The major source of SO2 emission are burning of fossil fuel in thermal power plant, smelting industries and other processes as manufacture of sulphuric acid and fertilizers. SO2 is involved in the erosion of building materials as limestone marble, the state used in roofing, mortar and deterioration of states. SO2 is also component of acid rain.
Chlorine destroys ozone molecules.
Oxygen and hydrogen are the main components of air and are not pollutant.