I'm guessing g the term you are looking for is volcanic winter. It's when volcanic ash and sulfuric acid droplets block out the sun after a large volcanic explosion. I would suggest a quick google search to figure out if this word is the right word you are looking for.
The answer, though i'm not quite sure' should be sulphate aerosols, though i think that is actually what the clouds are made out of. So the answer might be instead just stratospheric ash clouds
