Thanks for your response, Enthalpy.
Your idea that "the air carries dust that settles on the fresh paint" was my first thought, which is why, after cleaning the space thoroughly, I installed an air filtration unit which presumably exchanges the air about 4x/hour in my volume of space. But it's made no difference. And the filter is remarkably clean which shows the air in my studio to be generally very clean.
I would not consider these particles to be "dust" (i.e., the kind of dust one sees in, say, a living room through a sunbeam which, agreed, is everywhere). The largest particles I'm getting on my painted surfaces are about the size of the grit of 80 to 100 grit sandpaper, or the size of table salt. I've looked at these particles through 5x magnification and they are clearly sand (along with some other residual debris such as pulverized paint from the sandblasting).
I can't imagine "sand" and other particles this size and weight lingering in the air. I don't see them. I have spotlights over my work area(s) and I can look into them in the way you suggested by creating a beam of light. There is no debris in the air. And most perplexing is that, if this grit were being carried by the air, I would think it would settle everywhere in my studio: on the floor, on my large work table, on my desk (on which I'm writing this). There is none of this grit anywhere else in my studio—it simply appears, seemingly by magic, on the surface of a painting as soon as I begin to apply the paint. This is why I've concluded that it must be 1), attracted and 2), traveling long distances, perhaps from other parts of the ceiling I've not covered with plastic—15' away or so.
Adding to the mystery is that often the particles appear first at the bottom of the painting—as if they're leaping onto the wet painting from the floor! I've tried spritzing the floor with water, again, to lessen any possible static charge caused by low humidity. This seems to have helped at times but not at others.
Now you see why I'm so frustrated! The grit does seemingly appear out of nowhere. The source of the grit must be the nooks and crevices of the ceiling and elsewhere that still have retained some of the sand from the sandblasting. But this suggests the grit is traveling long distances and doesn't stop along the way to alight on any other surface.
And you're right, the answer would seem to be to seal off the areas likely to be the source of the grit. (That's going to be a little tricky in this space.)
Thanks again for reading all this and for your thoughts, greatly appreciated. I could give a few more details but this is the gist. I actually thought that, once the weather became warmer and I could open my windows, the humidity would increase and lessen the static charge, if that were the problem. But it's been getting warmer out and has made no difference to my problem.