Recent Developments in Determining Critical Loads for Sulphur and Nitrogen Deposition in Western Canada

Silvina Carou,
Atmospheric Science Assessment and Integration, Environment Canada, ON, Canada

Julian Aherne and Shaun Watmough
Environmental and Resource Studies, Trent University, Peterborough, ON, Canada

Dean Jeffries
Aquatic Ecosystem Impacts Research Division, Environment Canada, Burlington, ON, Canada


In Canada, the critical load approach has been adopted to assess where and by how much sulphur dioxide (SO2) and nitrogen oxide emissions need to be reduced in order to protect sensitive ecosystems from acid deposition damage. National acid rain assessments produced in the last 15 years have presented critical load information for eastern Canada. Estimates for this part of the country have evolved significantly from critical loads of wet sulphate deposition for all but the most sensitive lakes, to more refined and comprehensive estimates of total (wet plus dry) acid deposition (sulphur plus nitrogen) for lakes and forest soils combined. This information is also essential to track the implementation of domestic programs such as the Canada-wide Acid Rain Strategy for Post 2000 (The Strategy), which commits eastern provinces to reducing SO2 emissions in order to meet the long term goal of reducing acid deposition below critical loads. The Strategy also commits provinces and territories to “keep clean areas clean”. Although eastern Canada continues to be the largest area susceptible to continued acid rain damage, comparatively little information is available for western Canada and, as a result, deposition and ecosystem sensitivity are poorly understood. In an effort to assess the potential for acidification problems in western Canada, first cut estimates of the region’s critical loads and critical load exceedances are being completed for soils and surface waters. Critical load estimates available for forest soils in Saskatchewan (SK) and Manitoba (MB) show that approximately 2% of mapped soils in SK and 7% in MB receive damaging levels of sulphur and nitrogen deposition. Forest soils in Alberta (AB) and British Columbia (BC) are also being assessed. Some areas of AB and BC are characterized by alpine ecosystems but very limited deposition and ecosystem data are available at this time to properly assess their sensitivity. However, preliminary estimates of critical loads of nutrient nitrogen for alpine ecosystems in southern BC indicate they may be exceeded due to high levels of nitrogen deposition. In terms of aquatic critical loads of acid deposition, new estimates are available for Manitoba that span a broader range than previously available. Future plans include assessing the critical loads of total acid deposition over all of Canada for lakes and forest soils combined. The results will be used to evaluate the success of, and future needs for, emission reduction commitments under The Strategy.