Black dust on my rear bumper

[landrovered]

Drives me crazy on my yellow boxster, it is especially bad when there is dew out in the morning. I think it is a combination of road grime kicked up by the rear tires and perhaps a bit of exhaust smoke and brake dust thrown in for good measure. My car can be spotless except for the dirt on the rear bumper. Now I take a damp rag and wipe it off when I see the problem. It does not happen when I drive the car later in the day or when it is dry out.