Acid rain results when sulfur dioxide (SO2) and nitrogen oxides (NOX) are emitted into the atmosphere and transported by wind and air currents. The SO2 and NOX react with water, oxygen and other chemicals to form sulfuric and nitric acids. These then mix with water and other materials before falling to the ground.
This can include rain, snow, fog, hail or even dust that is acidic.
Causes of Acid Rain:
While a small portion of the SO2 and NOX that cause acid rain is from natural sources such as volcanoes, most of it comes from the burning of fossil fuels. The major sources of SO2 and NOX in the atmosphere are:
- Burning of fossil fuels to generate electricity. Two thirds of SO2 and one fourth of NOX in the atmosphere come from electric power generators.
- Vehicles and heavy equipment.
- Manufacturing, oil refineries and other industries.
Forms of Acid Deposition:
Wet Deposition: Wet deposition is what we most commonly think of as acid rain. The sulfuric and nitric acids formed in the atmosphere fall to the ground mixed with rain, snow, fog, or hail.
Dry Deposition: Acidic particles and gasses can also deposit from the atmosphere in the absence of moisture as dry deposition.
This image illustrates the pathway for acid rain in our environment: (1) Emissions of SO2 and NOx are released into the air, where (2) the pollutants are transformed into acid particles that may be transported long distances. (3) These acid particles then fall to the earth as wet and dry deposition (dust, rain, snow, etc.) and (4) may cause harmful effects on soil, forests, streams, and lakes.
Measuring Acid Rain
`1. Acidity and alkalinity are measured using a pH scale for which 7.0 is neutral. The lower a substance’s pH (less than 7), the more acidic it is; the higher a substance’s pH (greater than 7), the more alkaline it is.
2. Normal rain has a pH of about 5.6; it is slightly acidic because carbon dioxide (CO2) dissolves into it forming weak carbonic acid. Acid rain usually has a pH between 4.2 and 4.4.
The Effects of Acid Rain on Ecosystems
1. Acidic rainwater can leach aluminum from soil clay particles and then flow into streams and lakes. The more acid that is introduced to the ecosystem, the more aluminum is released.
2. Plants and animals that are acid-sensitive will be lost as the pH declines. Generally, the young of most species are more sensitive to environmental conditions than adults.
3. Nitrogen Pollution: Acid rain also contains nitrogen, and this can have an impact on some ecosystems. For example, nitrogen pollution in our coastal waters is partially responsible for declining fish and shellfish populations in some areas.
4. Effects of Acid Rain on Materials: The acidic particles corrode metal and cause paint and stone to deteriorate more quickly. They also dirty the surfaces of buildings and other structures such as monuments.
5. Visibility: In the atmosphere, SO2 and NOX gases can be transformed into sulfate and nitrate particles, while some NOX can also react with other pollutants to form ozone. These particles and ozone make the air hazy and difficult to see through.
6. Human Health: SO2 and NOX react in the atmosphere to form fine sulfate and nitrate particles that people can inhale into their lungs. In addition, NOX emissions also contribute to ground level ozone, which is also harmful to human health.
