Smog, a word created from combining smoke and fog, is a cloud of atmospheric pollution composed of particles released by combustion (coal fired power, exhaust gas) and tropospheric ozone.
Consequently, smog forms above cities that have a lot of traffic or industrial activity, especially during sunny weather when ultra-violet rays oxidise atmospheric particles and create ozone.
The word smog was first used in 1905, when the phenomenon overwhelmed London during the Industrial Revolution.
Sao Paolo, drowned in smog. © BuenosAiresPhotographer.com CC by-sa 2.0