Dirty Snow

Look at this faintly dirty snow photographed in a pristine area (not a parking area or highway) after our recent snowstorm - it shows that there are fine particulates in the air of which we're not usually aware. They're only visible here because of the contrast with the substrate on which they have landed. The uneven textured surface of the snow also highlights the discoloration.

The dirt could be soot from wood smoke, or dust and dirt from the roads, or other pollutants such as vehicle exhausts and old pesticides. Whatever it is, it's been carried down through the air by the snow and we're breathing it while our bodies do their best to filter it out.