MAIN COMPONENTS OF SMOG
Science and Technology Division
formed mainly above urban centres, is composed mainly of tropospheric ozone (O3);
primary particulate matter such as pollen and dust; and secondary particulate matter such
as sulphur oxides, volatile organic compounds, nitrogen oxides (NOx) and
ammonia gas. The severity of smog in an urban area is usually assessed by measuring
Sources, Composition, and Formation
ozone (O3) is found as a ground-level polluting gas. This paper discuses only
tropospheric ozone, not to be confused with stratospheric ozone, which forms a layer
around the earth, protecting it from the rays of the sun. Tropospheric ozone is produced
by the action of light and the chemical bonding of volatile organic compounds (VOCs) and
nitrogen oxides (NOx). The table below identifies the main sources of
a result of heat from the rays of the sun, the concentration of ground-level ozone is
highest in urban centres in the summer. Weather conditions also affect ozone formation;
masses of stagnant air can hold pollutants at ground level for several days. In addition
to the regions where pollution is greatest the Windsor-Quebec corridor, southern
Ontario, the Atlantic region, and the Lower Fraser Valley , other urban centres,
such as North York, London and Oakville, two or three times a year experience pollution
higher than the maximum permissible concentrations of 82 parts per billion (ppb) per hour.
Both Natural and Human-Made
|Volatile organic compounds (VOC)
||Tailpipe emissions, evaporation
of gasoline at service stations, surface coatings such as oil paints, solvents such as
barbecue starters, fuel combustion, vegetation
|Nitrogen oxides such as nitric
oxide (NO) and nitrogen dioxide (NO2)
manufacturing industries, electricity generating stations, fossil fuel powered plants, oil
refineries, pulp and paper plants, incinerators
|Sulphur dioxide (SO2)
||Non-ferrous metal smelting,
thermal electricity generating stations, oil refineries, pulp and paper plants,
||Tailpipe emissions, volcanoes,
wind erosion, forest fires, fossil fuel powered plants
Source: Anonymous, "Health Effects of Outdoor Air
Pollution," American Journal of Respiratory and Critical Care Medicine, Volume
153, 1996; Health Canada, summary of recent research on the effects of ambient air
pollution on health in Saint John, Health Canada Internet Site, 1997.
Composition, and Formation
normal concentrations, sulphur dioxide (SO2) is not toxic; however, the acid
pollutants into which it is chemically changed do have negative effects on health. Sulphur
dioxide is found in the atmosphere as a result of the combustion of fossil fuels, the
production of electricity, and the smelting of sulphur-containing ores (see table above).
Winds then carry away sulphur dioxide, sometimes over long distances. After mixing with
water vapour and undergoing complex changes including oxidation, sulphur dioxide turns
into sulphuric acid (H2SO4) and sulphate ions or sulphates (SO42-).(1) When these changes take place, some
pollutants form acid precipitation, while others remain suspended in the air as dust or
droplets. Sulphates account for a considerable proportion of all particulate matter in the
air smaller than three microns (µ m).
Canada, almost half of sulphur dioxide is produced by the smelting of non-ferrous metals;
in the United States, two thirds of sulphur dioxide is produced by thermal-electricity
generating stations. Of all emissions in both countries, 80% are concentrated in the east
of the continent, east of the Manitoba-Saskatchewan border in Canada and east of the
Mississippi River in the United States.
Sources, Composition, and Formation
other substances, particulate matter can come from natural sources, but industrial and
other human-produced factors are responsible for the high concentration of particulate
matter in the air (see table above). Sulphates account for most particulate matter, but it
also includes nitrates (NO31-) and other pollutants, such as metals.
Particulate matter is found in the air in solid and liquid forms. It is measured in total
suspended particulates (TSP) of all sizes; PM10, particulate matter measuring
10 microns in diameter or less; and PM2.5 and PM2.1, fine
particulate matter measuring 2.5 or 2.1 microns or less.(2) The smallest particulate matter easily penetrates
the respiratory tract, causing health problems. It can also carry chemicals such as
metals, polycyclic aromatic hydrocarbons (PAHs), and other pollutants into the lungs.
Eastern Canada has a high concentration of particulate matter in the form of sulphates,
while in western Canada there is a high concentration of nitrates.(3)
(1) Anonymous, "Health Effects of
Outdoor Air Pollution," American Journal of Respiratory and Critical Care Medicine,
Vol. 153, 1996, p. 3-50.
(2) Health Canada, "Air Quality and Health in
Saint John: A Summary of Recent Research on the Effects of Ambient Air Pollution on
Health," Health Canada Internet Site, 1997.
(3) Canada-United States Air Quality Agreement,
Scientific and Technical Activities and Economic Research, Progress Report, 1996.