Acid Precipitation

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WHERE DOES IT COME FROM?

Acidified precipitates (which can include rain, snow, other forms of atmospheric moisture, and dry acidic particles) are produced when sulfur oxides (SO 2 . and S0 3 ) or nitrogen oxides (NOx)—or, to a minor extent, hydrogen chloride (HCI)—react with air and water in the presence of sunlight. Though the actual mechanisms that produce acid precipitation are not thoroughly understood, there's little question that they result in sulfuric, nitric, and/or hydrochloric acid buildup that can render rainwater as sour as vinegar!

The primary sources of sulfur emissions (which are estimated to cause about two-thirds of all acid precipitation today) are coal-fired power plants (Ohio's electric utilities are the largest producers in the U.S.) and smelters (International Nickel in Sudbury, Ontario pumps out 1%-roughly 2,500 tons—of the world's daily total). Nitrogen emissions, on the other hand, come largely from transportation sources (about 40%), with effluents from power plants and industry making up the rest.

Though such pollutants usually remain in the atmosphere for no more than five days (NO hangs on longer than does SO 2 .), they've been known to show up as far as 700 miles downwind of their sources, in the form of sulfates and nitrates. These two substances are the major producers of acid rain. And ironically enough, if it weren't for the Clean Air Act of 1970, they probably wouldn't have gotten a chance to travel so far and do so much harm.

In order to comply with the act's standards, many utilities and industries—in the early 1970's—built tall smokestacks to disperse emissions (which, for the purpose of law enforcement, are measured at ground, or nose, level). Though no one anticipated the problem at the time, the superstacks (International Nickel's is 1,250 feet high) help sulfur and nitrogen stay airborne long enough to cause full-fledged acid precipitation.

Of course, coal-fired plants that have been built since the passage of the Clean Air Act are equipped with sulfur scrubbers and are relatively clean. The older (uncontrolled) plants now spew out most of the sulfur oxide emissions that enter the atmosphere. Consequently, it's of vital concern that the oil-fired power plants currently being converted to coal not be considered "old"—and thus unconrollable—for regulatory purposes.

Nitrogen oxide, the other major cause of acid precipitation, is much more difficult to deal with than is sulfur oxide. Because there is no suitable control technology yet, NO x production is likely to remain essentially unchecked for as much as a decade. Rollbacks and/or delays in clean air standards (both of which are advocated by the Reagan administration) could further postpone effective control of nitrogen oxide emissions.

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