The United States and Canada signed the 1991 United States–Canada Air Quality Agreement (AQA) to reduce the impact of transboundary air pollution.
The Acid Rain Annex, negotiated with the original 1991 agreement, committed both the United States and Canada to reducing acid rain-causing emissions of sulfur dioxide (SO2) and nitrogen oxides (NOX). Since this time, there have been large reductions in SO2 and NOXemissions on both sides of the border with subsequent reductions in ecosystem acidification and improvements in air quality. For example, the United States has reduced total SO2 emissions from covered sources by 67 percent in 2010 from their 1990 levels while Canada’s total SO2 emissions have decreased by 57 percent in 2010 from 1990 emission levels.
The Ozone Annex, added to the Agreement in 2000, committed the countries to reducing emissions of NOX and volatile organic compounds (VOCs), the precursors to ground level ozone, a key component of smog. Between 2000 and 2010, the United States has reduced total NOX emissions by 42 percent in the transboundary ozone region while Canada’s total NOX emissions decreased by 40 percent in the region.