Hello!
Europe (especially Scandinavia), United States (areas like New York), Southwest Canada, Southeast China, and Taiwan.
Things that cause it are SO2 emission by volcanoes and combustion of fossil fuels such as coal and oil. I would suspect in these areas that there are a lot of cars, power plants using coal or diesel, or manufacturing that is using
combustion on site.
Hope this Helps! Have A Wonderful Day! :)