The locals tell me that during winter, many people in Belgrade us really old coal ovens to heat their poorly insulated flats and houses. The coal is usually cheaply obtained from questionable sources and contains lots of particles. This leads to massive amounts of air pollution, making Belgrade during the winter rank among the worst cities in the world with regards to air quality.
However, the coal dust in the air makes the color of the sunlight shift more into red and orange tones, which makes sunset photos even prettier.
Now, to be honest, I do not know if all or any of this is true, but I was really happy how the sunset looked in reality and on the picture even before the post processing.