What is acid rain?
Acid rain is classified as any form of precipitation (rain, snow, fog, hail) that has a pH of 4 or lower. It is primarily a result of air pollution. Particular gases released into the atmosphere react with the water particles in clouds and combine to make a very weak form of acid. Acid rain can have harmful effects on plant life, water supplies and aquatic life and some forms of infrastructure. Acid rain is not acidic enough to burn your skin upon contact. The acid that is formed in the atmosphere can also fall as acidic dust and pollute the soil many thousands of kilometers from the source of the pollution.
How is acid rain formed?
There are three main causes of acid rain, but both are due to the same gases being released into the atmosphere. Acid rain is caused by the two main air pollutants, sulfur dioxide and nitrogen oxide. When these gases come into contact with the hydrogen and oxygen molecules in water they combine to form a weak acid. The major cause of acid rain is air pollution by burning fossil fuels, smelting metals and motor vehicles. Acid rain can also be caused naturally by the eruption of a volcano. When a volcano erupts it produces a large amount of sulfur dioxide and it is not uncommon to have acid rain. Acid rain can also be caused by lightning strikes as these naturally produce nitrogen oxide.
The oxides of nitrogen or NOx and sulfur dioxide or SO2 are the two main sources of acid rain.
Sulfur dioxide, which is a colorless gas, is released as a by-product when fossil fuels that contain sulfur are burned.
Industrial processes like the processing of crude oil, utility factories, and iron and steel industries.
Natural means and disaster can also result in sulfur dioxide being released into the atmosphere, such as rotting vegetation, plankton, sea spray, and volcanoes, all of which emit about 10% sulfur dioxide.
On the whole, industrial combustion is responsible for 69.4% sulfur dioxide emissions into the atmosphere, and vehicular transportation is responsible for about 3.7%.
When sulfur dioxide reacts with the atmospheric moisture, it undergoes oxidation to form sulfate ions.
SO2 (g) + O2 (g) = SO3 (g)
The sulfate ions then combine with hydrogen atoms from the atmosphere to form sulfuric acid in the aqueous state.
SO3 (g) + H2O (l) = H2SO4 (aq)
Sulfur dioxide affects the breathing capacity of lungs and cause permanent damage to them. Shortness of breath, asthma, recurring cough are some of the major problems related to constant exposure to this gas.
Oxides of Nitrogen
Nitrogen oxide is another major component of acid rain. Nitrogen compounds that contain oxygen atoms, are known as oxides of nitrogen. For example, nitrogen dioxide and nitrogen monoxide are oxides of nitrogen, and they are generically termed as NOx.
These gases are produced in combustion processes, which involve extremely high temperatures. For example, utility plants, automobiles and chemical industries such as in the production of fertilizers.
Five percent of nitrogen oxide is emitted by natural processes like lightning, volcanic eruptions, forest fires, and action of bacteria in the soil. Industrial processes emit 32% and vehicular transportation is responsible for 43%.
While reacting with atmospheric moisture, nitrogen oxide also undergoes oxidation reaction to give rise to nitric or nitrous acid.
NO2 (g) + H2O (l) = HNO3 (aq) + HNO2 (aq)
Nitrogen oxide, which is a dangerous gas in itself, causes damage to the respiratory organs by attacking the membranes in them; thus increasing the chances of respiratory diseases. It also causes smog and plays a critical role in damaging the ozone layer. The nitrogen oxide can be carried far away from the original location of its emission.
How does acid rain effect the environment?
In the 1970’s the effects of acid rain were at their worst. Many forests the world over were dying and there were many cases where marine life in lakes and rivers died out or became mutated. Governments the world over worked to lower the amount of sulfur dioxide and nitrogen oxide that they were adding to the atmosphere. Acid rain is often felt in countries many thousands of kilometers from major air pollution. Mountainous regions are more likely to suffer from greater acid rain fall due to the higher rainfall received in these areas.