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Acid Deposition Case Studies


Key facts on Canada

30.0 million (2001)

9 970 610 km²


United States of America, Alaska

Cool temperate, polar in far north; January (-30 to -5C), July (10-20C)

0-3000+m above sea level

Air Quality:
Generally good in rural areas but ozone levels may be high during summer months. High emissions of acidic pollutants in large urban areas from industry and transport can lead to poor air quality.

Acid Deposition:
Eastern Canada is most sensitive to the effects of acid deposition because of its thin granitic soil. The sensitive area covers 43% of Canada, over 4 million km². Most of the emission sources are also located in the east of the country.

Canada, 2001

Source: The Perry-Castañeda Library Map Collection, The General Libraries, The University of Texas at Austin, Austin, TX 78713-8916, USA

Acidic Pollutant Emissions

The major sources of acidifying pollutants in Canada arise from human activities. The largest source of sulphur dioxide in Canada is from metal ore smelting and fossil fuel power generation whilst most of the nitrogen oxides arise from transport and industry. Emissions of sulphur dioxide have been falling over recent years, resulting in emissions of 2.7 million tonnes of sulphur dioxide in 1995, a 41% reduction from 1980. Emissions of nitrogen oxides remained fairly static at around 2 million tonnes between 1980 and 1995. Canadian emissions of sulphurous and nitrogen oxides are considerable but are less than the emissions from countries such as Germany, the UK, the Russian Federation and the United States of America. Emissions of sulphur dioxide from the neighbouring USA totalled approximately 16 million tonnes in 1995.

Most of the sulphur dioxide in Canada comes from metal ore smelting and other industrial sources (61% in 1995) and power generation (21% in 1995). The major source of nitrogen oxides in Canada is transport (59% in 1995). Total emissions of sulphurous and nitrogen oxides in 1980 and 1994 and their reductions over this time are shown in Table 3.8.

Table 3.8: Emissions of Acidifying Air Pollutants in Canada


Emissions 1980

Emissions 1995

% reduction

Sulphur dioxide '000 tonnes




Nitrogen oxides '000 tonnes




Reductions in Canadian sulphur dioxide have occurred largely as a result of industrial process changes, installation of scrubbers, and fuel switching in the 1990s. Emission reductions of nitrogen oxides are being sought by the introduction of more stringent performance standards on exhaust emissions from new vehicles.

Acidic Pollutant Depositions

Fifty per cent of acid deposition in eastern Canada is estimated to originate from sources in the USA. Because of prevailing southerly winds during summer months, much of the pollution from the USA is exported to Canada. Some pollutants from Canada are transported to the US but on balance, Canada receives much more pollution than it exports. The import / export of pollutants across the border has therefore made acid rain a serious diplomatic issue between the US and Canada. The problem is compounded by the fact that the states in the US that are most heavily populated and have high air pollution emissions are located south of the sensitive eastern regions of Canada.

In a country as large as Canada, long range transport of pollutants means that pollutants emitted in one part of the country may be deposited hundreds or thousands of kilometres away in another part of Canada. With the continents of Europe and Asia being great distances away and separated from North America by the Pacific and Atlantic Oceans, acid pollutants from Canada are unlikely to be transported to countries other than neighbouring countries.

Eastern Canada generally experiences high levels of acid deposition due to the large quantities of acidic pollutants emitted into the atmosphere each year in the heavily populated areas (the southern border and the lakes) of Canada and from pollution imported from the US. Acid deposition is less serious in western Canada due to a less acid-sensitive environment and lower levels of acid deposition.

Effects of Acid Deposition on Forests

In 1995, the Acid Rain National Early Warning System indicated an absence of large scale decline in Canadian forests caused by atmospheric pollution. In certain areas however, a decline in health of one or more particular species was identified. A decline in the health of white birch (Betula papyrifera and B. cordifolia) in eastern Canada around the Bay of Fundy in New Brunswick and Nova Scotia was apparent as leaf browning and premature leaf fall. The area of concern experiences frequent acid fogs during summertime with pH 3.0 or less.

The Canadian sugar maple (Acer saccharum) has also been identified as having sustained reduction in growth with increasing levels of acid deposition. Maples on soils with low buffering capacity also show higher levels of dieback compared to those on well-buffered soils.

Effects of Acid Deposition on Water

Acidification studies in Canada have shown that freshwater acidification is a serious problem in Canada. Between 1986 and 1990 studies of 1 253 lakes in Quebec revealed that 19% had a pH level of less than 5.5, and 52% had a pH of less than 6.0. Acid deposition (mainly sulphuric acid) is thought to be largely responsible for the acidity of lakes in southwestern Quebec and natural organic acids the cause in northeastern Quebec.

Studies of lakes in Ontario, Quebec and the Atlantic Region between 1981 and 1994 have shown that lake acidity has improved in 33% of the 202 lakes studied, 56% remained stable and 11% have worsened. Lakes were noticeably less acidic in the Sudbury region of Ontario and this has been attributed to large reductions in sulphur dioxide emissions from Sudbury's nickel smelters. However, large reductions in sulphur dioxide in eastern Canada over recent years have not generally led to improvements in lake acidity in southern and eastern Canada. This is likely to be a result of the large quantities of pollutants being deposited which have originated in the United States.

Effects of Acid Deposition on Buildings

Acid deposition is known to accelerate normal weathering effects on buildings. The harsh climatic conditions in Canada cause degradation of materials because of the many freeze-thaw cycles that occur in winter and the high humidity in the summer. Acid deposition adds to this and effects can be seen for example on bronze statues in Montreal. The bronze becomes streaked with brown, black and blue-green surfaces which result from a combination of acid deposition, de-icing salts and corrosive dust.

Many historic monuments and buildings are slowly being eroded by acid rain, including the Parliament Buildings.

Effects of Acid Deposition on Health

There is concern in North America that human health is affected by acid deposition. The 1994 Canada - United States Air Quality Agreement have reported that long-term ambient exposures to acid aerosols have been linked to decrease in lung function in children. Acid aerosols are very tiny particles (less than 2.5 micrometres in size) which can enter the respiratory system and because of their size may filter through natural bodily defences. The basis of concern arises from a study of more than 10 000 children aged between 8 and 12 years in 24 North American cities. Most air pollution studies have been carried out on sensitive persons, such as asthmatics but this study identifies that acid aerosols have a detrimental effect on normal lung function.

Short-term exposure to poor air quality episodes may also cause breathing difficulties amongst for example asthmatics. These conditions may occur during summer months, particularly in cities where car pollution is the main source of nitrogen oxides.

Control and Policy

The long range transport of air pollutants is a problem which has long faced Canada. For many years Canada has been receiving much larger amounts of pollution from the United States than vice versa. After many research programmes were carried out during the 1980s, international agreements were set up between Canada and the US in 1991 for mutual reductions in sulphur dioxide and nitrogen oxide emissions. The history of control of acid rain in Canada and the US is outlined in Table 3.9.

Table 3.9: Canadian Acid Rain Control Chronology

Late 1970s

Long-Range Transport of Air Pollutants Programme established to investigate air pollution within and into Canada.


Canada signed the United Nations Economic Commission for Europe (UNECE) agreement to reduce and prevent long-range transboundary air pollution.


Extensive research conducted in US and Canada on acid deposition.


Canada signed the UNECE Helsinki Protocol, agreeing to reduce national annual sulphur dioxide emissions by at least 30% below 1980 levels by 1993.


Canada signed the UNECE Sofia Protocol to freeze nitrogen oxide emissions in 1994 at 1987 levels (approx. 2 million tonnes).


US Clean Air Act amended to include SO2 and NOx emissions controls.


Canada / US Air Quality Agreement signed. Mutual obligations for reducing SO2 and NOx.


Canada signed the UNECE Oslo Protocol. An emissions cap of 1.75 million tonnes of SO2 established for the main source region in eastern Canada.