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Nature of Particulate Matter



Particulate matter (PM) represents the collection of very tiny liquid and solid particles that are suspended in the air.  Individual particles are typically composed of a very complex mixture of chemical species, and some particles are also carriers of known toxic substances, such as polycyclic aromatic hydrocarbons, some of which are known carcinogens.  Many particles have a solid core surrounded by a liquid layer.
Particles come in a variety of sizes.  Two broadly monitored size fractions are particles with diameter less than or equal to 10 micrometres (µm), known as inhalable particles (or PM10), and those with diameter less than or equal to 2.5 µm, known as fine particles (PM2.5).
PM is emitted directly to the air (primary PM), and it also forms in the air (secondary PM) from precursor gases such SO2, NOx, VOC and NH3.  Sources of primary PM include soot (elemental carbon, or EC) emitted directly from combustion of fossil fuels; metals such as iron, lead, mercury and cadmium; elements of soil and road dust; bio-aerosols (i.e. particles containing or composed of living micro-organisms such as fungal spores and mould); and salt (e.g. road salt and oceanic sea-salt).
Secondary PM includes:  ammonium sulphate (produced in the air from emissions of SO2 and NH3); ammonium nitrate (produced in the air from emissions of NOx and NH3); and numerous carbon-containing substances (known as organic carbon, or OC), which may be emitted directly or formed in the air from emissions of VOC. 
PM is a very complex pollutant, not only because particles typically consist of a mixture of substances, but also because some of the substances that make up the particles are semi-volatile.  Semi-volatile substances can exist in the air both as particles and vapours (i.e. gases).  The mass of semi-volatile PM (e.g. ammonium nitrate and some organic compounds) is not static but can instead change frequently as the substances respond to the changing meteorological, physical and chemical conditions that they encounter while moving through the air.
Ambient levels of particles can be elevated year-round, and in urban areas the levels are typically higher in the mornings and evenings reflecting traffic patterns.  Particles can travel very large distances and affect areas thousands of kilometres away from the sources of the emissions.



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