Will be industrial fallout, minute particles of steel, are you near an industrial area or something?
You may be able to get rid of them by claying and polishing and making sure you put a good wax/sealant on your paintwork to try and stop them bonding on in the future.
If they are stubborn and dont want to come off quite so easy some Aquartz Iron-Cut may be required.
Spray on leave to work and wipe off.
Does similar to the claying but without the potential abrasion from the claying process.
HTH.