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Aircraft Exhaust Penner et al. (1984) suggested that emissions of 1 percent of the fuel mass of the commercial aviation fleet as particulates, between 40,000- and 100,000-foot (12- to 30-km) altitude for a 10-year period, would change the planetary albedo sufficiently to neutralize the effects of an equivalent doubling of CO2. They proposed that retuning the engine combustion systems to burn rich during the high-altitude portion of commercial flights could be done with negligible efficiency loss. Using Reck's estimates of extinction coefficients for particulates (Reck, 1979a, 1984), they estimated a requirement of about 1.168 × 1010 kg of particulates, compared with the panel's estimate of 1010 kg, based upon Ramaswamy and Kiehl (1985). They then estimated that if 1 percent of the fuel of aircraft flying above 30,000 feet is emitted as soot, over a 10-year period the required mass of particulate material would be emitted.

However, current commercial aircraft fleets seldom operate above 40,000 feet (12 km), and the lifetimes of particles at the operating altitudes will be much shorter than 10 years. An estimate (National Research Council, 1985) for the half-life of smoke is 1.4 × 10-7/s.14 This gives a half-life of 83 days, or a little less than one-quarter of a year. Thus the amount of fuel to be turned into soot continuously for complete mitigation (1012 t C) is closer to 40 percent than to 1 percent. That seems impractical. However, if the amount of mitigation required is equivalent to the 1989 U.S. emissions of greenhouse gases equivalent to CO2 (8 × 109 t CO2), the amount of soot required would be 500 times smaller, and the required soot corresponds to less than 0.1 percent of the fuel burned. If 1 percent of the fuel were used, about 25 × 109 t CO2/yr could be mitigated.

In 1987, 16 percent of the cash operating expenses of airlines were spent on fuel (U.S. Bureau of the Census, 1988). Because the operating revenue in that year was $45,339 million, the approximate cost of the particulate emissions from jet engines for mitigation of the 1989 U.S. CO2 equivalent emissions would be about $7 million, or about $0.001/t CO2/yr plus the capital costs of adjusting the aircraft engines.

This provides a cost range of $0.001 to $0.1/t CO2/yr. An alternate possibility is simply to lease commercial aircraft to carry dust to their maximum flight altitude, where they would distribute it. To make a cost estimate, a simple assumption is made that the same amount of dust assumed above for the stratosphere would work for the tropopause (the boundary between the troposphere and the stratosphere). The results can be scaled for other amounts. The comments made above about the possible effect of dust on stratospheric ozone apply as well to ozone in the low stratosphere, but not in the troposphere. The altitude of the tropopause varies with latitude and season of the year.

In 1987, domestic airlines flew 4,339 million ton-miles of freight and express, for a total express and freight operating revenue of $4,904 million