Here is a plot showing the AQI for each day in 2012:
Notice that the winter months only have good ozone AQI days. This seasonal trend is noticeable during all years. Ozone forms photochemically in the air when other pollutants react in the presence of sunlight. This explains the low ozone concentrations during the winter months. There is not sufficient energy from the sun to make the ozone form. The daily solar energy is lower during winter months for two reasons. First, days are shorter. And second, the earth's rotation is tilted away form the sun causing sunlight to spread over a larger area.
In comparison here is a plot of ozone AQI values for Salt Lake City in 2012:
Again, we see low ozone concentrations in the winter months. (Notice that there is missing data in October and December.) Salt Lake does not have many days that exceed the EPA's National Ambient Air Quality Standard (NAAQS). There are two other factors that can causes the difference in these two cities: emission amounts or type, and weather.
Salt Lake, however, has other pollution problems they deal with. Particulate Matter smaller than 2.5 microns, otherwise known as PM 2.5, is the troublesome pollutant along the Wasatch Front. Unlike ozone, PM 2.5 doesn't seem to follow an annual trend. Not shown in this chart is a long polluted episode throughout January 2013. A strong inversion kept air near the surface. The mountains act as barriers that prevented the pollution to mix away. Meteorology, topography, and emissions play a large role in PM 2.5 concentrations in Northern Utah. The year 2012 was fairly unpolluted, but the beginning of 2013 had several days with very unhealthy air quality. Those episodes occur during periods of strong inversions.