Acid Rain

Acid rain describes any form of precipitation with high levels of nitric and sulfuric acids. It can also occur in the form of snow, fog, and tiny bits of dry material that settle to Earth which is known as acid deposition.

Acid rain can occur due a natural reasons such as rotting vegetation and volcanoes which release chemicals, but the main cause is human activity. Burning fossil fuels, coal-burning power plants, factories and cars all release sulfur dioxide (SO2) and nitrogen oxide (NO2) into the atmosphere. These gases react with water, oxygen and other gases to form sulfuric acid, ammonium nitrate and nitric acid. The wind disperses the acids over hundreds of miles across the globe before they fall to Earth in some form of precipitation.

The deposition can occur in either one of two ways; wet or dry deposition.

- Wet deposition is where precipitation removes acid from the atmosphere and is deposited on Earth.

- Dry deposition is when polluting particles and gases stick to the ground via dust and smoke in the absence of of precipitation. Precipitation will then eventually wash pollution into streams, lakes and rivers.

Acid deposition increased dramatically after the industrial revolution and was first discovered in 1852 by a Scottish chemist, Robert Angus Smith. However public awareness to acid deposition did not grow until the 1960's and 70's.

Effect of Acid Rain

Acid rain has many ecological effects - aquatic environments such as lakes,streams and wetlands being the most clearly impacted areas. Acidic precipitation falls directly into them along with water run off and ground water all flowing into them. The water becomes acidic, at first it is a diluted form of acidity but eventually it all builds up. This causes aluminium and magnesium to be released from the soil, decreasing the waters pH further, while also being toxic to aquatic animals such as crayfish, clams and fish. If the pH of a lake drops below 4.8, it's plants animals are at risk of death. It is estimated that 50,000 lakes in the USA and Canada have pH levels below normal, and several hundred of these have a pH level too low to support aquatic life.

Furthermore, the effect on aquatic life can also lead to diminishing numbers of other animals. This is due to the food chain being altered, so predators such as birds have less food due to fish dying in lakes and streams.

Forests can also be extremely damaged by acid deposition. Especially forests at higher elevations where precipitation is more frequent and suffer problems induces by the acidic cloud cover as the moisture in the clouds blanket them. The acid in water droplets can damage leaves and bark on the trees and cause stunted growth. This means they are more vulnerable to disease, cold temperatures, extreme weather and insects, while also having the possible effect of inhibiting the ability to reproduce. In addition, the release of aluminium in the soil makes it hard for trees to take up water. The acid kills micro-organisms in the soil, disrupting nutrients and can cause calcium deficiency.  Some soils can neutralise acid, the lower the 'buffering capacity', the larger the effect of acid rain on the soil and forests.

A forest damaged by acid rain

The most advance cases of forest damage is occurring in Eastern Europe. It is estimated that half of the forests on Germany and Poland are damaged by acid deposition, while it is roughly 30% of forests effected in Switzerland.

Architecture and art can also be damaged. Acid has the ability to corroded materials, especially limestone. The acid reacts with materials in the stone which disintegrate and wash away. Modern buildings as well as pipes above and below ground, cars and railway tracks can all be damaged.

Statues eroded by acid rain

Preventing Acid Rain

Due to these problems and the adverse effects on human health, methods are being used to reduce damage caused by acid rain. One is by cleaning smoke stacks by using scrubbers which trap pollutants before they are released into the atmosphere. Other ways are by having catalytic converters in cars to reduce emissions, promoting alternative energy sources and conservation of energy. Funding has also been given to restore ecosystems damaged by acid rain all over the world.