Oleum and liquid SO3 is stored in tanks and when the tanks are being filled or emptied, there is an increase in vent vapour. Similarly, when loading into or from trucks & wagons there will be a SO3 fugitive vapour. This SO3 vapour will condense to form a white plume of sub-micron sized H2SO4 mist. Some H2SO4 plants connect the vent pipe back into the plant, but this can cause problems such as mist being introduced into a tower where mist was not formerly a problem.
The vapours are drawn from the tank into a 2-stage scrubbing & filtration system by a suction fan. The vapour is irrigated with water or 96% H2SO4, thus the SO3 is hydrolysed or absorbed into H2SO4, and the quantity of water or H2SO4 is dependent on the system size & vapour strength.