Does anyone have insight into the chemistry of these nanoparticles?

Because, clearly, cars don't emit gold. Cars do emit platinum and palladium from degrading catalysts, but these don't condense from a gas phase, as the article implies. The article also mentions particles formed from NO2, but these particles would be nitrate salts, which dissolve in blood and therefore aren't particles anymore. The same is true for much "kicked up dust"; calcium, magnesium, iron compounds would dissolve, too. All of these ions are present in blood anyway, so a tiny bit more makes no difference.

So what are we really talking about? Soot? Anything else?