The balcony of the flat where I live at is unusable because of the major street next to it. There's this same greasy dust everywhere. My guess is that it is soo

The balcony of the flat where I live at is unusable because of the major street next to it. There's this same greasy dust everywhere. My guess is that it is soot, covered with unburnt hydrocarbons, from car or oil heating exhaust. It's really disgusting. I have resigned long ago to not let plants intended for consumption grow there...