Aerosols aren't gases in the classial sense and reflect sunlight. This works especially well high up in the atmosphere.

https://science.nasa.gov/science-research/earth-science/climate-science/aerosols-small-particles-with-big-climate-effects/

There are studies that collect data around volcano eruptions and coal power plants getting online and offline. Long story short: Climate is complicated; I'm not a climate scientist and not to be trusted; it would work great at cooling the planet; we definitely shouldn't do it (yet?) because it masks the temperature problem and could lead to us not reducing CO2 because we "wouldn't have to", but it could be a tool if we might be on the edge of a catastrophic runaway effect that causes too much water to evaporate into the atmosphere.

Update: Btw, you are right about dark particles low in the atmosphere, those typically warm the planet. It's mainly sulfur dioxide aerosols byproduct that cool the planet (also mentioned in the NASA article)