You mean sulfate — the sulfur in the volcano oxidizes when it hits air at lava temperatures, if it wasn't oxidized already. That's what I was referring to with "stratospheric acid". It's true that it would be cheaper, but we're talking about the equivalent of one Krakatoa eruption every couple of years, for centuries. Nothing like that has happened since the advent of life on earth, so although it might work and be safe, or it might have unexpected and undesired effects, for example due to sulfuric acid in rain.