Origin of Sulphuric Acid in the Atmosphere I often see claims that volcanic emissions of sulphur dioxide, on Earth and on the planet Venus, creates sulphuric acid when it combines with water vapour. It doesn't. Sulphur dioxide creates sulphurous acid, which is less corrosive than sulphuric acid. To make sulphuric acid you need sulphur trioxide. Is there a natural process which converts sulphurous acid into sulphuric acid, or has one  acid been confused with the other?
Oxidation states of sulphur vary between -2 and +6. Sulphur compounds are generally
  emitted in reduced form. They are then oxidised in the Earth’s atmosphere, generally to
  SO2
  , a +4 oxidation state. Around 65% (the remainder is removed by dry deposition) of this
  sulphur dioxide is eventually oxidised to the +6 state of H2SO4
  , where SO2 –
  4
  is the sulphate
  ion. Sulphur is stable in the presence of oxygen in this state. The higher-state compounds
  also generally have a greater affinity to water, meaning they are more readily removed from
  the atmosphere by wet deposition. 


The natural atmospheric sulphur cycle and its response to anthropogenic perturbations, Angus J. Ferraro