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I have been to different places with nearly the same freezing temperatures, around -2°C, but only at some specific places I repetitively experienced freezing rain. One common factor at these places was relatively more air pollution. Could it be that pollution is a contributing factor that causes freezing rain to occur?
If not, what are the main factors that cause freezing rain, and the regions on Earth that often experience them, especially in Europe?
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.
Pollutions won't influence Freezing Rain.
Rosemary Peters 'Freezing Rain Fact',by April 25,2017
Freezing rain starts its journey in the atmosphere as snow. The snow encounters a deep pocket of warm air, melts and becomes rain. The rain then passes through a layer of air close to the surface where the droplets cool to a temperature to just below freezing; however, the droplets don’t actually freeze over. When the “supercooled” raindrops hit the surface, like a road, tree or power line, they immediately freeze and form a thin glaze of ice.
