Was going to ask the same question. There also is a spot in Poland that’s not in a large city - was curious about that one as well. Some sort of NO2-generating production? Also between DRC and Angola.
https://en.wikipedia.org/wiki/Be%C5%82chat%C3%B3w_Power_Stat...
https://www.bing.com/maps?v=2&cp=51.266111~19.330278&style=h...
SMOG WARS - Polish Air Pollution Documentary (Full International Version)