Photochemical smog or oxydizing type of pollution is characterized by the presence of large concentration of ozone, oxides of nitrogen and various hydrocarbons. It occurrs in Los Angeles. Nitrogen dioxide splits into nitric oxide and nascent oxygen. Nascent oxygen combines with molecular oxygen to form ozone. Ozone reacts with carbohydrates (unsaturated hydrocarbons) to aldehyde and ketone. Nitrogen oxides, oxygen and ketone combine to form Peroxy Acetyl Nitrates (PAN).