Ferretti, vice president at the Chicago Climate Exchange, extolled emissions trading, noting that the practice had helped bring sulfur dioxide and acid rain under control much less expensively than traditional command-and-control regulation.

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However, it is also applicable in locations where smelter acid is not available by the purchase of sulfuric acid or the manufacture of sulfuric acid from sulfur or pyrite.

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Back then, the concern was sulfur dioxide, the culprit behind acid rain.

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The limit is now set at about half of the amount of sulfur (the leading cause of acid rain) released in 1980.

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Sulfur dioxide produces fine particulate matter and acid rain.

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Acid-treated lignin emits sulfur dioxide when burned.

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The sulfur dioxide and soot thrown off results not only in a polluted atmosphere but also acid rain that falls on a third of China's cities.

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Media-fueled alarm about acid rain provided a basis for legislation to create markets for buying and selling excess sulfur dioxide (SO2) and nitrogen dioxide emission credits, and Project 88 became the Clean Air Act of 1990.

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It also emits the most carbon dioxide and sulfur dioxide, making it the world's No. 1 contributor to global warming as well as acid rain.

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