The main source of NO X emissions is the combustion of fuels in motor vehicles, residential and commercial furnaces, industrial and electrical-utility boilers and engines, and other equipment. Overall, NO X emissions amounted to 2.0 million tonnes in 1995. The influence of transboundary flows of air pollutants from the United States into Canada is significant. So far, Canadian NO X emissions have declined slightly, from 2.1 million tonnes in 1990 to 2.0 million tonnes in 1995. This is largely a result of industrial process changes, retrofitting of fossil-fueled power plants, and provincial and federal programs targeting mobile sources.
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. At a pH below 6, fish and other aquatic species begin to decline. Under the Eastern Canada Acid Rain Program, Canada committed to cap SO 2 emissions in the seven provinces from Manitoba eastward at 2.3 million tonnes by 1994.