Passenger cars in North America saw incredible advances in emissions almost entirely courtesy of California, that state spearheading demands to respond to the LA smog crisis that peaked in the 1980s. Other jurisdictions, like my home province of Ontario, just went along with the giant. It's why diesel cars are so rare, and as we know often lied (quite aside from the VW thing, every single instance where a diesel vehicle has been measured under actual use has seen emissions dramatically worse than claimed).

Big trucks, however, couldn't make the transition as quickly, and for the necessary torque still rely upon diesel. And they've gotten away with much, much worse, though again California is taking the lead and in the next few years older trucks will be banned from the roads.

Regarding the location of particulate sources, I've read that even in London, England, something like 75% of the particulate matter is from outside the city, blown in from farms and rural areas.