Photochemical smog (also called the 'Los Angeles Smog' as it was first observed in Los Angeles in the 1940s) is formed when air mixture containing NO_x (nitrogen oxides) and hydrocarbons are exposed to sunlight. The energy provided by the UV radiations emitted by the sun results into formation of various products like NO₂, O₃, peroxyacetyl nitrate (PAN) and other volatile organic compounds containing aldehyde groups.