History

Although NOx was recognized as a contributor to acid rain in the 1960's and 1970's when scientific studies began to link SO₂ to the acidification of aquatic systems in Scandinavia and North America, serious attention to NOx only occurred in the 1980's when understanding of its contribution to the formation of ground-level ozone became better understood. Early ozone management programs in the United States in the 1970's focused mainly on volatile organic compounds (VOCs), the other primary pollutant involved in ozone formation. The 1990 Phase I NOx/VOC Management Plan endorsed by the Canadian Council of Ministers of the Environment (CCME) in 1990 and the 1990 amendments to the U.S. Clean Air Act both recognized the significant regional contribution of NOx to ground-level ozone. Canada's rationale for its commitments in the 1988 NOx Protocol under the United Nations Economic Commission for Europe (UN ECE) Convention on Long Range Transboundary Air Pollution (LRTAP) were based on the role of NOx in ozone formation.

In the 1990's a new dimension was added with the improved understanding of the contribution of NOx to the secondary formation of fine particulate matter (PM_2.5). It also became evident that some of the progress made in reducing the contribution of SO₂ to acidification was being replaced with acidification due to NOx. With NOx recognized as a significant contributor to both ground-level ozone formation and PM_2.5 formation, and to acidification, commitments to reduce NOx emissions have been included in a number of recent Canadian domestic and international agreements and programs (e.g. Ozone Annex to the Canada-U.S. Air Quality Agreement (2000), Canada-wide Acid Rain Strategy for Post-2000 (1998)).