Many years ago I worked at a power plant while I was in college.
One of the things they did every day was to "blow soot". The procedure was to send a huge blast of compressed air up the stacks to remove soot which had accumulated during the day.
This was always the first task of the graveyard shift operator, which meant that it happened at midnight every single day, effectively preventing anyone from seeing the huge cloud of soot being released. Even on the clearest nights, the stars could not be seen for 1/2 hour after blowing soot.
Golly, do you think it is possible that pollution controls might have improved things since "many years ago?"
Soot blowing is always after dark. Fresh snow presents a problem with the evidence...