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Acid Rain

Issue context

Why is acid rain an issue?

Acid rain became a Canadian environmental issue in the 1970s, after scientists noted losses of fish populations in some highly acidified lakes in northern Ontario. Acid rain is caused by pollutants such as sulphur dioxide (SO₂) and nitrogen oxides (NO_x), which in the atmosphere are chemically converted to sulphuric acid and nitric acid, respectively. Diluted forms of these acids fall to earth as rain, hail, drizzle, freezing rain, or snow (wet deposition) or are deposited as acid gas or dust (dry deposition). Normal rain is slightly acidic, but acid rain can be as much as 100 times more acidic.

More than 90% of the SO₂ and NO_x emissions occurring in North America come from human activity. The largest Canadian sources of SO₂ are the smelting or refining of sulphur-bearing metal ores and the burning of fossil fuels for energy. NO_x are formed during the combustion of fuels. Transportation (cars, trucks, trains, etc.) accounts for over 50% of Canada’s NO_x emissions, power generation contributes 10%, and industrial, commercial, and residential combustion processes combined amount to 30%. Once released into the atmosphere, acidic pollutants may be transported great distances by the prevailing winds and weather systems before being deposited. More than 50% of the acid rain that falls in eastern Canada comes from U.S. sources.

About 4 million square kilometres, or 46% of Canada’s total surface area, are highly sensitive to acid rain. Much of this area is in eastern Canada, which has little ability to neutralize acidic pollutants because of its thin, coarsely textured soil and granitic bedrock, characteristic of the Canadian Shield. This region receives more acidic deposition than any other in Canada. Acid rain is a less serious problem in western Canada because of lower overall exposure to acidic pollutants and a generally less acid-sensitive environment (excluding the Canadian Shield region).

Acidic deposition can contribute to declining growth rates and increased death rates in trees. Instances of dieback and deterioration caused by acid fog and acidic cloud water have been noted in white birch in southeastern New Brunswick and in red spruce in the higher-elevation areas of the eastern United States, respectively.

High levels of sulphate deposition result in the acidification of sensitive lakes, rivers, and streams and cause metals to leach from surrounding soils into the water system. High acidity and elevated levels of metals (notably aluminum) can seriously impair the ability of water bodies to support life, resulting in a decline in species diversity. Recent studies suggest that when lakes acidify and their water becomes more transparent (due to a decrease in dissolved organic carbon), the lakes become more susceptible to harmful ultraviolet-B (UV-B) radiation, which can penetrate more deeply and harm more species. Further research is required to better understand the combined effects of acidic deposition and UV radiation on aquatic ecosystems.
SO₂ and its by-products also cause deterioration in building materials, such as cement, limestone, and sandstone. Some significant historic structures such as the Canadian Parliament Buildings are slowly being eroded by acidic pollutants.

Human exposure to particulate matter, including sulphate and acidic aerosols, leads to increased respiratory problems. Recent research indicates a relationship between decreased lung function, increased cardiorespiratory mortality, and long-term exposure to ambient acidic aerosols.

What is being done?

1985–87: The Canadian Acid Rain Control Program is established. Federal–provincial agreements are signed with the seven eastern provinces to reduce their combined SO₂ emissions to 2.3 million tonnes per year by 1994 (a 40% reduction of the 1980 level).

1991: The Canada–United States Air Quality Agreement is signed to reduce SO₂ and NO_x emissions. Canada is committed to a permanent national limit on SO₂ emissions of 3.2 million tonnes per year by the year 2000 and a 10% reduction in projected NO_x emissions from stationary sources by the same year. The 2.3 million tonne per year limit on SO₂ emissions for eastern Canada is extended to the year 2000.

1994: A Sulphur Oxide Management Area (SOMA) is established in the main source region in eastern Canada. An SO₂ emissions limit for the area is set at 1.75 million tonnes a year beginning in the year 2000.

1997: A report on protecting acid-sensitive ecosystems, human health, and air visibility in Canada, entitled Towards a National Acid Rain Strategy, is submitted to the National Air Issues Coordinating Committee (the federal-provincial-territorial body charged with addressing air issues in Canada). The report recommends further SO₂ emission reductions of up to 75% from current commitments in targeted regions of eastern Canada and the United States, to prevent critical loads for acidic deposition from being exceeded across eastern Canada.

1998: A Canada-wide Acid Rain Strategy for Post-2000 is signed by federal and provincial/territorial Ministers of Energy and Environment in October. It provides a framework for further SO₂ emission reduction commitments in eastern Canada beyond 1999.

Acknowledgements

Data and advice provided by the following agencies are gratefully acknowledged:

Canadian Heritage
    Parks Canada

Environment Canada (DOE)
    Meteorological Service of Canada
    Environmental Protection Service
    National Water Research Institute
    DOE Regions

Fisheries and Oceans Canada

Ontario Ministry of Environment and Energy