Valley snow leads to stronger and more persistent cold pools (a.k.a. inversions). The buildup of pollution over the past couple of days is quite evident looking south over the Salt Lake Valley this morning.
The morning sounding from the Salt Lake City airport shows a stable, near isothermal (i.e., constant temperature) airmass extends from the valley floor to about 750 mb, or about 1000 m above the valley floor. We're pretty much cutoff from the clean air above.