Put it this way, although cars are allegedly better than they were, fuel consumption hasn't dropped considerably. The cars are more numerous than ever, and, although there are EVs, there are still more ICE cars than there were in the good old days when petrol came with lead in it.
I am not sure that most people in urban areas even know what good air tastes and smells like. I take a canal path through lush countryside, far from any cars for most of the way. This canal has an aqueduct (or is it a viaduct?) over a motorway and the contrast is incredible. You go from basically smelling flowers to air pollution and back to clean air again quite quickly, so the filth is totally noticeable. Note the cars on the motorway are going at speed, so they should be working efficiently (until a few decades ago 56 mph was what engines were optimised for regarding efficiency in the UK).
If just living in a major city then you don't get this instant switch from bad to good air. So you just don't notice it. If you could see the filth, you would prefer a swimming pool that was pissed in, it is that toxic.
If you do have to live in a city, my top tip is to find out if there are any meteorologists in town. If there are, buy a house next to where they are living. Anecdotal, however, I used to work with meteorologists and they would always live to the West of the city centre, to get cleaner air than those living in the east of the city, or further downwind.
Again anecdotal, however, due to the canal and motorway experience described above, in post-industrial countries such as the UK, it is definitely the vehicles rather than any other source. Given the choice of microparticles that just get in your blood or clumps of big particles that you can eventually cough up and spit out, I would much prefer the latter. My hunch is that the legislation to improve vehicle emissions has optimised the exhaust for nanoparticles. Please prove me wrong!