Which is always present in photochemical smog?

Photochemical smog occurs at high temperature over cities and towns due to stull air, emission of nitrogen oxides and hydrocarbons from automobile exhausts and solar energy. Nitrogen dioxide splits into nitric oxide and nascent oxygen. Nascent oxygen combines with molecular oxygen to form ozone.Ozone reacts with hydrocarbons to form aldehydes and ketones.Nitrogen oxide,oxygen andketones combines to formPeroxy Acyl Nitrates (PAN) which is responsible for the eye irritation. This smog was first observed in Los Angeles, so called asLos Angeles smog.Classical (London) Smog was reported in 1952 in London. It occurs at low temperaure, containssulphur gases (hydrogen sulphide, sulphur dioxide), smoke and dust particles.