Acid Precipitation

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JUST HOW ACID DOES IT RAIN?

The acidity or alkalinity of a solution—as many of you already know—is measured numerically (the levels range from 0 to 14) on what's called a pH scale. A pH of 7 is neutral, while smaller numbers are acid and larger ones are basic (alkaline). The rating is actually a measure of the concentration of hydrogen ions ... and it builds logarithmically (that is, pH6 is 10 times more acidic than pH7, pH5 100 times more than pH7, and pH4 1,000 times more than pH7).

The pH of pure rainfall happens to be slightly lower than neutral (about pH5.6), because the moisture reacts with naturally occurring carbon dioxide in the atmosphere to produce dilute carbonic acid. Hence, precipitation that has a pH of less than 5.6 is generally considered to have been made acid by some human activity. Historic records, obtained from core samples of polar ice, indicate that world precipitation has been slowly growing more acid over the last 100 years. (Episodes of acid rain have occurred naturally for eons—caused by sulfur released from volcanic eruptions, for example—but these sources contribute a very small part of this century's growing total.)

Though Swedish scientists were the first to pinpoint the harmful effects of acid precipitation, U.S. researchers have been keeping track of rainfall pH since the 1950's. As far back as the middle of that decade, precipitation with a pH of 4.5 was occasionally recorded in a few areas of the Northeast. But by 1979 the average pH of rainfall in the entire eastern U.S. had fallen to below 4.5 . . . and many areas now regularly report pH readings as low as 3.4 (close to the acidity of vinegar). Nor is acid precipitation limited to the eastern U.S. In the Los Angeles basin, for example, typical pH levels have fallen from around 7 in the 1950's to between 4.5 and 5 at present.

WHAT DOES IT DO?

The overall effect that acid precipitation has on a particular ecosystem depends largely upon the soil and water's ability to tolerate the incoming low pH liquid. Areas rich in calcium or magnesium carbonates (limestone and dolomite, respectively) and/or organic matter are, as yet, able to buffer the acidity of the rain, in much the same fashion that an antacid tablet relieves a sour stomach. Ironically, the Midwest (where a large portion of the sulfur oxide emissions are produced) has good buffering capability overall. But down wind—in New York, Pennsylvania, and West Virginia—the soils are formed mostly from igneous rock that's low in buffering compounds.

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