The United States and Canada committed to addressing transboundary air pollution issues in the 1991 AQA, a bilateral executive agreement. The AQA established a framework for addressing shared concerns relating to transboundary air pollution, and, in the first annex to the agreement, set out objectives for each country to reduce emissions leading to acidic deposition. An additional annex to the Agreement addressed Scientific Cooperation and in 1997, the Parties signed a "Commitment to Develop a Joint Plan of Action for Addressing Transboundary Air Pollution" to jointly address the shared problems of ground-level ozone and particulate matter (PM). While science assessment activities were under way to better understand transboundary PM issues, the Parties moved ahead in 2000 to establish an annex addressing ground-level ozone in the eastern border region.
Both Parties have made excellent progress in achieving reductions in emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx), the two major pollutants leading to acidic deposition. Since 2000, the Parties also have made good progress in reducing emissions of volatile organic compounds (VOCs) and further reducing emissions of NOx, to address ground-level ozone in the border region.
In 2007, the United States and Canada achieved the emissions reduction targets laid out in both the acid rain and ozone annexes, and these emissions have continued to decrease in the subsequent years. These emissions reductions have led to lower levels of acid rain and ground-level ozone, as well as decreased PM. Because of the successful completion of the AQA’s initial targets, the two countries are currently exploring options for updating the AQA.