That's why I used the qualifier "gasoline-fueled": the particulates are coming mostly from _diesel_ engines. (I didn't belabor that point earlier because I ass

That's why I used the qualifier "gasoline-fueled": the particulates are coming mostly from _diesel_ engines. (I didn't belabor that point earlier because I assumed that people here would already know that because the VW diesel-emissions cheating scandal was such a big story.) During a busy commute in a major US metro area, the vast majority of the vehicles on the road are gas-powered (because a large fraction of trucks will avoid busy commutes). I realize the situation is different in Europe where diesel-powered cars are much more common.