Origin of Sulphuric Acid in the Atmosphere
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