Injection of species into the stratosphere associated with these launches includes emissions not only from the rocket exhaust but also from ablation of the solid rocket motors, the paint on the outer hulls, and hardware from satellites and discarded portions of rockets in the atmosphere (Zolensky et al., 1989). The increase in launches of such vehicles has led to a significant increase in particles associated with solid rocket use. Figure 12.10, for example, shows the concentration of large (>l-yu,m diameter) solid stratospheric particles in the 17- to 19-km altitude region from 1976 to 1984,

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