What is Acid Rain?

Acid precipitation threatens our land, food sources and livelihoods. Read about the causes of acid rain and how to prevent more acid rain.

| January/February 1982

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    A simple, inexpensive tool can be used to measure the pH of water.
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    Large factory smokestacks contribute to the increasing acidity of rain.
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    This graph demonstrates different regions' abilities to deal with acid rain based on the existing soil.

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Next to air, without which we could survive for only a few minutes, water is the compound most necessary to human life. Not only do we drink it — to help our bodies perform a number of vital functions — but we cleanse ourselves with it, grow food with it and harvest the bounty of its lakes and streams. In fact, the single most important reason for a human's picking a spot to live has usually been the presence of an adequate supply of clean water.
Of course, Western civilization's attitude toward the precious liquid has changed somewhat in the last 100 years. Water is now delivered to most people in developed countries through mazes of pipes and faucets, rather than directly from wells or streams. It's entirely possible that the attitudes engendered by take-it-for-granted tap water have helped cause the pollution and shortage problems we face today. In little more than a decade, the threat to our drinking supplies has become a top-priority concern.

In fact, 10 years ago few people even recognized the danger in what was then a puzzling new phenomenon ... acid precipitation. Acid rain, as it's often called, is a direct result of the burning of fossil fuels, and it's the first water-quality problem that's managing to cross state and national boundaries on a daily basis. 

Where Does Acid Rain 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 SO 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 build-up 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 percent — roughly 2,500 tons — of the world's daily total). Nitrogen emissions, on the other hand, come largely from transportation sources (about 40 percent), 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.

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 1970s 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.


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