SO2 is a gas that mixes with water to form sulfuric acid. The droplets can form clouds like water does (Venus is covered with them making it bright). Sulfuric acid has a high albedo (it reflects light), since it reflects the light, the energy it blocked from coming down. Therefore the electromagnetic radiation (aka light) cannot is not converted into thermal energy (it converts from light to thermal when it smacks into things). Therefore since the light cannot make it to the surface, the thermal energy will decrease (molecules will slow down) and the temperature will theoretically drop. :) I said theoretically because I\'m not quite sure how strong of a greenhouse gas CO2 is (I\'m not sure if it would trap any heat that did manage to get in) as compared to water vapor, but it doesn\'t sound like they care about that. :) Anyway sorry if I wrote that in a confusing way, basically the light will reflect off the clouds of sulfuric acid and so the light can\'t heat up the earth. Also, its better in the stratosphere because the sooner the light is blocked, the less it will heat up molecules it smacks into on the way down. Solution SO2 is a gas that mixes with water to form sulfuric acid. The droplets can form clouds like water does (Venus is covered with them making it bright). Sulfuric acid has a high albedo (it reflects light), since it reflects the light, the energy it blocked from coming down. Therefore the electromagnetic radiation (aka light) cannot is not converted into thermal energy (it converts from light to thermal when it smacks into things). Therefore since the light cannot make it to the surface, the thermal energy will decrease (molecules will slow down) and the temperature will theoretically drop. :) I said theoretically because I\'m not quite sure how strong of a greenhouse gas CO2 is (I\'m not sure if it would trap any heat that did manage to get in) as compared to water vapor, but it doesn\'t sound like they care about that. :) Anyway sorry if I wrote that in a confusing way, basically the light will reflect off the clouds of sulfuric acid and so the light can\'t heat up the earth. Also, its better in the stratosphere because the sooner the light is blocked, the less it will heat up molecules it smacks into on the way down..