Particulate matter is a complex mixture of organic and inorganic substances, present in the atmosphere as both liquids and solids. Coarse particulates can be regarded as those with a diameter greater than 2.5 micrometres (µm), and fine particles less than 2.5 micrometres. Coarse particles usually contain earth crustal materials and dust from road vehicles and industries. Fine particles contain the secondarily formed aerosols, combustion particles and re-condensed organic and metallic vapours. The acid component of particulate matter generally occurs as fine particles. A further distinction that can be made is to classify particulates as either primary or secondary, according to their origin. Primary particulates are those emitted directly to the atmosphere while secondary particulates are those formed by reactions involving other pollutants. In the urban environment, most secondary particulate matter occurs as sulphates and nitrates formed in reactions involving sulphur dioxide and nitrogen oxides.
Particulate matter is emitted from a wide range of man-made sources, the most significant primary sources being road transport (25%), non-combustion processes (24%), industrial combustion plants and processes (17%), commercial and residential combustion (16%) and public power generation (15%). Natural sources are less important; these include volcanoes and dust storms. In urban areas typical annual mean values are 10 - 40 µgm-3. Background levels in rural areas range form 0-10 µgm-3 . During pollution episode, particulate levels can rise to several hundred µgm-3 .
Particulates may be seen as the most critical of all pollutants, and some estimates have suggested that particulates are responsible for up to 10,000 premature deaths in the UK each year. Fine particles of less than 10 micrometres (µm) in diameter can penetrate deep into the lung and cause more damage, as opposed to larger particles that may be filtered out through the airways natural mechanisms.