Acid Rain Facts

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 1995, for instance, U.S. SO2 emissions were measured at 16.8 million tonnes—a  full six times greater than Canada’s—2.7 million total tonnes. But the sources of SO2 emissions from the two countries are quite different. While 61% of Canada’s emissions come directly from industrial sources, 66% of the U.S.’ emissions are from the electrical utilities.
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 Muskoka-Haliburton and Quebec City 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 to 4.2 millions of tonnes per year.

Where do NOX 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 1995, Canada’s largest contributor of NOX was the transportation sector, which accounted for approximately 60% of all emissions. Overall, NOX emissions amounted to 2.25 million tonnes in 1995. By comparison, U.S. NOX emissions for 1995 amounted to 21.7 million tonnes—10 times more than Canada’s. 

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 occur simultaneously in 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 

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.
The critical load varies across the Canada. It depends on the ability of a particular ecosystem to neutralize acids. The critical load for aquatic ecosystems is defined 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 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 . The program’s objective was to reduce wet sulphate deposition to a target 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.