They're torn up from the road and rubbed off from the tyres themselves. The majority is obviously coming from the road.

There's been studies done here in Sweden since we've banned studded winter tyres in some areas of Stockholm in an effort to reduce road wear and improve air quality. I don't know of any sources in English off the top of my head though.

The last time I read an article about it though (2+ years ago, during Diesel-gate), the findings was that the danger in the particles comes from their size with smaller particles being more dangerous for humans. And that road particles were something like an order of magnitude larger than exhaust fume particles from diesel engines.

We've since banned diesel engines inside Stockholm as well so the whole thing is moot here.