Is pollution a contributing factor to freezing rain?
Pollution shouldn't contribute to produce freezing rain. It might have an impact in the precipitation rate and location but not on the likelihood of freezing rain.

Freezing rain requires not only that the near-surface temperature is below freezing, but also that there is a temperature inversion that provides above zero temperature in the atmospheric layers above the surface.

Usually temperature drops as you go higher into the atmosphere. But in some areas, due to climate, geography and local weather patterns, you can often find that the temperature rises as you go up, that's called a temperature inversion (eventually as you go high enough the inversion dies and temperatures drops as you go even higher).

Such condition can melt the snow high in the atmosphere and then the water droplets enter an area near the surface that is below zero and cools them down but not for long enough to freeze solid (something that would produce sleet). Water droplets in such conditions gets below zero but remain liquid, a phenomena called supercooling. When the supercooled drops hit the surface (that is also below zero), they instantly freeze forming a thin layer of ice, hence freezing rain.

This figure from mprnews.org nicely summarize the phenomena:



Regarding why some places can be mor suitable for freezing rain, I'll highlight here the comment by @Anirudh pointing to this link that deals with the case of Guizhou Province in China.