Sulfur trioxide (also spelled sulphur trioxide) is the chemical compound with the formula SO3. In the gaseous form, this species is a significant pollutant, being the primary agent in acid rain. It is prepared on massive scales as a precursor to sulfuric acid.
In process plant environment, SO3 gas is mixed into flue gas from combustion to make the ashes charged up before flowing through electrostatic precipitators. The electrostatic precipitators will then trap the ashes, making cleaner process emission possible.
Sulfur trioxide will cause serious burns on both inhalation and ingestion since it is highly corrosive and hygroscopic in nature. SO3 should be handled with extreme care as it reacts with water violently and produces highly corrosive sulfuric acid.