My 8th grade Physical science teacher had told me that the pollution can be lowered with lower temperatures.
I lived in Alaska at the time. He explained that the air was cleaner because 1) their wasn't as much pollution. 2) what ever pollution was their went away with the cold weather (-10's and mostly bellow freezing temps in the winter)
I have been thinking about Acid rain. Its polluted rain, so does snow just bring all the pollutants down with it?