Some studies indicate that less than half the sulfur is instantaneously converted to sulfate aerosols or is converted during the lifetime of the plume that is formed. The IPCC study reported that only a few percent were converted immediately after combustion and during plume life (27), while a study involving analyses of plumes found at least 32% conversion (33).
Surprisingly little work has been done on tracking what happens to the sulfur oxides or sulfate aerosols after a plume dies out (34). Indeed, there is a lack of understanding of the fate and significance of aviation emissions in general and as such this is considered to be an important research area (35) especially with regard to the effects on climate change since aviation’s contribution to greenhouse gas warming is growing much faster than other areas.
Since the water vapor content of the stratosphere varies, it could be assumed that in certain areas, most of the sulfur would eventually react with water vapor and form sulfuric acid. But this is an assumption that requires verification. Thus, the actual benefit to be achieved may be much less than that due to the 80% of the total tons of sulfur burned in jet fuel in the stratosphere. This may negatively impact the value of this option as well as others involving SO2 injection, since low conversion rates (<5%) would require more than 10 times as much sulfur than is predicted here.
Obviously, large quantities of SO2 have made their way to the stratosphere and been converted into sulfuric acid aerosol as evidenced by the results of various volcanic eruptions (Pinatubo, Tambora, etc.). But can one compare the volcanic eruptions with man-made release in the stratosphere from emissions or deliberate mass releases like those described here?
Volcanic eruptions pass through the troposphere on the way to the stratosphere and thus can pick up water vapor as they rise. Water from the volcano is often included in the eruption column as well. So the SO2 from a volcano has several sources of the water needed to form sulfuric acid. In the relatively dry environment of the stratosphere, there may not be enough water vapor to convert SO2 into sulfuric acid. And the higher the release, the lower the water content of the atmosphere. So releases at 50,000-100,000 feet may offer aerosols a longer lifetime, but if there is no water vapor, then they will never form in the first place. At lower altitudes like 35,000-50,000 feet, their formation is also problematic and dependent on the water vapor present.
Thus, whether or not the remaining sulfur compounds from man-made emissions are eventually also converted to sulfate is not known at this time. However, if it were all converted within a few hours, then the amount of sulfur required to produce the desired effect could be decreased. With the above assumptions, the expected amount of sulfur in jet fuel that would form sulfate aerosol in the stratosphere would be about 40% of the total (50% of 80%) and this 40% factor is reflected in my subsequent calculations.
Crutzen and Wigley provided estimates of the amount of sulfur as sulfate aerosol required to be injected into the stratosphere to achieve forcing offsets equivalent to about the last 50 years (1 million metric tons) and for a doubling of CO2, about 5 million metric tons (1700-2100 or sooner). This, no doubt left the impression with most readers, including the media, that massive pulse injections of sulfur, equivalent to volcanic eruptions would be required annually or somewhat less frequently, although Crutzen did mention continuous injection.
