Sulphur dioxide and small amounts of sulphur trioxide are formed during combustion of sulphur-containing materials such as fossil fuels.
Sulphur dioxide (SO2)
SO2 is a non-flammable, colourless gas with a pungent smell. It readily dissolves in water to give an acidic solution, and it tends to form aerosols.
Sulphur trioxide (SO3)
SO3 is a non-flammable, colourless solid that is easily melting and evaporating. It reacts vigorously with water to give sulphuric acid tending to form aerosols. Thus, It creates an easily visible blue haze at the chimney.