This report refers to several types of pollutants. The following definitions are taken from Ambient Air Quality POST Note 458, Parliamentary Office of Science and Technology, Feb 2014 and Defra's UK-AIR website:

Particulate matter (PM) includes:

primary particles-those directly emitted from a source, including combustion and mechanical sources, such as traffic emissions.

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)-less than 100nm diameter (100 millionths of millimetre or nanometre), which are emitted in large numbers from diesel engine exhaust.

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 nitrogen dioxide (NO2). Nitrogen dioxide has a variety of environmental and health impacts. It is a respiratory irritant which may exacerbate asthma and possibly increase susceptibility to infections. In the presence of sunlight, it reacts with hydrocarbons to produce photochemical pollutants such as ozone. NO2 can be further oxidised in air to acidic gases, which contribute towards the generation of acid rain.

Ozone (O3) is 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'.

previous page contents next page

© Parliamentary copyright 2014
Prepared 16 December 2014