What causes acid rain?

Acid deposition is a general term that includes more than simply acid rain. Acid deposition primarily results from the transformation of sulphur dioxide (SO₂) and nitrogen oxides into dry or moist secondary pollutants such as sulphuric acid (H₂SO₄), ammonium nitrate (NH₄NO₃) and nitric acid (HNO₃). The transformation of SO₂ and NOx to acidic particles and vapours occurs as these pollutants are transported in the atmosphere over distances of hundreds to thousands of kilometers. Acidic particles and vapours are deposited via two processes - wet and dry deposition. Wet deposition is acid rain, the process by which acids with a pH normally below 5.6 are removed from the atmosphere in rain, snow, sleet or hail. Dry deposition takes place when particles such as fly ash, sulphates, nitrates, and gases (such as SO₂ and NOx), are deposited on, or absorbed onto, surfaces. The gases can then be converted into acids when they contact water.

What does acid mean?

An acid is a substance with a sour taste that is characterized chemically by the ability to react with a base to form a salt. Acids turn blue litmus paper (also called pH paper) red. Strong acids can burn your skin.

What is pH?

A pH scale is used to measure the amount of acid in a liquid-like water. Because acids release hydrogen ions, the acid content of a solution is based on the concentration of hydrogen ions and is expressed as "pH." This scale is used to measure the acidity of rain samples.

• 0 = maximum acidity 
• 7 = neutral point in the middle of the scale
• 14 = maximum alkalinity (the opposite of acidity)

The smaller the number on the pH scale, the more acidic the substance is. Rain measuring between 0 and 5 on the pH scale is acidic and therefore called "acid rain." Small number changes on the pH scale actually mean large changes in acidity.

For example, a change in just one unit from pH 6.0 to pH 5.0 would indicate a tenfold increase in acidity. Clean rain usually has a pH of 5.6. It is slightly acidic because of carbon dioxide which is naturally present in the atmosphere. Vinegar, by comparison, is very acidic and has a pH of 3.

Where is acid rain a problem?

Acid rain is a problem in eastern Canada because many of the water and soil systems in this region lack natural alkalinity - such as a lime base - and therefore cannot neutralize acid naturally. Provinces that are part of the Canadian Precambrian Shield, like Ontario, Quebec, New Brunswick and Nova Scotia, are hardest hit because their water and soil systems cannot fight the damaging consequences of acid rain. In fact, more than half of Canada consists of susceptible hard rock (i.e., granite) areas that do not have the capacity to effectively neutralize acid rain. If the water and soil systems were more alkaline - as in parts of western Canada and southeastern Ontario - they could neutralize or "buffer" against acid rain naturally. 

In western Canada, there is insufficient information at this time to know whether acid rain is affecting these ecosystems. Historically, lower levels of industrialization - relative to eastern Canada - combined with natural factors such as eastwardly moving weather patterns and resistant soils (i.e., soils better able to neutralize acidity), have preserved much of western Canada from the ravages of acid rain.

However, not all areas in western Canada are naturally protected. Lakes and soils resting on granite bedrock, for instance, cannot neutralize precipitation. These are the conditions found in areas of the Canadian Shield in northeastern Alberta, northern Saskatchewan and Manitoba, parts of western British Columbia, Nunavut and the Northwest Territories . Lakes in these areas are as defenseless to acid rain as those in northern Ontario. If sulphur dioxide and nitrogen oxide emissions continue to increase in western Canada, the same sort of harmful impacts that have happened in eastern Canada could occur.

Visit the Atlantic region web site for more information on acid rain in this region.

Visit The NatChem Website for information on how to obtain deposition data and maps.

Where do sulphur dioxide emissions come from?

Sulphur dioxide (SO₂) is generally a byproduct of industrial processes and burning of fossil fuels. Ore smelting, coal-fired power generators and natural gas processing are the main contributors. In 2000, for instance, U.S. SO₂ emissions were measured at 14.8 million tonnes - more than six times greater than Canada's 2.4 million tonnes. But the sources of SO₂ emissions from the two countries are different. In Canada, 68% of emissions come from industrial sources and 27% comes from electric utilities (2000). In the U.S., 67% of emissions are from electric utilities (2002).

Canada cannot win the fight against acid rain on its own. Only reducing acidic emissions in both Canada and the U.S. will stop acid rain. More than half of the acid deposition in eastern Canada originates from emissions in the United States. Areas such as southeastern Ontario (Longwoods) and Sutton, Quebec receive about three-quarters of their acid deposition from the United States. In 1995, the estimated transboundary flow of sulphur dioxide from the United States to Canada was between 3.5 and 4.2 millions of tonnes per year.

SO₂ Emissions from Canada and the United States

Air Pollutant Emissions, Canada
Air Pollutant Emission Trends, U.S.

Have SO₂ emission levels changed at all?

Initiated in 1985, the Eastern Canada Acid Rain program committed Canada to cap SO₂ emissions in the seven provinces from Manitoba eastward at 2.3 million tonnes by 1994, a 40% reduction from 1980 levels. By 1994, all seven provinces had achieved or exceeded their targets. In 1998, the provinces, territories and the federal government signed The Canada-Wide Acid Rain Strategy for Post-2000, committing them to further actions to deal with acid rain. Progress under both the Eastern Canada Acid Rain Program and under the Post-2000 Strategy, including data on emissions, is reported in the respective annual reports of these two programs. Between 1980 and 2001, emissions of SO₂ declined by approximately 50% to 2.38 million tonnes. In eastern Canada , emissions of SO₂ declined by approximately 63% between 1980 and 2001.

Eastern Canada Acid Rain Reports
Annual Progress Reports on The Canada-Wide Acid Rain Strategy for Post-2000
National Pollutant Release Inventory

Where do NO_X emissions come from?

The main source of NOx emissions is the combustion of fuels in motor vehicles, residential and commercial furnaces, industrial and electrical-utility boilers and engines, and other equipment. In 2000, Canada's largest contributor of NOx was the transportation sector, which accounted for approximately 60% of all emissions. Overall, NOx emissions amounted to 2.5 million tonnes in 2000. By comparison, U.S. NOx emissions for 2000 amounted to 21 million tonnes - 8 times more than Canada 's emissions.

The influence of transboundary flows of air pollutants from the United States into Canada is significant. Overall about 24% of the regional-scale ozone episodes that are experienced in the United States also affect Ontario. An analysis of ozone concentrations at four sites in extreme southwestern Ontario taking wind factors into account provides an estimate that 50 to 60% of the ozone at these locations is of U.S. origin (Multi-stakeholder NOx/VOC Science Program 1997b).

NO_x Emissions from Canada and the United States

Air Pollutant Emissions, Canada
Air Pollutant Emission Trends, U.S.

Have NO_X emission levels changed at all?

In Canada , total NOx emissions have been relatively constant since 1985. As of 2000, stationary sources of NOx emissions have been reduced by more than 100,000 tonnes below the forecasted level at power plants, major combustion sources and metal smelting operations. In 2000, as part of the Ozone Annex to the Canada-US Air Quality Agreement, Canada committed to an annual cap on NO₂ emissions from fossil-fuel power plants of 39,000 tonnes in central and southern Ontario and 5,000 tonnes in southern Quebec. It also committed to new stringent emission reduction standards for vehicles and fuels and measures to reduce NOx emissions from industrial boilers. These commitments are estimated to reduce annual NOx emissions from the Canadian transboundary region (defined as central and southern Ontario and southern Quebec) by approximately 39% from 1990 by 2010.

Canada-U.S. Air Quality Agreement and "Progress Reports"
Air Pollutant Emissions
Eastern Canada Acid Rain Reports
Annual Progress Reports on The Canada-Wide Acid Rain Strategy for Post-2000

What is the difference between a target load and a critical load?

The critical load is a measure of how much pollution an ecosystem can tolerate; in other words, the threshold above which the pollutant load harms the environment. Different regions have different critical loads. Ecosystems that can tolerate acidic pollution have high critical loads, while sensitive ecosystems have low critical loads.

Critical loads vary across Canada. They depend on the ability of each particular ecosystem to neutralize acids. Scientists have defined the critical load for aquatic ecosystems as the amount of wet sulphate deposition that protects 95% of lakes from acidifying to a pH level of less than 6. (A pH of 7 is neutral; less than 7 is acidic; and greater than 7 is basic.) At a pH below 6, fish and other aquatic species begin to decline.

A target load is the amount of pollution that is deemed achievable and politically acceptable when other factors (such as ethics, scientific uncertainties, and social and economic effects) are balanced with environmental considerations. Under the Eastern Canada Acid Rain Program, Canada committed to cap SO₂ emissions in the seven provinces from Manitoba eastward at 2.3 million tonnes by 1994. The program's objective was to reduce wet sulphate deposition to a target load of no more than 20 kilograms per hectare per year (kg/ha/yr), which our scientists defined as the acceptable deposition rate to protect moderately sensitive aquatic ecosystems from acidification.

Under the Canada-Wide Acid Rain Strategy for Post-2000, signed in 1998, governments in Canada have adopted the primary long-term goal of meeting critical loads for acid deposition across the country. Recently, maps that combine critical load values for aquatic and forest ecosystems have been developed. These maps indicate the amount of acidity (reported as acid equivalents per hectare per year (eq/ha/yr)) that the most sensitive part of the ecosystem in a particular region can receive without being damaged.

Terrestrial or Aquatic Critical Loads

Click to enlarge

The maximum amount of acid deposition that a region can receive without damage to its ecosystems is known as its critical load. It depends essentially on the acid-rain neutralizing capacity of the water, rocks, and soils and, as this map of Canada shows, can vary considerably from one area to another. Critical loads were calculated using either water chemistry models (i.e., "Expert" or "SSWC") or a forest soil model (i.e., "SMB"). The index map (lower left) indicates the model selected for each grid square: red = Expert (aquatic), yellow = SSWC (aquatic), green = SMB (upland forest soils).

Would acid rain remain a problem without further controls?

Yes. Scientists predicted in 1990 that a reduction in SO₂ emissions from Canada and the U.S. of approximately 75% beyond commitments in the 1991 Canada-U.S. Air Quality Agreement (AQA) would be necessary to eliminate the acid deposition problem in Canada. This science was based on the effect of sulphur-derived acids in wet deposition on aquatic ecosystems. New science presented in the 2004 Acid Deposition Science Assessment assesses the capacity of aquatic and terrestrial ecosystems to receive acids derived from both sulphur and nitrogen in wet and dry deposition. Improved estimates of dry deposition (the sum of gaseous SO₂, particle sulphate, nitric acid, particle nitrate and other nitrogen species) indicate that past estimates of critical loads for aquatic ecosystems are too high, implying that past predictions of the impact of proposed control strategies have been overly optimistic. In some regions, the critical loads for forest ecosystems are even more stringent that those for aquatic ecosystems. Canada still needs to evaluate the sustainability of forest ecosystems for various levels of acid deposition given the new critical loads for terrestrial ecosystems. It is likely that new science will continue to support the need for further SO₂ emission reductions of this scale or somewhat greater.

That is why The Canada-Wide Acid Rain Strategy for Post-2000 calls for further emission reductions in both Canada and the United States. Without further controls beyond those identified in the 1991 Canada-U.S. Air Quality Agreement, areas of southern and central Ontario, southern and central Quebec, New Brunswick and Nova Scotia would continue to receive mean annual sulphate deposition amounts that exceed their critical loads. The critical load would be exceeded by up to 10 kg/ha/yr of wet sulphate in parts of central Ontario and central and southern Quebec. As a result, about 95,000 lakes would remain damaged by acid rain. Lakes in these areas have not responded to reductions in sulphate deposition as well as, or as rapidly as, those in less sensitive regions. In fact, some sensitive lakes continue to acidify. 

In total, without further controls, almost 800,000 km² in southeastern Canada-an area the size of France and the United Kingdom combined-would receive harmful levels of acid rain; that is, levels well above critical load limits for aquatic systems.

Predicted wet sulphate deposition in excess of critical loads in 2010, without further controls (in kg/ha/yr).

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