Acid Rain and Our Ecosystem | EcoCenter: Air | Smithsonian
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The region's legacy of acid rain is clearly visible in the black crust on the gravestones at the Madison Street Cemetery in Hamilton, New York. (Cassandra Willyard)

Acid Rain and Our Ecosystem

More than 150 years after acid rain was first identified, scientists now see success in recovery from its damaging effects

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Geologist Rich April climbs the small hill behind Colgate University and makes his way into the cemetery. He stops before a white marble pillar erected in 1852. The inscription is nearly illegible. Over time, any stone exposed to the elements will weather, April explains, but this marble has weathered unnaturally fast. The culprit? Acid rain.

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April pulls a vial of acid from his pocket to demonstrate. He unscrews the cap and lets a few drops leak onto the stone, where they fizz and bubble. The rain that fell throughout the Northeast in the latter half of the 20th century wasn’t as acidic as the liquid in April’s vial, but the principle is the same. Acid eats marble. Given enough time, it can erase even words meant to last an eternity.

The effects of acid rain extend far beyond graveyards. Acid rain destroyed fish populations in lakes and streams, harmed fragile soils and damaged millions of acres of forest worldwide.

These far-reaching effects illustrate the profound impact air pollution can have on the land. But the story of acid rain is also a tale of how understanding air pollution can lead to solutions. Due to overwhelming scientific evidence linking power plant emissions to acid rain and acid rain to the death of lakes, new regulations have dramatically cut emissions and cleaned up the rain that falls on the United States.

The term ‘acid rain’ was coined in the mid-1800s, when Robert Angus Smith, a Scottish chemist working in London, noticed that rain tended to be more acidic in areas with more air pollution and that buildings crumble faster in areas where coal is burned. But it took another century for scientists to realize that acid rain was a widespread environmental problem. Scandinavian scientists began to document acidic damage to lakes and streams in the 1950s. In 1963, Gene Likens, then at Dartmouth, and colleagues began collecting and testing the pH of rainwater in New Hampshire’s White Mountains as part of an ecosystem study. They were surprised to find that it was quite acidic, but they didn’t have much basis for comparison; at that time, scientists weren’t regularly measuring the pH of rainwater.

Likens took a job at Cornell a few years later and set up instruments to collect rainwater in the Finger Lakes region and soon observed that the rain in New York was roughly as acidic as rain in New Hampshire. “That was the first clue that we had that this might be some kind of a regional phenomenon,” he says. But neither Likens nor his colleagues had a clear idea what the cause might be.

Likens won a fellowship that took him to Sweden in 1969, a serendipitous event, he says, because he met Svante Odén, a scientist at Uppsala University who had observed the same trends in Sweden that Likens had been observing in the Northeastern United States. Odén had his finger on a potential cause. “He was trying to build a case that [acid rain] might be due to emissions coming from the more industrialized areas of Europe,” Likens recalls.

Likens and his colleagues traced the emissions from coal-fired power plants and examined satellite and aircraft data, and they found a similar long-distance link. “Sure enough, the emissions were coming primarily from Midwestern states like Indiana, Ohio, Illinois and Kentucky,” Likens recalls. “They were making their way literally thousands of kilometers to New England and southeastern Canada and coming back down as acids.”

He reported his findings in Science in 1974, and the story was immediately picked up by newspapers. The phone didn’t stop ringing for months, Likens recalls. “It was that media exposure that really put acid rain on the map in North America.”

Acid rain occurs, Likens and Odén and other scientists realized, when sulfur dioxide and nitrogen oxide enter the atmosphere and react with water to form sulfuric and nitric acids. Natural sources of these gases exist—volcanoes, for instance, belch out sulfur dioxide—but the vast majority comes from the burning of fossil fuels, especially by coal-fired power plants. The tall smokestacks allow pollution to travel long distances. According to studies conducted by Likens and his colleagues, normal rainwater has a pH of 5.2. During the 1970s and 1980s, when acid rain was at its worst, scientists recorded pH levels as low as 2.1, roughly 1,000 times more acidic.

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