Parliamentary Research Branch

PRB 98-4E

MAIN COMPONENTS OF SMOG

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 (NO_x) and ammonia gas. The severity of smog in an urban area is usually assessed by measuring ground-level ozone.

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 (NO_x). 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.

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 (SO₄^2-).(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).

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.

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 (NO₃^1-) 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 PM_2.5 and PM_2.1, 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)

(3) Canada-United States Air Quality Agreement, Scientific and Technical Activities and Economic Research, Progress Report, 1996.