An acid-deposition critical load (CL) is a quantitative measure of the acid buffering capacity of an ecosystem. The CL will vary geographically since some locations have a lower acid buffering capacity than others and hence are more sensitive to acid deposition. An acid-deposition CL exceedance is the difference between atmospheric acid deposition to that ecosystem and its CL value. If the difference is positive, then the atmospheric acid deposition to that ecosystem is either larger than its acid buffering capacity, an indication that an ecosystem is being damaged by acid deposition or is at risk of damage, depending upon the atmospheric acid-deposition input that was chosen. An acid-deposition CL field has been available for eastern Canada since the 1990s, but more recently a new CL field has been developed that covers most of sub-Arctic Canada from coast to coast. The availability of this new CL field has made it possible to evaluate the impact of acid deposition across Canada for both current and future conditions.

A one-year simulation has been performed with a size- and composition-resolved, off-line regional particulate-matter model named AURAMS (A Unified Regional Air-quality Modelling System). The year simulated was 2002, the modelling domain covered most of North America, and the horizontal grid spacing was 42 km. The hourly gridded anthropogenic emissions files used for the simulation were prepared from the 2000 Canadian, 2001 U.S., and 1999 Mexican national emissions inventories. Hourly biogenic emission fields were predicted using BEIS v3.09. AURAMS predictions from this 2002 simulation have been processed to obtain annual sulphur total deposition and annual oxidized and reduced nitrogen total deposition fields based on the wet and/or dry deposition of SO₂, H₂SO₄, p-SO₄, NO₂, HNO₃, RNO₃, p-NO₃, PAN, NH₃, and p-NH₄. Two CL exceedance calculations were then performed based on the AURAMS total deposition predictions, one for sulphur-only annual total deposition and one for sulphur-plus-nitrogren annual total deposition. As expected, sizeable areas of eastern Canada were predicted to be in exceedance of CL, but so too were some parts of western Canada. This latter finding is significant as SO₂ and NO_x emissions are projected to increase in western Canada over the next two decades.