Pure water has a pH of 7. Rainwater is usually slightly acidic and has a pH of less than 6. Rain is considered acid rain if it has a pH of below 5.5. In some heavily industrialized areas with large amounts of pollution, the pH of rain can fall as low as 4. Acid rain is caused when sulfur dioxide and nitrogen oxides are released into the air, bonding with oxygen to form sulphuric acid (H2
), ammonium nitrate (NH4
) and nitric acid (HNO3
). The primary sources of sulphur dioxide are industrial processes and the burning of fossil fuels. The primary source of nitrogen oxides is the internal combustion engine.
Once these acids are in the atmosphere, they react with and are removed from the air through precipitation like snow, rain, or hail. Due to the introduction of the acid, the pH of the rain water drops and becomes far more acidic, sometimes below a pH of 4. The acids are carried to the ground where they produced undesirable effects on the environment. Acid rain is not something from a science fiction movie. It is not as if acid is falling from the sky. For instance, lemon juice is very acidic, with a pH of about 2. The pH scale is similar to the richter scale. An increase in acidity from a 5 pH to a 4 pH indicates a tenfold increase in acidity.
Acid rain causes the greatest problems in areas where the soil is naturally acidic. In most areas where the soil is naturally alkaline, it can easily neutralize the acid rain. However, in some areas such as the East coast, where the soil is more naturally acidic, it is not neutralized.
The U.S. Geological Survey (USGS) established0 220 acid rain monitoring stations throughout the US, and supports about 72 of these stations. It is hoped that by collecting data over the long term, scientists will be better informed about the causes and prevention of acid rain.