Parliamentary Research Branch


PRB 98-4E


Prepared by:
Christine Labelle
Science and Technology Division
October 1998

Smog, 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 ground-level ozone.

Tropospheric Ozone: Sources, Composition, and Formation

Tropospheric 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 smog-forming pollutants.

As 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.


Sources, 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) Tailpipe emissions, 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, incinerators
Particulate matter 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.

Sulphates: Sources, Composition, and Formation

In 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 ( m).

In 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.

Particulate Matter: Sources, Composition, and Formation

Like 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.