Option 2: Direct Injection of Sulfur Dioxide Gas Using Dedicated Fleet of Jet Aircraft
This is also an old idea (both Budyko and Broecker proposed it in the 1980’s), but one which is possibly doable. Sulfur dioxide gas is used on a large scale to bleach paper and as one of the feedstocks in the manufacturing of nearly 40 million tons of sulfuric acid in the U.S. alone. It is obtained by burning sulfur. Thus, there is no shortage of sulfur dioxide. It is available in 2000lb tanks as a 99.5% pure liquid. Converting this to a gas onboard an aircraft for purposes of release into the ambient air is a process that will have to be studied. It remains a liquid below 14°F and might stay one if simply bled out into the air behind a plane carrying it. If so, then it will have to be converted to a gas onboard and then bled out.
Using the 1.612Tg per year in the 2030 example from the table and considering that sulfur is half the weight of sulfur dioxide and assuming as in Option 1 that half the SO2 is converted to sulfuric acid aerosol, this requires roughly 17,600 tons per day must be released globally. Assuming a single plane can release 40 tons per day or 2.5 tons/hr, this requires a fleet of 440 planes. Each plane flies 3, 6 hour shifts per day and is in the stratosphere for 16 hours each day. This requires that 92lbs be released per minute from each plane.
The aircraft can likely be obtained from those retired from service that are stored in the U.S. in the Mojave Desert. These planes can probably be purchased for a few million dollars each and retrofitted for the sulfur dioxide release program. The sulfur dioxide cost is also borne by the government. This plan could also be carried out by a single country, most likely the U.S. or Russia, although to effectively distribute the sulfur dioxide would require use of the airspace of other countries, thereby reducing its utility as a unilateral approach.
Some of the same concerns as with the sulfur in jet fuel option apply here as well. Burning of jet fuel produces water that will react with SO2 emitted from the turbines and form sulfuric acid. In the case of direct injection of SO2 gas into the stratosphere, the conversion to sulfuric acid may be much slower. Likewise, it is not known how much SO2 is actually converted to sulfuric acid. Thus, it will be necessary to monitor the release to determine how much is converted. A co-release of water vapor along with the SO2 might speed the process of conversion.
Direct spraying of sulfuric acid is not practical, since dilutions on the order of 20-100:1 with water would be required and aerosolizing of the acid solution would probably not be possible or might result in ice formation or simply droplets too large to stay suspended.
Use of a dedicated fleet also frees up the planes to fly the most advantageous routes and times for release. As noted earlier, releases along the equator are more likely to become evenly distributed globally, while those at high latitudes tend to stay there. By not being locked into existing flight schedules, this release program will have much more flexibility.
The potential for civilian casualties is also greatly reduced, since the dedicated flights will carry no more than a dozen individuals vs. hundreds on commercial aircraft should a catastrophic failure result. Unlike the commercial fleet, these planes will not use high sulfur fuel, so any accidents would have to come from some other cause.
