I'm more interested in all of the other parts - If you can feel the particles, I would think you ran through an area that those particles were airborne and then stuck to your bike, but why just certain parts?
An example would be on new vehicles that get shipped across country by rail road, The wheels on the rail cars get hot and tiny particles of the wheels fly off and land on the vehicles. these tiny metal particles are so hot that they embed themselves into the paint of the vehicles that are being hauled - It is called rail dust and the cars have to be sanded & repainted, any unpainted parts (like chromed plastic or whatever) have to be replaced