- Acid rain is rain or any other form of precipitation that is unusually acidic, i.e. elevated levels of hydrogen ions (low pH). It has harmful effects on plants, aquatic animals, and infrastructure. Acid rain is mostly caused by emissions of compounds of sulfur, nitrogen, and carbon which react with the water molecules in the atmosphere to produce acids. However, it can also be caused naturally by the splitting of nitrogen compounds by the energy produced by lightning strikes, or the release of sulfur dioxide into the atmosphere by phenomena of volcanic eruptions.
- A major environmental impact of acid deposition is the lowering of pH in lakes and rivers. Most aquatic life is disrupted as the pH drops. Phytoplankton populations are reduced, and much common water – dwelling invertebrates, such as May flies and stone flies, cannot survive when the pH falls below about 6.0. Some sensitive species of fish, including trout and salmon, are harmed when pH levels fall below 5.5. Acidity has a deleterious effect on the reproductive cycle of fish; when the pH is less than 4.9, reproduction of most fish species is unlikely. Acid dead lakes have pH below about 3.5. Acid rain also causes pitting and corrosion of metals and the deterioration of painted surfaces, concrete, limestone, and marble in buildings, monuments, works of art, and other exposed objects.
- Once a body of water contains too much acid, the creatures in the water's food chain begin to die. Eggs and larvae are sensitive to low pH and unable to survive. As water becomes more acid, the fertility of eggs is reduced, fewer hatch, and fish may not grow to adult sizes. Eventually, fish or insects, the fish's food, may no longer be able to live in water with a low pH.
- The amount of acid in liquids is measured on a scale from 0 to 14. This is called the "pH" scale. A pH of 7.0 (distilled water) is in the middle of the scale and is considered neutral - neither acidic nor alkaline. Things below 7.0 such as lemon juice (pH of 2.0) are acidic. Things above 7.0, like ammonia (pH 11.0) are alkaline. The pH scale is logarithmic. This means that a pH of 6 is ten times more acidic than a pH of 7. A pH of 5 is 100 times more acidic than a pH of 7 and a pH of 4 is 1,000 times more acidic than a pH of 7.
- Because carbon dioxide and water found naturally in the atmosphere have a pH of 5.0 to 5.6. Natural rain is slightly acidic. A natural buffering ability present in most soils that contain limestone can neutralize acidity. However, several regions of the country are damaged by acid rain because they have thin soils and granite bedrock. Granite is low in limestone and cannot neutralize (buffer) acid precipitation. Acid rain is a worldwide problem because it can be carried in the atmosphere for great distances before falling back to earth.
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