What are Particulates?Published by MAC on 2005-08-10
What are Particulates?
The Gallon Environment Letter
Canadian Institute for Business and the Environment
Vol. 10, No. 14, August 10, 2005
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Dust (particulate matter) is a significant component of air pollution. Particulates are not a single pollutant but a mix of particles of different sizes, origin, and chemical composition. Regulations have tended to address only size rather than the other aspects of the particle mix.
Coarser particulates may be found near sources of pollution such as power plants, industrial and agricultural operations, and roads. They include cement dust, foundry dust, wind-blown soil dust and coal dust. Finer particulates may be found long distances from sources. Chemical reactions between air pollutants and also between natural compounds such as water and seawater can lead to formation of different types of particles.
Among the features of particulates are:
- Size variations. Particles in the air may range in size from a grain of sand which can be seen with the naked eye down to as small as individual molecules. In between, still too small to be seen, are colloidal particles. Those particles which are 10 micrometers or less in diameter are called PM10 where PM is particulate matter. Smaller particles are 2.5 micrometers or less in diameter and are called PM2.5. PM10 includes the set of PM2.5. (1 micrometre = 1 millionth of a metre)
- Chemical composition. This affects size, for example, sulphur tends to be in fine particles while heavy metals particles are coarser. (See below for the range of material particulates may contain.)
- Sources affect size and chemical composition. Coarse PM is usually from a source due to abrasion and crushing. Particulates may also be altered or created by chemical and physical reactions with gas or liquid pollutants.
- Size affects how particles disperse. Coarse particles settle closer to where they were created than fine particles. Fine particulates can be airborne for days or months and transported 1000 km or 10,000 km.
- Secondary particle creation. It isn't just a matter of what goes up must come down. Particles are also formed by chemical reactions in the air. Fine particles are often created from gas to particle conversion reactions, from evaporation of water from contaminated fog or cloud droplets and condensing of volatile organic compounds, metals and incomplete combustion.
- Different types. Particles are variously described as aerosols (very small particles, liquid or solid which are formed by reactions of gases atomization of liquids or dispersion of dusts), haze (contains particles which reduce visibility), mists (contain liquid particles), smoke (contains particles from incomplete combustion) and biological.
- Particles may be made up of a wide range of material including: Aluminium, iron, calcium, silicon from coal combustion, rock mining, soil erosion, vehicle debris such as engine and brake linings, automobile and truck tire fragments, general debris and waste remnants.
- Soot and carbon black. These particles are emitted from car and truck exhausts, furnaces, incinerators, power generation, and steel making. These types of pollutants are particularly bad because they may play a role in further chemical reactions. Carbon black is a carrier of gases and other particulate pollution.
- Fly ash. When coal is burned, the mineral content which is not combustible is converted to ash. The bottom ash, a heavy part, is deposited and collected.
- Fly ash particles which are partially collected by stacks and scrubbers also escape to the atmosphere in small particles which harm plants and people. Asbestos. When particles of this material are breathed in, it causes asbestosis, a type of pneumonia, as well as cancer. International programs have been put in place to reduce use and exposure to asbestos although Canada is still a producer and exporter.
- Toxic metals. Lead particles are less present in the air in many developed countries where leaded fuel has been banned but they a growing concern in other countries. Mercury from power plants and volcanoes in the atmosphere is toxic, travels long distances and some is in particulate form. Beryllium with its high toxicity is a growing problem because of its use in electronic equipment and instrumentation.
- Radioactive. Naturally occuring radon decays to form products which adhere to atmospheric particles. Burning fossil fuels introduces radioactivity in the fly ash as do nuclear testing and nuclear accidents.
- Biological material: Particulates may contain plant and insect fragments, pollen, fungus, bacteri, virus, and mould spores.
Effects of Particles
The effects depend on the nature of the particles such as their size and toxicity as well as on how long the particles stay in the air and the results of chemical and physical interactions.
Among the effects are:
- Reduction of visibility.
- Dust covering. For example, Michaelangelo's David was recently cleaned of 500 years of accumulated dust. Curators found that, soon after the cleaning, dust brought in by the many visitors was accumulating again. Restoration of art can change the fundamental nature of the artistic piece.
- Fostering chemical reactions in the atmosphere affecting weather and air pollution impacts.
- Certain sized particles scatter light affecting weather and climate.
- Health effects. Particulates are associated with higher rates of circulatory and respiratory diseases. Effects can include heart attacks, pneumonia, harmful impacts on chronic lung diseases, increased risk of lung cancer and reduced survival rates from cardio-respiratory effects. Coarser particles damage nasal and upper respiratory passages. Small particles may enter deep into the lungs and the blood stream or lymph system damaging organs. Increases in hospital visits and more frequent and severe asthma may also result.
Controlling Particulate Matter
Many companies offer air pollution control equipment and services to remove particles from operations and waste streams but not all are effective for all particulate matter. Cost is often a barrier to the take-up of technology.
Among the control measures are:
- Sedimentation. Gases are stored in large spaces and over time, the larger particles settle out through the force of gravity.
- Dry centrifugal collectors. Particles are separated from the air stream by spinning or fans so that the particles are forced outward to be collected.
- Filtration. Fabric filters collect dust in tube bags in baghouses.
- Scrubbers. Gas is passed through a section with a narrow waist. A scrubbing liquid is broken by the passing gas into droplets which capture particles.
- Electrostatic Precipitors. The particles gain a charge when the gas is passed through current and are attracted to a surface for later removal. This is similar to the principle used by household air cleaners and the problem is that the process may also produces ozone which is toxic.
- Pollution prevention to avoid creation of air pollution in general and particulates in particular is another approach. Energy conservation and alternate energy sources may have a role to play in reducing particulates.
Manahan, Stanley. Fundamentals of Environmental Chemistry. Boca Raton, Florida: Lewis Publishers, 2000.