Actually “snow tourism” has little to do with causing the pollution. And most holiday skiers are little impacted by it, since the pollution is mostly in the valley, and the they’re at some ski station up on a mountain.
I’m not an expert about it, but my understanding it that it’s mostly caused by lack of circulation of the air. There was an article on NYTimes.com about how it happens in Salt Lake City (where if anything it is worse and more frequent than in the Alps).
I guess it’s especially bad this year because there’s snow down to such a low altitude, which keeps the valley air cool—so it does not rise and mix with the higher air to disperse the pollutants (especially particles). Then as particles accumulate low in the valley, they block sunlight from melting the snow. But the higher elevations get more sunlight so they stay warm. So the vertical circulation of air is blocked even more.
I think substantial precipation can help—by cleanses some of the particles out of the air.
What was amusing once in the last couple of years was when it was starting to get better into the last March—and then farmers started _burning_ their fields to clear debris for plowing. Particle emission on a grand scale.
Hope somebody else can explain it better.