in atmospheric chemistry
The term originated in Great Britain as a popular derivation of 'smoke-fog' and appears to have been in common use before World War 1. It originally referred to the heavy pollution derived largely from coal burning (largely smoke filled air, rich in sulfur dioxide), and it probably was largely a reducing atmosphere. More common today in cities is an oxidizing atmosphere which contains ozone and other oxidants.
See: photochemical smog
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2214 (