Salt Lake City is one of the most polluted cities in the United States during the winter months. It's unfortunate, because there are some incredible views that get covered in haze. The reason for the pollution is not because of factories or cars... but rather a "Winter Inversion". Inversions occur during the winter months when normal atmospheric conditions (cool air above, warm air below) become inverted. Inversions trap a dense layer of cold air under a layer of warm air. The warm layer acts much like a lid, trapping pollutants in the cold air near the valley floor. The Wasatch Front valleys and their surrounding mountains act like a bowl, keeping this cold air in the valleys. The snow-covered valley floors reflect rather than absorb the heat from the sun, preventing the normal vertical mixing of warm and cold air. Fog exacerbates the problem, facilitating chemical reactions that create even more particles and higher pollutant concentrations. The longer the inversion lasts, the higher the levels of pollution trapped under it. The warm inversion air layer is usually displaced by a strong storm system which restores air quality to healthy levels.
One positive effect of the inversion is that it makes for some unique sunsets! The air almost glows... and it makes it seem like you're looking at an abstract painting. These are a few photos I took just as the sun had dipped below the mountains.