Acid Rain

Acid rain is the wet or dry deposition of acidic substances and their precursors on the Earth's surface. Wet deposition refers to RAIN, SNOW, HAIL, drizzle and other familiar forms of visible precipitation. Dry deposition, mostly invisible, occurs through gravitational settling of large particles and uptake of gases and small particles at the Earth's surface. Rain and other precipitation may be defined as acidic or alkaline (basic) depending on chemical composition. The degree of acidity is usually measured on the pH scale, a logarithmic measure of the concentration of hydrogen ions (H+) in precipitation. A neutral solution has a pH of 7. Acidic solutions have values below 7 and basic solutions have values above 7. For each change of one pH unit, the hydrogen ion content changes by a factor of 10. A clean water sample in equilibrium with atmospheric carbon dioxide will have a value of 5.6 and this is often used as a definition of "clean" rain. When values are different from this, it means that other substances, either natural or man-made, are present in the rain.

Current annual measurements of the average pH of precipitation in the northern hemisphere range from about 4.0 to 7.0. The lower, highly acidic values occur primarily over and immediately downwind of urban and industrialized areas in North America, Europe and Asia. Higher pH values in precipitation are found over less industrialized regions where the atmosphere contains larger amounts of alkaline dust. The primary cause of low pH in precipitation over northeastern North America is sulphuric acid (H₂SO₄) resulting from industrial and urban emissions of sulphur dioxide (SO₂). Nitric acid (HNO₃) generated from emissions of nitrogen oxides (NOx) is a significant contributing factor in this region. The annual emissions of SO₂ for 2006 were 12.0 million tonnes in the US and 2.2 million tonnes in Canada, down from 1980 levels of 23.4 and 4.7 million tonnes respectively. Coal-fired thermal electric power plants produce about 70% of US emissions and a little more than 20% of Canadian emissions. Nonferrous smelters, producing such metals as nickel and copper, are the largest source of Canada's SO₂ emissions. NOx emissions for 2006 were 15.8 million tonnes in the US and 2.9 million tonnes in Canada compared to the 1980 levels of 20.9 and 1.8 million tonnes in the US and Canada respectively. Transportation sources contribute the majority of NOx emissions. The acid rain precursors, SO₂ and NOx, can be transported thousands of kilometres through the atmosphere, returning to earth as dry deposition or in wet acid form.

Degree of Effects
When acid rain reaches the Earth's surface it can cause damage to aquatic ECOSYSTEMS and buildings. Acid rain and the associated pollutants (SO₂, NOx, sulphate particles and ozone) can also damage forests and crops, and there is evidence of adverse human health effects. The degree of effects depends on the acid-reducing capability of the receptor (eg, vegetation, soils and rock, lakes and streams). In areas where this buffering capacity is low (eg, the Canadian SHIELD), acidic deposition over several years has led to increased acidity of rivers and lakes and to accelerated leaching of aluminum from soils. In Canada, surface waters exhibiting these effects have been largely restricted to the southeastern portion of the country where acid rain levels are highest. However, SO₂ emissions in western Canada have increased to the point that vulnerable lakes in this region may be threatened also.

As the pH of surface waters falls below about 5.5, the diversity of aquatic life such as vegetation, zooplankton, amphibians and fish is reduced. The aluminum leached from soils may also be in a form that is toxic to aquatic organisms. Most fish populations are eradicated by reproductive failure or disappearance of suitable food sources when the average pH of a lake drops to about 4.5. Fish populations in thousands of lakes in eastern North America and Scandinavia have declined or disappeared because of water acidification; hundreds of thousands more are threatened. Rivers, too, have shown adverse effects such as the marked decline of ATLANTIC SALMON in the Maritimes and in Scandinavia. Birds and other fish predators may decrease in numbers because of reduced food supply.

Reductions in North American SO₂ emissions promote an expectation that aquatic ecosystems will soon recover from acidification. Only lakes located near smelters that have dramatically reduced emissions approach this expectation. Most lakes are only affected by long range emissions, and so far, they show relatively small increases in pH. This delay in the chemical recovery of lakes is due to several geochemical factors related to the storage or release of acids or bases from the forest soils and wetlands that surround these lakes. Biological recovery in lakes necessarily follows chemical recovery. The only extensive evidence of biological recovery occurs in lakes from the Sudbury/Killarney region of Ontario.

The effects of acid rain, and the associated pollutants, on forests and agriculture are not as clear cut but are potentially serious. These include direct damage to plant foliage, seed germination failure, retardation of growth particularly at early life stages, deterioration of plant roots associated with the leaching of soil constituents and, possibly, increased plant susceptibility to insects and diseases.

There are several potential effects of acid rain on human health. Acidified drinking water supplies may become contaminated by leaching of copper, lead and other metals from delivery pipes. Increased concentrations of heavy metals in fish in acidified rivers and lakes can pose a problem for populations consuming significant quantities of these fish.

Control Methods
Methods available to reduce SO₂ emissions include the use of low-sulphur coal and oil; the removal of sulphur from fuel and feeder ore; the use of flue-gas desulphurization techniques; energy conservation; and the use of alternative energy sources. North American techniques for controlling acid rain precursors have been aimed primarily at reducing near-source air concentrations to levels necessary to avoid immediate and short-term impacts on human health (See AIR POLLUTION). The installation of pollution control devices and the building of taller emission stacks were effective in achieving the goal of improved air quality in North American cities. However, the result of taller stacks was to disperse SO₂ and NOx emissions over large regions, and the emission standards for the short-term protection of human health are inadequate for the protection of impacted regional environments and longer-term human health.

Emissions of SO₂ in both Canada and the US decreased between the early 1970s and the present as a result of the increased use of pollution control devices, the use of more low-sulphur fuels and the introduction of some nuclear power plants. These decreases in SO₂ emissions resulted in reduced acid rain levels and the chemical recovery of some lakes in specific locations in eastern Canada, thereby illustrating the potential effectiveness of further control actions. In the absence of new controls, or the expansion of SO₂ emission sources (eg, in western Canada), the cumulative acidification effects on regional environments remain a serious problem. In addition, there has been little reduction in NOx emissions over North America.

Control Targets
As a first step in controlling the effects of acid rain on surface waters, in 1983 Canada adopted a target loading of 20 kg of wet sulphate per hectare per year. It was estimated that a reduction of deposition rates to this value would protect moderately sensitive LAKE ecosystems and could be achieved by reducing North American SO₂ emissions by about 50%. The eastern Canadian provinces and the federal government agreed to reduce emissions by 50% by 1994; several formal federal-provincial agreements were signed in 1987. Since 1990, Canada has used a more precise deposition standard called the "critical load," which is the highest deposition rate that an ecosystem can tolerate without exhibiting negative ecosystem effects. For lakes located on the Canadian Shield, the critical load is almost always less than the 1983 target load, and it varies spatially depending on the acid sensitivity of the surrounding terrain.

About one-half of the sulphate deposition in eastern Canada comes from SO₂ sources in the US. Therefore, control action in the US was needed for Canada to achieve its target loading goal. After years of pressure from Canada, in November 1990 the United States government passed a new Clean Air Act promising to reduce SO₂ emissions by 50% by the year 2000. The following year, the 2 countries signed the Canada-US Air Quality Agreement, which further codified the reductions in S0₂ and N0x emissions. In 1998, the federal, provincial and territorial Ministers of Energy and Environment agreed to "The Canada-Wide Acid Rain Strategy for Post-2000," which has the long-term goal of reducing acid rain to meet the critical load standard. What this means is that much greater SO₂ emission reductions than those presently required by legislation will be needed to promote widespread chemical and, latterly, biological recovery.

At the international level, Canada signed, in 1985, the United Nations Economic Commission for Europe (ECE) Helsinki Protocol to reduce their sulphur compounds (or the export of these compounds to other countries via the atmosphere) by 1993. In 1994, Canada signed the Oslo Protocol to cap sulphur emissions at 1.75 million tonnes.

Acid rain is but one manifestation of the increasing effects of man-made chemicals on the composition of the global atmosphere. Other anthropogenic effects associated with growing industrialization and the "chemical society" include ARCTIC HAZE, CLIMATE CHANGE and the depletion of the stratospheric OZONE LAYER (see OZONE DEPLETION). These changes in regional and global environments and their socio-economic impacts are attracting increasing international attention.

See also SUDBURY, GREATER.

Author H.L FERGUSON and D.S. JEFFRIES

Links to Other Sites
WATERLIFE
Explore this Webby award-winning website about the ills besetting the waters of the Great Lakes. From the National Film Board of Canada.

Acid Rain
An introduction to environmental issues related to acid rain. From Environment Canada.

Science and Technology into Action to Benefit Canadians
"S&T into Action to Benefit Canadians" tells the story of Environment Canada's success in generating tangible environmental, social and economic benefits. From Environment Canada.

Canada-United States Border Air Quality Strategy Border Projects
This site features updates about joint projects operated under the Canada-United States Border Air Quality Strategy. From Health Canada.