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Acid Rain

Indicator: Area of exceedance of wet sulphate deposition above critical loads

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• The area in eastern Canada receiving wet sulphate deposition in excess of critical loads for aquatic ecosystems declined by over 40% between 1991 and 1996, from about 910 000 to about 540 000 square kilometres.
• The critical load for aquatic ecosystems is the amount of wet sulphate deposition that must not be exceeded in order to protect at least 95% of lakes in a region from acidifying to a pH level of less than 6.0. Many studies suggest that a pH of at least 6.0 is needed to protect most aquatic organisms.
• Scientists predict that large areas in eastern Canada, encompassing tens of thousands of lakes, will continue to receive wet sulphate deposition above critical load limits for aquatic ecosystems, even after the Canadian and U.S. SO₂ emission controls are fully implemented in the year 2010.

Notes:
i)     The National Atmospheric Chemistry (NAtChem) Database was used for the analysis of acid precipitation data.
ii)    Exceedance is the term used to describe the difference between the annual deposition and the “critical load” at any given location. The area of exceedance represents the sum of the areas experiencing exceedances.

Source:
R. Vet, C.-U. Ro, and D. Ord, National Atmospheric Chemistry Database and Analysis Facility, Atmospheric Environment Service, Environment Canada, Downsview, Ontario.

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