"Acid Rain"

"Acid Rain" is a fairly recent addition to our language. But this phenomenon--acidic precipitation and deposition--may, in fact, date back to the industrial revolution of the late 18th century.

It has become a recognized problem for scientists since the 1950s and for the last decade has been a major concern as it continues to defile major areas of our planet.

Acid Rain: A Summary

+Acid rain - the label given to both wet and dry acidic deposits- occurs within, and downwind of, areas of major industrial emission of sulphur dioxide (SO2) and the oxides of nitrogens (NOx).

+1980 sulphur dioxide emissions; Canada 4.7 million tonnes, United States 24.1 million tonnes

+1980 nitrogen oxides emissions; Canada 1.8 million tonnes, United States 20.0 million tonnes

+After SO2 and NOx are emitted into the atmosphere, they are transformed into sulphate or nitrate particles and by combining with water vapour, into mild sulphuric or nitric acids. They are transported, sometimes great distances, by the prevailing winds. In eastern North America these winds come mainly from the West and South-West.

+If there is no tightening of current environmental regulations, emissions are expected to increase. Any reductin of emissions would reduce future damage.

+The technology does exist to substantially reduce emissions of SO2 and NOx now.

+The pH scale is used to determine the degree of acidity or alkalinity of a solution. "Clean rain" is slightly scidic with a pH level of about 5.6. Below this level precipitation is considered abnormally acidic.

+More than two million square kilometres of North America now receive rain with a pH of 4.6 or lower, that is 10 or more times as acidic as "clean rain".

+Lake and river waters with a pH below 4.5 will not support fish populations. Even at pH 5.0 only limited fish populations can; survive; frogs, salamanders and many other creatures are decimated.

+Some 75 federal and 133 provincial stations form the integrated acid rain monitoring network in Canada. These stations provide a regular assessment of rain acidity and deposition.

+Relatively high sulphate and nitrate deposits in remote wilderness areas demonstrate the reality of long range transport of airborne pollutants.

+In the eastern United States and Canada, emissions from man-made sources of SO2 are 10 to 20 times greater than emissions from natural sources.

+Right now it is the SO2 emissions that cause the most measurable damage to the environment.

+The vast majority of vulnerable areas would be protected if wet sulphate depostition was at levels of less than 20 kilograms/per hectare/per year (18lbs per acre per year). Canadian emission reductions alone cannot achieve this in sensitive areas of Canada. U.S. emission reductions are needed as well.

+A 1980 Study prepared at the University of Wyoming for the U.S. Environmental Protection Agency estimated the cost of damage to natural and man-made resources from acid rain, in the eastern on-third of the United States, at $5 billion per year.

+The U.S. Congress Office of Technology Assessment estimates the direct cost of a 10-million-ton reduction in SO2 emissions at about $2.5 to $4.7 billion per year, representing $9 to $20 for each American.

+The direct cost of controlling SO2 emissions in Canada is estimated at perhaps $1 billion per year or $40 for each Canadian.

+By 1990 Canada is already committed to reduce by 25 percent the total 1980 allowable SO2 emissions in the eastern provinces, and will undertake additional reductions--up to 50 percent--to achieve the environmental target in concert with control programs in the U.S. This is in addition to the SO2 controls that both countries undertook in the 1970s to meet local air pollution standards.

+In eastern Canada, major resources threatened with damage from acid rain are sport fishing, tourism and the forest products industry. These sectors generate eight percent of Canada's GNP.

+Fish taken from waters which are becoming acidic exhibit high concentrations of mercury and other toxic metals in their tissues--presumably leached out of soils and bedrock. Drinking water from acidic lakes and rivers have elevated concentrations of toxic metals too.

+Important evergreen species and other plants exposed to high doses of acid rain show damage to their foliage, sometimes at levels such as pH 4.6. Growth of the whole plant is adversely affected.


SO2 Emissions

In eastern North America, man-made emissions of SO2 account for about 90 percent of the total sulphur moving through the atmosphere.

The largest sources of these emissions are electric utilities, industrial, commercial and residential heating, and smelters. Diesel engines, marine and rail transport are also significant sources.

The below table shows actual levels of emissions in 1980 from different source sectors in the United States and Canada. About two-thirds of the total emissions occur in eastern North America particularly in the highly industrialized central portion.

NOx Emissions

In eastern North America, man-made emissions of NOx account for most nitrogen oxides moving through the atmosphere. Natural sources of nitrogen emissions do exist in North America, but these are thought to be only a small percentage of the total.

The largest sources of the emissions are the transportation sector (cars, trucks, planes, etc.), electric utilities, and non-utility fuel combustion facilities. The majority of NOx emissions occur in the highly industrialized areas of North America.

Long Range Transport

Some pollutants fall to earth as dry fallout or deposition. Others react with water vapor in the atmosphere to form sulphuric and nitric acid and return to earth as dew, drizzle, fog, sleet, snow and rain.

Pollutants may be carried hundreds or even thousands of kilometres by the prevailing winds. This phenomenon is known as the Long Range Transport of Airborne Pollutants (LRTAP).

Environment Canada estimates that more than 50 percent of the acid rain that falls in Canada comes from American sources. Rather more than 10 percent of the acid rain that falls in the northeastern United States comes from Canadian sources.

The dots indicate the sources with SO2 emissions between 100 and 500 tonnes per year. The squares are sources of more than 500 kilotonnes. Smaller sources in shaded areas also account for a significant portion of total emissions.

The shaded areas are vulnerable to acidification. The lines depict the incidence of wet sulphate deposition, the numbers indicate the levels of deposition in kg/ha/yr. Levels of deposition exceeding 20 kg/ha/yr (18 lbs/acre/year) are generally regarded as threatening in vulnerable areas. Nitrate deposition, not shown on the map, threatens the same areas.

Effects on surface water and fish

The most vulnerable areas of our continent are those where the soils are thin and the bedrock is granite. They cannot counteract the action of the acids. The hardest hit areas are Ontario, Quebec, and Nova Scotia, Maine, Vermont, New Hampshire, New York, Massachusetts, New Jersey and Pennsylvania.

Effects are also appearing in Michigan and Minnesota, as well as a few more westerly states. Major parts of the southeastern States are vulnerable, and mountain lakes in Colorade and California are becoming acidic.

+About a dozen rivers in Nova Scotia--far removed from local upwind pollution sources no longer support healthy populations of Atlantic salmon.

+About 200 lakes in the Adirondacks no longer support fish life and thousands more are slowly losing their capacity to buffer acid rain.

+To date, 4016 lakes have been tested in the province of Ontario. Of these, 155 or four percent, were found to be acidified with their ability to support aquatic life extremely limited. A total of 2896 lakes had some susceptibility to acidification.

+A U.S. government study estimated that 55 percent of the lakes and 42 percent of stream-miles inthe eastern U.S. are currently being subjected to acidic deposition which will eventually lead to deterioration.

+Canadian and U.S. scientists cooperated in the Technical Work Groups established under the 1980 Memorandum of Intent concerning transboundary air pollution. The Memorandum states the intention of both nations to vigorously enforce air pollution legislation and to work for deelopment of a bilateral agreement on transboundary air pollution.

The scientists agreed that damages are observed in sensitive areas where sulphate is above the level of 20 kg/ha/yr (18 pounds per acre per year).

Acid Shock

In Canada and the northeastern United States sudden and intense acid doses often occur in the spring. As snow melts, pollutants stored in the snowpack are abruptly released. The meltwater has been measured to be as much as one hundred times more acidic than normal. This happens at the worst possible time for most fish and amphibians--at spawning time.

Studies have clearly demonstrated that trout and Atlantic salmon are particularly sensitive to low pH levels which interfere with reproductive processes and frequently lead to skeletal deformities.

Many species of amphibians--frogs, toads and salamanders--breed in temporary pools formed by spring rains and melted snow. The eggs and developing embryos are exposed to the acids and deformity or death occurs.

Field work has established that 80 percent of salamander eggs failed to hatch in waters with a pH level below 6.0. For the cricket frog and northern spring peeper an exposure to waters with a pH level about 4.0 resulted in more than 85 percent mortality.

Amphibians are important members of both water and land ecosystems. They are major predators aof aquatic insects and in turn serve as high protein food for many birds and mammals--important links in the food chain.

Toxic Metals

Acid rain leaches metals, such as aluminum, from the soils and sediments of watersheds, separating solid particles from soluble components and discharging these metals into lakes and rivers. Elevated levels of aluminum have serious effects including clogging of the gills, causing fish to gradually suffocate and die. Metals such as mercury, cadmium and lead in the tissues of fish can be toxic when eaten, to people, birds and animals, even at low concentrations.

History of a Lake

This large experiment lake is referred to simply as LAKE 223, a lake in northwestern Ontario, typical of thousands in North America. Since this experiment started 10 years ago, scientists have been artificially increasing the acidity of this lake while studying in great detail the attendant chemical and biological changes. By adding sulphuric acid to LAKE 223, scientists have lowered its pH value and have documented the acidification stages as follows.

1976 pH 6.8

1977 pH 6.1

-Increase in bacterial activity

-Increase in invertebrate acquatic animals, small animals which are food for larval fish, insects and large crustaceans

-Increase of green algae

-Decrease of brown algae, which is normally dominant

-Increase of insects

1978 pH 5.8

-Disappearance of one type of copepod, a crustacean species

-Reproductive failure of the fathead minnow

-Increase in deaths of lake trout embryos

-Decrease in slimy sculpin

1979 pH 5.6

-Increased plant production in depths of lake

-Development of mats of algae along shoreline

-Disappearance of opossum shrimp, a major fod source for lake trout

-Decrease in hardness of crayfish exoskeleton

-Severe decline in fathead minnow

-Decrease in white sucker abundance

-Decrease in lake trout abundance

1980 pH 5.4

-Disappearance of another copepos

-Infestation of parasites in crayfish

-Decreased reproduction and abundance of crayfish

-Increase of pearl dace, a small minnow

-Reproductive failure of lake trout

1981 pH 5.1

-Reproductive failure of white sucker

At present, LAKE 223 acidity is being maintained at pH 5.1 to study further developments. Although the experiment is artificial manipulation and does not occur in nature, it provides very valuable guidance in the process which might be expected in thousands of lakes now becoming acidified.

How Acid Rain Affects our Forests

The environmental and economic value of Canada's forests cannot be overestimated:

-they regulate the flow of many of our lake and river systems

-they prevent soil erosion

-they are a home to a highly diversified range of wildlife.

In addition, our forest provide an unique setting for a multi-billion dollar recreation and tourism industry.

Canada's forestry sector produces $23 billion worth of shipments annually. Net exports amount to $13 billion per year. No other industry contributes so much to Canada's balance of payments. Forestry employs, directly and indirectly, one million persons. This is approximately one in every 10 jobs in Canada and is the support of some 3000 one-industry communities.

Any threat to Canadian forests must be carefully watched. Acid rain poses an insidious and potentially devastating threat to our forests. A recent study has shown that seedlings can be damaged by moderately acidic rain (pH 4.6). The cumulative effect of sulphuric acid in rain and snow building up season after season in the forest soils is now being studied.

Researchers are beginning to evaluate the role of acid rain in increasing the vulnerability of trees to disease and insects. Acid rain may also interfere with the decomposition of plant letter on the forest floor. This would alter the natural cycle of growth and decay which replenished the soil and feeds the trees.

Many forestry scientists in both Canada and the United States are expecting serious, possibly irreversible, soil and forest effects oer the next 25 to 100 years if acid levels being deposited remain constant or increase.

Man-Made Environment

Stone buildings, statues, and monuments are eroded by a number of airborne pollutants including acid rain. Building materials including steel, paint, plastics, cement, masonry, galvanized steel, limestone, sandstone and marble also risk damage. The frequency with which structures need to have new protective coatings replaced is increasing, with resulting additional costs--estimated at billions of dollars annually.

The effects of the various pollutants cannot yet be reliably separated from each other. However, it is generally accepted that the major single corrosive agent of building materials is sulphur dioxide and its by-products.

Food Threat

Some lakes which are becoming acidified show evidence of increasing mercury contamination in the food chain. When fish with elevated concentrations of mercury in their tissues are part of the regular diet, human health may be threatened.

As the acidic level of recreational lakes and rivers increases, human activities in these waters are questioned. Scientific evidence, to date, indicates no adverse reactions should be expected from recreational activities in acidified water.

Acid deposition liberates toxic metals in some groundwater, in drinking water supply systems and in cisterns. While no adverse health effects from drinking such water have yet been reported, this requires careful research and attention.

There are signs that we are entering a new stage in the acid rain issue--an increasing conviction that the cost of the damage to our environment outweighs the cost of cleaning the rain.

There can be no quick solutions. The clean-up may take decades even if we start today. In the past few years we have established the fundamental requirements for action

-the recognition that acid rain is a serious problem

-the knowledge that reduction of emissions is the best solution.

Control Technology

Sulphur oxides are produced from the burning of fuels, smelting of ores and other industrial processes.


Sulphur oxide emissions can be reduced by taking measures before, during or after combustion.


-fuel switching . . . changing from higher sulphur content fuels to lower

-fuel blending . . . blending fuels with higher and lower sulphate content to produce a fuel with a medium sulphur content

-oil desulphurization . . . removing sulphur during refining process by hydrogenation (breaking chemical compositions and adding hydrogen)

-coal washing (physical coal cleaning) . . . crushing and removing sulphur and other impurities from coal by placing it in a liquid. The clean coal floats; the impurities sink

-chemical coal cleaning . . . dissolving sulphur in coal with chemicals


-fluidized bed combustion (FBC) . . . mixing finely ground limestone with coal and burning it in suspension

-limestone injection in multi-stage burners (LIMB) . . . injecting finely ground limestone into a special burner


-flue gas desulphurization (FGD) or scrubbing . . . mixing a chemical absorbent such as lime or limestone with the flue gas to remove sulphur dioxide.

The costs and ability of each method to contain sulphur oxides vary.

Non-Ferrous Smelters

Sulphur oxide emissions can be reduced by a variety of means.

-mineral separation . . . removing some of the sulphur-bearing minerals from the metal-bearing minerals before smelting

-process change . . . using smelting processes that produce less SO2 or that produce waste streams that are more easily controlled

-by-product production . . . capturing SO2 after the smelting process, to produce: sulphuric acid (used in many industrial processes and in making fertilizer); liquid SO2 (used in pulp and paper processing); or elemental sulphur (used in industrial processes).

Alternate Production Processes

There are sources of electricalf energy which do not cause acid rain.



There are also emerging technologies which have increasing acceptance, such as

SOLAR, WIND, TIDAL, BIOMASS (energy from waste vegetation) and GEOTHERMAL (heat from the earth's interior). These present choices for the future as finite energy supplies are depleted.


Transboundary air pollution, including acid rain, is of widespread international concern. Over the past decade it has become increasingly clear that pollution emissions in one country, borne by the winds, can cause serious environmental damage to another. Because the plllutants cross borders, no nation can unilaterally clean the rain that falls within its boundaries. It is a dilemma faced by all industrialized countries and their neighbours.

Canada/United States

In 1980, Canada and the United States signed a Memorandum of Intent (MOI) to negotiate a transboundary air pollution agreement "including the already serious problem of acid rain", in keeping with the growing climate of concern and mounting evidence of damage. Both countries agreed to negotiate an agreement as soon as possible.

Under the MOI joint Technical Work Groups, involving hundreds of scientists in both countries, were set up to compile scientific and technical data on the long-range transport of airborne pollutants.

The Work Groups laid the groundwork for development of joint solutions to the problem of acid rain on the North American continent.

Final reports of the Canada/United States Work Groups, released in February 1983, concluded that

(1) acid rain occurs in eastern North america within, and downwind from, major industrial regions

(2) damage in both the short and long term is occurring in areas vulnerable to acid rain as a result of sulphur deposition

(3) wet sulphate deposits greater than 20 kg/ha/yr (18 lbs per acre per year), in moderately sensitive areas, cause damage in lakes and rivers; in areas with deposits of below 20 kg/ha/yr no damage has been recorded

(4) the damage is primarily caused by sulphur deposits and the solution is to reduce them

(5) technology does exist to reduce emissions substantially

(6) if there are no changes in abatement programs it is forecast that emissions will increase through the remainder of this century.

In each country independent Peer Review experts have indicated the need for action based on what we now know.

In January 1984 the United States rejected a joint pollution control program. In March 1984 Canada committed itself to unilateral emission reductions of 50 percent by 1994.


In 1979, member governments of the United Nations' Economic commission for Europe, including Canada and the United States, signed the Long-Range Transboundary Air Pollution Convention, a UN agreement to deal with acid rain internationally.

In June 1982, the Ministerial Conference on Acidification of the Environment was held in Stockholm. It was attended by Canada and the U.S. and representatives of 19 other signatories to the Convention.

The Conference concluded that acid deposition from airborne pollution, including long-range transboundary air pollution, is a major environmental problem requiring policies for further urgent action at the national level and concerted international efforts.

In June 1983, the Executive Body for the Convention on Long-Range Transboundary Air Pollution met in Geneva to consider setting sulphur emission limits. Canada, Sweden, Norway, Denmark, Finland, West Germany, Switzerland and Austria endorsed a proposal to reduce national emissions by 30 percent. France subsequently endorsed the proposal. The final decision called for all parties to the Convention to decrease total annual emissions of sulphur compounds or their transboundary fluxes by 1993 (using 1980 emission levels as a basic). Programs for emission reductions at the national level and further plans for international action will be considered at future meetings of the Executive Body for the Convention.

What You Can Do to Help


More efficient use of fossil fuels means less acid rain. Efficiency can reduce the emissions which cause acid rain. We can all contribute, with

-car pools

-improvement and increased use of mass transit

-proper maintenance of vehicles and pollution control devices

-efficient home-heating

-electricity conservation


Development and marketing of sulphur by-products could help to offset the cost of installing control technology. Present by products of SO2 emission controls include sulphur, sulphuric acid, gypsum and pozotec (a road surfacing material).

Public Awareness

As we gain understanding of the complex acid rain issue and begin to cope with underlying causes, we realize the importance of public awareness.

An April 1983 public opinion survey showed 79 percent of Canadians consider acid rain the most serious environmental threat facing Canada today.

In the United States at 194 of 224 town meetings held in New Hampshire in April 1983 citizens voted in favor of reducing SO2 emissions by 50 percent.


In both Canada and the United States some of the hardest hit regions are those where tourism is a major contributior to the regional economy. Many resort owners and fishing camp proprietors are worried about their future.

In Canada sport fishing generated $1.1 billion in direct revenues during 1980 and an additional $10 billion in related tourism business earnings.

The Canadian forest industry produces $13 billion worth of exports annually. Any threat to our forests must be taken seriously.

In the past nature tended to balance our destructive actions, the depredations of man, the thoughtless ravaging of our environment, the plundering of our natural resources, as we raced headlong to establish a modern and material Utopia.

Now, today we have a choice, a challenge. We are confronted with what is probably the world's most destructive man-made pollution in the form of acid rain. Yes, acidification is laying waste major areas of our planet.

We, the inhabitants of Earth, have created this menace. But we have also created the technology to diminish or even eliminate it. Certainly it will be costly. But the cost of not coping with it will be far greater.

We owe it to the future of Earth, to future generations of mankind. It will be a relatively small investment for each of us -- and the dividends will be substantial.

Further Information

Please feel free to contact: Information Directorate, Environment Canada, Ottawa, Ontario K1A 0H3, (819)997-2800, ISBN 0-662-13047-2

Clean Air Strategic Alliance, 9th Floor, Sterling Place, 9940 - 106 Street, Edmonton, Alberta, Canada, T5K 2N2, ph. 403/427-9793, fx. 403/422-3127, em:

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