Is it just me or is there nothing wrong with McDonell's tweet? Maybe I'm missing something but I don't really see what the fuss is about.
It's the fact that it's some stupid off-hand comment that doesn't really have any bearing on the fact that it's a picture of a snowstorm.
Like saying "I'm sure there's some illegals mixed in there, but it's really just a cinco de mayo party"
On the technical side, the snow storm (or a rain storm) would have a pollution clearing effect. Due to the large surface area presented by the many particles of snow/water droplets swirling around, particulate pollution is quickly adsorbed by them. That's the "fresh air feeling" after a rain/snowfall.