You need quite a bit, and it needs to be continually deployed. It wouldn’t be hard to tell who is launching like 25,000 tons of sulfur dioxide into the stratosphere every year. It would leave effects clearly visible by satellite: https://earthdata.nasa.gov/esds/competitive-programs/measure...

But I said $50 billion per year. That’s what probably way too high. Other estimates mention $18 billion per year, I found one that mentioned $2 billion per year, and just doing the calculation by hand based on fuel and per hour operating costs, I get only like $100 million, although that depends on altitude (I think 20km is usually ideal, although most cargo jets can’t go much above 13km).

I even wonder what the effect would be of just using sulfur-rich jet fuel on regular commercial long-haul flights. The cost would be minimal or even negative (as sulfur-rich crude is cheaper). I guess in this way any nation could just decide to do sulfur-rich fuel in their long haul fleet and few would immediately notice.

It’s such a crappy solution to the problem, though, and will doubtless cause a bunch more problems. But it’d be insanely cheap to do.