Winter Walking and Cycling

[tomh009]

At least the brown is more of an indication of the particulate pollution caused by cars...I suspect what you're looking at there is particle-ized tires and brake pads, and a bit of soot from diesels.  If we're lucky, maybe some sand from sanding roads instead of salting.

And plain old "dirt" … the dust particles that accumulate on the roads in the summer, too, and make our cars look dusty (and which the rains then wash away). Although admittedly I have done zero research into the origins of said "dirt"!