Incidently, that SO2 won't stay in the stratosphere forever.  Where does it go?  What happens THEN?

You are entirely right, and you ask basic and important questions that must be addressed.

For particle lifetimes in the stratosphere, please see diary section 2, paragraph 1.
(Lifetime before returning to the ground: roughly 2 years)

For the effects of particles leaving the stratosphere, please see diary section 3, paragraph 3.
(Effect: about a year's setback in ongoing global SO2 emission reduction, which is currently few percent per year.)

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