That's pollen, and I have a coating of it most days here in Vegas leaving work. If it rains a little it makes things look worse (like yours).
If you're in a developed area (assuming with buildings you are), you can say "fk you humans" because we plant the trees in developed areas and there's been a lot of fallout for bad decisions made in planning in decades past as the trees we tend to plant (not always the same ones that were native) also tend to produce a boatload of pollen, which is leading to increased allergy issues for people in cities as the trees matured/grew bigger and now produce that boatload extra of pollen.