U.S.–Canada Air Quality Agreement

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The U.S.–Canada Air Quality Agreement (AQA) is a 1991 treaty between Canada and the United States aimed at reducing air pollution that crosses the border. It was often called the Acid Rain Treaty because it focused on pollutants that cause acid rain, mainly sulfur dioxide (SO2) and nitrogen oxides (NOx).

Why it was made
- Both countries wanted to protect health, ecosystems, and the economy from cross-border air pollution.
- They agreed that cooperation and coordinated actions could reduce pollution more effectively.

What the agreement does
- It creates an Air Quality Committee that reports progress every two years.
- It has two main parts: the Acid Rain Annex and the Ozone Annex.
- The Acid Rain Annex (1991) set targets to cut SO2 and NOx.
- The Ozone Annex (added in 2000) aims to reduce ground-level ozone and smog by cutting NOx and volatile organic compounds.

Key programs and progress
- In the United States, the Acid Rain Program (started in 1995) established the first cap-and-trade system for SO2.
- Canada pushed its own measures, including the Eastern Canada Acid Rain Program and later a Canada-wide strategy to protect health and reduce emissions.
- The Ozone Annex defined areas in the United States (18 states plus the District of Columbia) and in Canada (parts of Ontario and Quebec) that are targeted, affecting about 40% of Americans and 50% of Canadians.
- By 2007, both countries had reached or exceeded the targets in the Acid Rain Annex and the Ozone Annex, showing lower emissions and strong cooperation.

Context and impact
- Transboundary pollution is a global concern, and the AQA builds on decades of environmental cooperation.
- Canada is more vulnerable to acid rain due to geology, and a sizable portion of Canadian pollution comes from the United States, highlighting the importance of cross-border action.