The word smog was coined to describe the 'smoke fog' prevalent in London.

  Classical smog-

The odd combination of particulates (solid pollutants like carbon, dust particles etc.) with gaseous oxides of sulphur present in the atmosphere constitutes is nowadays known as classical smog. The principal source of classical smog is the combustion of industrial and household fuels (coal and petroleum). Because of the presence of SO2 and carbon (soot) particles, classical smog has a reducing character. It occurs in winter months particularly in early morning hours. It causes severe lung and throat irritation.

"London smog"

Photochemical smog-

There is another type of smog, viz., the Los Angeles type of smog, which is distinctly from the London type of smog. The principal constituents of the Los Angeles type of smog are ozone and oxides of nitrogen. Because of the presence of O3, NO2 and some photochemical oxidants, this type of smog is oxidising in character. It causes irritation in eyes due to the presence of ozone. It is now known that this type of smog is formed through photochemical reactions involving solar radiations. Hence, this smog is named as photochemical smog. It is formed around mid day of summer months when the intensity of solar radiation is very high.

"Los Angeles smog"

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written by - @iambijoy
picture sources - Google.