Option 3: Direct Injection of Sulfur Dioxide Gas Using High Altitude Jet Aircraft
The higher the release of the sulfate aerosol precursor compound, the longer the aerosol will stay in the stratosphere. By having the release occur at 50,000 feet instead of 35,000-40,000 feet, the aerosol may stay aloft for 3-5 years instead of 1-2. This will reduce the number of flights and total quantity of precursor compound necessary. However, to date only relatively small aircraft have been designed that can fly at these altitudes. Lack of sufficient thrust and lift limits the operational ceilings and the total payload. If Option 2 is ever considered, it may be worthwhile to begin developing aircraft with this capability.
Option 4: Direct Injection of Ammonium Sulfate Aerosol Using Dedicated Fleet of Jet Aircraft
One of the concerns about release of SO2 and the subsequent creation of sulfuric acid aerosol is that eventually, the sulfuric acid will reach the surface of the Earth in the form of precipitation. If the buffering capacity of the soil or water on which it falls is insufficient, the pH may be lowered and environmental damage done. The amount of sulfuric acid produced from these strategies would probably not adversely impact the oceans, where most of the rainout would occur and there is no evidence that there would be a problem with land impacts either.
In the event that it is determined that there is a problem with acid rain, it has been previously suggested that ammonium sulfate be used instead (22). Ammonium sulfate has a pH of 5.1, somewhat acidic, but far less so than sulfuric acid.
Ammonium sulfate is produced as a granular solid commercially from petroleum refining and steel production waste streams and by reacting anhydrous ammonia with sulfuric acid. It is used primarily as a source of sulfur and as fertilizer (42, 43).
Ammonium sulfate aerosol is formed naturally by the reaction of sulfuric acid with ammonia, a gas present at around 5ppbv in the troposphere. Some of the sulfuric acid released in these hypothetical programs would be converted to ammonium sulfate before reaching the surface.
To create ammonium sulfate in the stratosphere would require that both SO2 and ammonia gas are released simultaneously and that the SO2 is instantly converted into sulfuric acid which then reacts with the ammonia to form ammonium sulfate. This seems highly unlikely. Therefore, a direct release of ammonium sulfate is the only option worth considering.
Ammonium sulfate is soluble in water up to a 70% solution at room temperature and 100% in boiling water. The more concentrated the solution, the less the weight that has to be carried aloft. However, spraying an aerosol form of this aqueous liquid into the low temperature of the stratosphere will be problematic for several reasons. First, it is difficult to aerosolize a viscous solution. Also, regardless of the water content, ice crystals would be produced, trapping the ammonium sulfate and causing it to fall from the stratosphere rapidly.
A nanoparticle form of ammonium sulfate probably cannot be created due to its hygroscopic nature. Thus, a dry particle release is also probably not possible. The potential for this should, however, be investigated.
