SO2 is the first of the criteria air contaminants (CACs) to receive significant attention from an environmental perspective. Scientific studies in the 1960's and 1970's began to link SO2 to the acidification of aquatic systems in Scandinavia and North America and to demonstrate that long-range transport of SO2 was taking place. Acid rain, caused primarily by SO2, was the prominent air issue in many countries including Canada in the 1970s and 1980s. The first major domestic and international agreements on air issues focused on SO2 and acid rain. They began with the 1979 Canada-U.S. Memorandum of Intent on Transboundary Air Pollution and the 1979 UNECE Convention on Long-Range Transboundary Air Pollution (LRTAP). Other agreements and programs addressing acidification and SO2 continued to emerge from then until present, one of the most recent for Canada being the Canada-wide Acid Rain Strategy for Post-2000 endorsed by the Canadian Council of Ministers of the Environment (CCME) in 1998.
In the 1990's, the role of SO2 in smog formation and visibility deterioration began to receive increasing attention. Evidence of significant health effects of fine particulate matter (PM) began to mount, along with improved understanding of the significant role of SO2 in secondary formation of fine PM, a primary component of smog. Smog and visibility protection are now parallel drivers for SO2 management agreements and programs along with the continuing concern with acidification.
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