[Robert A Cook PE]

I agree: atmospheric dust (from very large volcanoes) creates a noticeable cooling effect for a few years after the big eruptions.

But do the sulphur oxides act as a net reflector, or a concentrator of sunlight?

[cogitator]

Actually, the main cooling effect is from sulfur dioxide aerosols in the stratosphere (reflection). The dust settles out a lot quicker, so its cooling effect is short-term.