Q.From where does ozone come in the photo chemical smog?

Smog, a combination of smoke and fog, occurs in two chemically distinct forms. Classical smog forms in cool, humid climates as a reducing mixture of smoke, fog and sulphur dioxide. Photochemical smog forms in warm, dry, sunny climates when sunlight drives a chain reaction between unburnt hydrocarbons and nitrogen oxides from vehicles and factories: NO2 photolyses to NO and atomic oxygen, the oxygen atom combines with O2 to form ozone, and ozone reacts back with NO to regenerate NO2, while ozone and NO2 also attack hydrocarbons to produce formaldehyde, acrolein and peroxyacetyl nitrate (PAN) — making it an oxidising smog. Both ozone and PAN are powerful eye irritants; NO and O3 irritate the nose and throat, and …