Chemical processes

Chemical processes

    • Combustion of fuels creates sulfur dioxide and nitric oxides. They are converted into sulfuric acid and nitric acid.

    Gas phase chemistry

    • In the gas phase sulfur dioxide is oxidized by reaction with the hydroxyl radical via an intermolecular reaction
    SO2 + OH → HOSO2
    Which is followed by:
    HOSO2 + O2 → HO2 + SO3
    • In the presence of water, sulfur trioxide (SO3) is converted rapidly to sulfuric acid:
    SO3 (g) + H2O (l) → H2SO4 (l)
    Nitrogen dioxide reacts with OH to form nitric acid:
    NO2 + OH → HNO3

    Chemistry in cloud droplets

    • When clouds are present, the loss rate of SO2 is faster than can be explained by gas phase chemistry alone. This is due to reactions in the liquid water droplets.


    Sulfur dioxide dissolves in water and then, like carbon dioxide, hydrolyses in a series of equilibrium reactions:
    SO2 (g) + H2O SO2 .H2O
    SO2H2O H+ + HSO3−
    HSO3− H+ + SO32−


    • There are a large number of aqueous reactions that oxidize sulfur from S (IV) to S (VI), leading to the formation of sulfuric acid. The most important oxidation reactions are with ozone, hydrogen peroxide and oxygen (reactions with oxygen are catalyzed by iron and manganese in the cloud droplets).

Last modified: Wednesday, 29 February 2012, 9:18 PM