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similar reactions occur on the surface of sulfuric acid solutions (and presumably would occur on the surfaces of sulfuric acid and dust particles), but are 100 to 1000 times slower (Tolbert et al., 1988). Given the rapid alternation of light and dark at mid-latitudes compared to the 6-month cycling at the poles, these reactions are estimated to account for a 1 percent depletion of ozone at present. However, in the presence of enhanced concentrations of sulfuric acid (or, presumably, dust) in the stratosphere, the reactions could become much more important.

The El Chichon volcanic eruption in 1982 is estimated to have released 1.2 × 1010 kg of sulfur compounds, compared to the release of 1010 kg of dust or aerosol discussed above, leading to a concentration of 0.03 g/m2, compared to the target of 0.02 g/m2 discussed above, about 10 times the background concentration of 0.002 g/m2. After this eruption the ozone concentration within the eruption plume in the stratosphere decreased by amounts up to 20 percent. However, since the volcano also emitted enormous quantities of hydrochloric acid (HCl) (equivalent to 9 percent of the existing HCl in the entire stratosphere), it is not clear how much of the depletion was caused by reactions involving the dust and aerosol, and how much was due to the increased Cl from the HCl (Hoffman and Solomon, 1989).

It appears that destruction of stratospheric ozone due to chemical reactions on the surface of added dust or aerosol in the stratosphere is a possible side effect that must be considered and understood before this possible mitigation option can be considered for use.

A National Research Council (1985) report cites papers by Cadle et al. (1976) and Mossop (1963, 1965) that give the amount of silicate particles from the 1963 Mount Agung eruption with sizes between 0.2 and 2.0 µm as 1 × 1010 kg, about the loading the panel assumed would have to be added. The half-life of this dust is not given, but the life of a sulfate aerosol with a size of 0.2 to 0.45 µm and a column height of 23 km is given as roughly 1 year, consonant with the panel's lower estimate.

Note that the dust can be expected to produce visible optical effects, such as spectacular sunsets, as in the case of volcanic dust.

Delivery Scenarios

Naval Rifles A 16-inch naval rifle fired vertically could put a shell weighing about 1 t up to an altitude of 20 km. With larger propellant loadings, some sacrifice in payload, or the use of sabots (a device fixed to the shell so that it will fit properly in the rifle barrel), higher altitudes could be achieved. Note that any launch technology could be used, but so much less is known about items such as rail guns that system and cost estimates based on existing launch technologies seemed the best choice.