Since its my experience its anecdotal. That said higher particulate from diesel engines its pretty common when not at highway speeds. Combine significant traffic with highways/interestates close to buildings and residential areas, its a bit nasty. Sure rush hour in Saigon is without a doubt worse but the bay area is pretty bad on a localized level. Keep in mind I am saying localized, just the immediate areas around these high traffic areas. I can recount a number of buildings I have been in where you would get significant soot buildup on a weekly basis.

CA is fairly unique in this respect in that there are major road ways with residential close by.