The term 'particulates' includes a range of chemically and physically different material in sizes ranging from a few nanometres (millionths of a millimetre) in diameter, up to a few tens of micrometres (thousandths of a millimetre)in diameter. Smaller particles are transported through the atmosphere by turbulence. Larger particles may be suspended from the ground by the wind, and then settle under the force of gravity.
Fossil fuel combustion, in vehicles, open fires and industrial processes, produces a wide range of particle size and composition, depending on the fuel, the combustion conditions, and (for industrial processes) scrubbing of the stack gases.
Particles may be emitted directly from many industrial processes, although most of these are controlled in the U.K. by regulatory agencies. However, there are many 'open' processes such as mining and quarrying which are not so easily controlled.
Particles (e.g. from soil) may also be suspended from the ground by wind or traffic turbulence. Road sources contribute organic particles from tyres, and metal-containing particles from brake linings. Dust emissions from unpaved roads are typically 100 times greater than for paved roads.
The effects of dust on vegetation have been reviewed (Farmer, 1993). For specific effects of cement dust, fly ash, quarry dust and soot, see the separate entries in APIS.
Particulate concentrations are measured by weight per unit volume of air (SPM=suspended particulate matter). Air sampling is often restricted to a particular size range of particles, e.g. PM10 is that fraction of the SPM for which the aerodynamic diameter is less than 10 micrometre. In general, the chemical composition of SPM is not measured.
Ecosystem specific information
Woodland and hedgerows - because of their size and structure, increase air turbulence. This leads to greater deposition of particles on these vegetation typoes than on short vegetation. Trees have been suggested as environmental 'screens' around industrial sources or next to roads to remove particles especially 'dust' from the atmosphere (Freer-Smith et al., 1997).
|Critical Load/ Level|
No estimate available