[Old Professer]

The general level of airborne aerosols and soot particles must have been far higher in the late 19th and early 20th century, even accounting for the increase in population, due to dependence on coal and wood for heat to warm buildings and drive steam engines and electric power plants. In my lifetime, I have watched the skies clear. Once, while driving through Steubenville, Ohio I asked someone there why the buildings were so blackened on the walls and rooftops and he simply said, "The steel plant."

[cogitator]

Coal use in the post-WWII industrial age exploded. The level of sulfate aerosols from coal was higher in the 1950s than any other decade. London suffered from killer fogs in that decade; St. Louis, Missouri also had a notable killer fog event. Due partly to concerns about air quality in general and partly due to acid rain concerns, sulfur emissions in industrialized countries, particularly the U.S., were markedly reduced in the 1970s.

Your observations are quite valid.