>>43368
SO₂ freezes out in the stratosphere into aerosol and forms a haze that reflects part of sunlight. Since there's no weather going on in the stratosphere it takes very long for this haze to get cleaned out; it takes a couple years to clean up to normal levels.
But it's important that there's a strong temperature inversion at the top of the troposphere (similar to how air cannot rise out of basins in winter), so unless the eruption cloud is strong enough to punch through that, majority of it is washed out of the atmosphere quickly as acid rain.
The stratosphere starts at about 16-18km at the equator, and lower further north you go (cold air is more compact) down to about 8km in polar regions. Majority of Soufriere's plume stayed below that, but some punched through.
https://earthobservatory.nasa.gov/images/148190/tracking-la-soufrieres-plume