Formation of Acid Rain

In the wake of development of major environmental problems like global warming, the issue of acid rains remains a bit neglected. Acid rains have destroyed forests, damaged buildings and also caused health problems. The only solution to resolve this problem is reducing industrial and automobile emissions.

Instances of acid rains have increased after the onset of Industrial Revolution. John Evelyn, an Englishman however, noted that acid rains had occurred in as early as 17th century. Studying more about formation of acid rains should educate us about the underlying chemical processes involved in this phenomenon.

The formation of acid rain takes place as a result of many interrelated processes and chemical reactions. However, a short explanation of this phenomenon would be as follows: reaction of nitrogen and sulfur oxides with moisture in the atmosphere forms acid rains.

Precipitation of these gases takes place in the form of sulfuric and nitric acids. The acid rains occur when oxides of sulfur and nitrogen come in contact with water. Industries and volcanoes are prime sources of sulfur and nitrogen oxides. Other factors that contribute considerably to the problem are automobile emissions, lightning and forest fires.

A series of complex reactions take place in the process of acid rain formation. Before getting into the details of chemical reactions, it is important to understand the causes and effects of acid rains.

Oxidation of SO2
In this process, sulfur dioxide reacts with OH (hydroxyl radical) to form HOSO2. Immediately after this intermediate process, HOSO2 reacts with oxygen; this reaction leads to the formation of water and sulfur trioxide. These reactions can be depicted in a better manner with formulas presented below.

SO₂ + OH → HOSO₂

HOSO₂ + O₂ → SO₃ + H₂O

Sulfur Trioxide Converts to Sulfuric Acid
The reaction of sulfur trioxide with water forms sulfuric acid.

SO₂ + H₂O → H₂SO₃

Nitric Acid Formation
Formation of nitric acid takes place from the reaction between nitrogen dioxide and hydroxyl radical. This reaction can be depicted as below.

NO₂ + OH → HNO₃

All the reactions illustrated above take place in the gaseous form. Reactions which take place in humid atmosphere are different from that in a dry atmosphere. Continue reading to known more about the chemical processes which take place in liquid state (i.e., inside the clouds).

Hydrogen dioxide reacts with water molecules and enters into a series of reactions presented below.

SO₂ (g) + H₂O is in equilibrium with SO₂·H₂O

SO₂·H₂O is in equilibrium with H⁺ + HSO₃⁻

HSO₃⁻ is in equilibrium with H⁺ + SO₃²⁻

The acid rains occur in two different forms i.e., wet and dry. The wet acid rains occur as a result of precipitation of acids contained in clouds. Snowfall and rainfall are the two forms of wet acid rains. In dry acids, the gases containing sulfur and nitrogen compounds deposit on the surface of plants, buildings and ground surface. When dry acids get washed off the ground surface by rainwater, dry deposition takes place.

Acids formed the by emission of volcanic ash and other materials have a pH of around 4. The acid rain which result from these emissions have a corrosive effect. Sources of drinking water too, are polluted by the occurrence of acid rains.

The pH of acids precipitated by emissions of the infamous Poas Volcano is 2. Acid rains caused by this volcano are responsible for health problems and destruction of forests; irritation of eyes and lungs also is caused by these rains.

Understanding the process of formation of acid rains is important since it is one of the major environmental problems. Acid rains are considered to be far more dangerous than other environment issues because of the damage they cause. We should all try our bit to reduce air pollution through judicious use of petrol and other fuels.