Some of the cameras show either a lot of fog or a lot of smoke. Earlier it really didn't look like fog because of how quick it was moving and it was not spread out like fog would be.
Live Webcam 1 - Reagan National Airport - Washington D.C.
Live Webcam 2 - 4K - Reagan National Airport - Washington D.C.
Live Webcam 3 - 4K - Reagan National Airport - Washington D.C.
Hunter smoking too much crack
It's a temperature inversion creating a ground-hugging "pea-soup" fog mixed with car exhaust, jet exhaust, house chimney smoke, ground heat rising from sewers, fish stink from the river, etc. They used to be a lot, lot worse.
I actually watched as the smoke started and increased.