Ammonia (NH₃): a byproduct of agriculture, particularly livestock manure, slurry management, and fertilizers. Smaller amounts can be derived from transport and waste disposal. It is not harmful to humans or mammals but is damaging to terrestrial and aquatic ecosystems. It is a precursor to secondary particulate dispersion.
Nitrogen oxides (NOx): combustion processes (e.g. inside motor vehicles) emit a mixture of nitrogen oxides (NOx), primarily nitric oxide (NO) which is quickly oxidised in the atmosphere to form nitrogen dioxide (NO₂). NO₂ has health impacts from penetration of the lungs and physiological systems.
Ozone (O₃): not emitted directly from any sources. It is a secondary pollutant formed through the reaction of volatile organic compounds with NOx and hydrocarbons in the presence of sunlight. Whereas nitrogen dioxide acts as a source of ozone, nitric oxide (NO) destroys ozone and acts as a local sink (NOx-titration). For this reason, O₃ concentrations are not as high in urban areas (where high levels of NO are emitted from vehicles) as in rural areas. Ambient concentrations are usually highest in rural areas, particularly in hot, still and sunny weather conditions which give rise to summer ‘smogs’.
Particulate matter (PM) includes:
primary particles: those directly emitted from a source, including combustion and mechanical sources, such as traffic emission;
secondary particles: those formed in the atmosphere as a result of chemical reactions between gases such as ammonia, nitrogen oxides or sulphur dioxide.
PM is conventionally defined and measured by size:
Coarse particles (PM₁₀–PM₂.₅): particles smaller than 10 μm (10 thousandths of a millimetre or a micron) in diameter but greater than 2.5 μm diameter. Coarser particles arise from re-suspended road dust, brake and tyre wear, sea salt, quarries and soil;
Fine particles (PM₂.₅–PM₀.₁): particles less than 2.5 μm diameter, which include most combustion particles such as those emitted from diesel engine exhaust, waste burning, bonfires, and domestic biomass burning; and secondary particles of ammonium sulphate or nitrate;
Ultrafine particles (PM<₀.₁): particles less than 100nm diameter (100 millionths of millimetre or nanometre) which are emitted in large numbers from diesel engine exhaust.
PM has health impacts with smaller particles considered particularly harmful.
Sulphur dioxide (SO₂): produced by industrial process such as combustion of fossil fuels for energy production. Exposure causes constriction of the lung’s airways, particularly concerning for those suffering from asthma and/or chronic lung disease. As SO₂ is typically a precursor to secondary PM exposure, it contributes to the negative health effects of PM. Environmentally, SO₂ exposure harms plants by degrading chlorophyll, reducing photosynthesis, increasing respiration rates and changing protein metabolism. Deposition of SO₂ pollution can acidify soil and water resulting in a loss of biodiversity often in places distant from the source of the emissions.