Nina Shen Rastogi in Slate:
Back in the 1980s, when the Lantern herself was just a little penlight, acid rain was the environmental scourge of the day. Canada's environmental minister proclaimed it an “insidious malaria of the biosphere“; it menaced the Transformers; it turned Kimberly's hair bright green in an episode of Diff'rent Strokes. Toxic precipitation fell off the radar in 1990, when Congress passed an amendment to the Clean Air Act calling for major reductions in the types of emissions that lead to acid rain. Emissions have dropped significantly since then, but the problem is far from gone.
Acid rain occurs when sulfur dioxide and nitrogen oxides—gases released by the burning of fossil fuel—form acidic compounds in the atmosphere. These fall back to earth in rain, snow, or sleet or as dry particles or gases. (At high altitudes and along coastlines, bits of acid suspended in clouds or fog pose an additional threat.)
Back in acid rain's heyday as a public menace, scientists focused on how it wrecked lakes and streams, making the water toxic to fish and other organisms and threatening sensitive tree populations like the red spruce in the Northeast mountains. In later years, they began to understand how acidification can also cause imbalances in soil chemistry, exacerbating problems for watersheds and plant life.