A method of controlling air pollution arising from sulphur trioxide and or sulphuric acid in coal combustion gases, combustion gases passing through the demister region of a wet flue gas desulphurization unit are exposed to a sulphide based reducing agent such as hydrogen sulphide, such that metal catalysts promoting the oxidation of ammonia leading to the formation of nitrosylsulfuric acid are poisoned and so prevented from catalysing the formation and discharge of sulphuric acid in aerosol emissions in stack gases.