Acid Rain

Acid Rain, 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 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. On the pH scale, 7 represents a neutral solution. Acidic solutions have values below 7 and alkaline 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 manmade, are present in the rain.

Keywords

Environment

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 industrialized areas in northeastern North America and Europe. Higher pH values 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. Current annual emissions of SO₂ amount to about 20 million t in the US and about 2 million t in Canada. Coal-fired thermal electric power plants produce about 70% of US emissions and about 20% of Canadian emissions. Nonferrous smelters, producing such metals as nickel and copper, account for about 50% of Canada's SO₂ 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.

Acid Rain
Acid rain has wiped out fishes, molluscs and frogs in thousands of Eastern Canada lakes (photo by Don E. McAllister).

Acidity Scale
The pH scale measures the degree of acidity (artwork by Michael Lee).

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, SO₄ 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-reduction capability of the receiving surfaces (eg, vegetation, soils and rock structures). In areas where this buffering capacity is low (eg, the Canadian Shield), acidic deposition over several years had led to increased acidity of rivers and lakes and to accelerated leaching of aluminum from soils. The aluminum can be in a form which is toxic to aquatic organisms. 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. Most fish populations are eradicated when the average pH of a lake drops to about 4.5. Thousands of lakes in eastern North America and Scandinavia are devoid of fish 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.

Acid Rain, Map

The effects of acid rain, and the associated pollutants, on forests and agriculture, are not as well documented 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 susceptibility to insects and diseases. Since the Canadian forest industry is valued at $30 billion annually, even a small reduction in productivity is significant.

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 quantities of these fish.

Control Methods

Available control methods include the use of low-sulphur coal and oil, 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 early 1990s as a result of 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 partial recovery of some ecosystems in specific locations in eastern Canada, thereby illustrating the potential effectiveness of further control actions. In the absence of such new controls, it is anticipated that SO₂ emissions will begin to rise again, and even at current levels the cumulative acidification effects on regional environments remains a serious problem. In addition, there has been no decrease in NOx emissions over North America.

Control Targets

As a first step in controlling the effects of acid rain on surface waters, Canada has adopted a target loading of 20 kg of wet sulphate per hectare per year. It is estimated that a reduction of current deposition rates to this value would protect moderately sensitive LAKE ecosystems and could be achieved over North America by reducing 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.

About one-half of the sulphate deposition in eastern Canada comes from sources of SO₂ in the US. Therefore, control action in the US is 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 sulphur dioxide emissions by 50% by the year 2000. The following year the 2 countries signed an agreement for reductions in S0₂ and N0x emissions.

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

Author H.L FERGUSON

Links to Other Sites

Acid Rain: Pollution and Politics
This extensive multimedia CBC website covers issues related to acid rain pollution.

Acid Rain
An extensive information source about causes and effects of acid rain. From the Environment Canada website.

Dr. David Schindler
This University of Alberta site offers a brief bio of distinguished ecologist Dr. David Schindler. The link to the Experimental Lakes Area (ELA) project offers an excellent overview of research into the effect of pollution on aquatic ecosystems.

Feature Articles

Invention of Standard Time

Time waits for no man… and neither do trains...