Acid Deposition

Perhaps you've heard about acid rain killing fish and poisoning trees. Acid rain is part of the phenomenon of acid deposition. It's made up of raindrops that have been acidified by air pollutants. Fossil fuel burning releases sulfur dioxide (SO₂) and nitric oxide (NO₂) into the air. The SO₂ and NO2 readily combine with oxygen and water in the presence of sunlight and dust particles to form sulfuric and nitric acid aerosols, which then act as condensation nuclei. The tiny water droplets created around these nuclei are acidic, and when the droplets coalesce in precipitation, the resulting raindrops or ice crystals are also acidic. Sulfuric and nitric acids can also be formed on dust particles, creating dry acid particles. These can be as damaging to plants, soils, and aquatic life as acid rain.

What are the effects of acid deposition? In Europe and in North America, acid deposition has had a severe impact on some ecosystems. In Norway, acidification of stream water has virtually eliminated many salmon runs by inhibiting salmon egg development. In 1990, American scientists estimated that 14 percent of Adirondack lakes were heavily acidic, along with 12 to 14 percent of the streams in the Mid-Atlantic states. Forests, too, have been damaged by acid deposition. In western Germany, the impact has been especially severe in the Harz Mountains and the Black Forest.

Since 1990, the United States has significantly reduced the release of sulfur oxides, nitrogen oxides, and volatile organic compounds, largely by improving industrial emission controls. But acid deposition is still a very important problem in many parts of the world—especially Eastern Europe and the states of the former Soviet Union. There, air pollution controls have been virtually nonexistent for decades. Reducing pollution levels and cleaning up polluted areas will be a major task for these nations over the next decades.