What's the dust that coats everything stored in a parking garage? Is that tire dust or something else?

Tire and brake dust, sand, little bit of grease, some soot settling out of the smog, maybe some salt (if you're by the sea)

I used to have an apartment abutting a parking garage and also above a busy three lane road.. the worst black dust and grime everywhere. Pretty sure moving out of that apartment probably added years back to my life.