Formation of Secondary Particulate Matter

Atmospheric sulphur dioxide (SO₂) forms fine particles when chemical reactions convert it into sulphuric acid and sulphate compounds. These are secondary particulate matter because they form in the atmosphere from a gaseous precursor rather than being emitted directly as particles.

Oxidation in the gas phase

SO₂ does not become particulate matter simply by cooling. Its conversion generally begins with oxidation, which changes sulphur from its +4 oxidation state in SO₂ to its +6 state in sulphuric acid and sulphate.

• In the gas phase, the hydroxyl radical (OH) initiates a reaction sequence that converts SO₂ into sulphur trioxide (SO₃).
• SO₃ then reacts with atmospheric water vapour to form sulphuric acid (H₂SO₄), whose very low volatility favours its transfer into particles.
• Sunlight supports this pathway by driving the atmospheric photochemistry that produces OH radicals.

Oxidation within atmospheric water

SO₂ also dissolves in cloud droplets, fog droplets and aerosol water. In water, it exists as dissolved SO₂, bisulphite and sulphite, with their relative amounts depending on acidity.

• Oxidants such as hydrogen peroxide (H₂O₂) and ozone (O₃) convert these dissolved sulphur compounds into sulphate.
• The rate of aqueous oxidation depends on the available oxidants and the droplet’s pH; different oxidants respond differently to acidity.
• When cloud or fog droplets evaporate, the sulphate they contain can remain in residual aerosol particles.

Formation and growth of fine particles

The resulting sulphuric acid and sulphate contribute to secondary inorganic aerosol, an important component of fine particulate pollution.

• Sulphuric acid can condense onto existing particles or participate in the formation of new particles, which may subsequently grow.
• Atmospheric ammonia (NH₃) can neutralise sulphuric acid, producing ammonium bisulphate or ammonium sulphate according to the availability of ammonia.
• These sulphate-containing particles commonly occur in PM₂.₅, particles with an aerodynamic diameter of 2.5 micrometres or less.
• Sulphate salts take up water under humid conditions, increasing particle size and light scattering and contributing to haze.
• The amount formed depends on SO₂ availability, oxidants, atmospheric water, ammonia and reaction time; therefore, particulate pollution can develop downwind of the original emission source.