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Think of the surface of a sphere out from the source. The surface area of this sphere goes like the square of the distance. There's a constant amount of stuff distributed over this surface, and because you breathe in through a constant sized area you get an inverse square relationship of particles fluxing into your lungs.
wence
Definitely all of this. Pollutant levels fall off with a polynomial (I think it's the square, but can't work out why it's not the cube, after all, we're talking a volume here) of the distance from the source. That means 2m away is 4 times better than 1m away, and 10m away (on an adjecent side road) is way better than a mask for the bad stuff, which the mask barely touches.
Also means an extra couple of meters stopped behind a car is well worth it, rather than resting your tyre against the exhaust pipe, as seems popular...