Julia, I don't know much about that idea - only what I read about that in popular press. While sulfate aerosols do indeed posses sun-blocking capacity, they are also known to deplete ozone layer. I don't know if this effect is significant for stratospheric implantation, but I'd be careful not to trade warming to ozone depletion.
Also, what comes up must eventually come down -- and having sulfur rain does not sound very appealing. Also, I don't know if one can produce a homogeneous layer of that stuff...
So, it does not sound very appealing to me, but what do I know...