Oxidising smog (photochemical smog) gets increased during the day time and in the presence of O3. It occurs in warm, dry and sunny climate. When fossil fuels are burnt, hydrocarbons (unburnt fuels) and nitric oxide (NO) are produced. Ozone reacts rapidly with the NO to generate NO2. NO(g)+O3(g)⟶NO2(g)+O2(g) Ozone is a toxic gas and both NO2 and O3 are strong oxidising agents and can react with the unburnt hydrocarbons in the polluted air to produce chemicals such as formaldehyde, acrolein and peroxyacetyl nitrate (PAN).