Environmental Science (2+1)

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.

Hydrolysis
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−


Oxidation

• 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).