Anyone who's spent a winter in the Treasure Valley knows the pattern. A layer of cold air settles into the valley floor, warmer air caps it from above, and everything that gets emitted down here stays down here. The foothills look clear and bright while the valley sits under a gray lid for days at a time. During those stretches, most people do the sensible thing and stay inside with the windows shut. Which raises a question worth thinking about: what's the air like in there?