Acid Rain

The term acid rain is used to describe all precipitations- rain, snow, fog, dew- which are more acidic than normal water. The normal rain is slightly acidic having a pH of about 5.6 as carbon dioxide gas reacts with it to form a weak carbonic acid. For example:

CO2 + H2O → H2CO3

Rainfall is declared as acid rain when its pH is less than 5.6 because natural and unpolluted rainwater actually has a pH between 5.6-6.5 (acidic) due to the reaction of water with the presence of CO2. The acidity of the rainwater comes from the natural presence of three substances (CO₂, NOx, and SO₂) found in the troposphere (the lowest layer of the atmosphere) which are emitted from the combustion of coal in power plants and gasoline in vehicles. The pH of acid rain can range between 5.6 to 3.5 and in some cases, pH can go even lower than 2.

History 

Robert Angus Smith was the first to point out the relationship between atmospheric pollution and acid rain in Manchester, England, in 1852. And in 1872, it was he who coined the term “acid rain”. 

Scientists in Europe gradually began to study the phenomenon of acid rain from the 1960s. And after that, in the following decade, the systematic study of acid rain started to take place in the United States and then in Canada as well. 

Chemicals in Acid Rain

Chemicals in acid rain are acids like nitric acid and even sulphuric acid present in the polluted air. These chemicals in acid rain are formed when oxides of nitrogen and sulphur come in contact with rainwater.

Acid Rain Causes

The main acid rain causes are the formation of mineral acids like carbonic acid, nitric acid and sulphuric acid.

The Atmospheric Reaction of Acid Rain

• Carbon dioxide gas molecules react with water molecules to form carbonic acid (H2CO3). The ability of carbonic acid (H2CO3). The HC2O3 (carbonic acid) molecule has the ability to deliver H⁺, this capacity of carbonic acid molecule classifies this molecule as an acid. Therefore, it is responsible for lowering the pH of a solution. 

CO2 + H2O → H2CO3

H2CO3 → H⁺ + HCO3-

• Nitric Oxide (NO) molecule contributes to the natural lowering in pH (acidity) of the rainwater, it is formed during lightning storms by the reaction of nitrogen and oxygen, two common atmospheric gases. In the air, nitric oxide (NO) is oxidised to nitrogen dioxide (NO2), this, in turn, combines with water to give nitric acid (HNO3). This acid dissociates in water to yield hydrogen ions (H+) and nitrate ions (NO-2) in a reaction analogous to the dissociation of carbonic acid shown in the equation below, again lowering the pH of the solution.

NO + ½ O2 (g) → NO2 (g)

N2(g) + O2 (g) → 2NO (reaction takes place during the lightning in the atmosphere)

3NO2 (g) + H2O → 2HNO3 (aq) + NO (g)

About one- fourth of the lowering in pH of the rain is accounted for by nitric acid (HNO₃).

• In most of the water bodies, the lowering in pH is accounted for by the presence of sulphuric acid (H2SO4) in rainwater. Although there is a natural source of sulphuric acid production. Sulphuric acid is produced naturally in trace amounts from biological decay and volcanic activity, it is produced almost entirely by human activity, especially the combustion of sulphur-containing fossil fuels in power plants.

When these fossil fuels are burned, the sulphur contained in them reacts with water to form sulphuric acid.

SO2(g) + O2 → SO3 (g) + H2O → H2SO4

Sulphuric acid is a very strong acid. Therefore, it is considered a strong electrolyte and readily dissociates in water, to give an H⁺ ion and HSO-4 ion. The HSO-4 ion may further dissociate to give hydronium ion (H+) and sulphate ion (SO-24) thus, the presence of H2SO4 causes the concentration of H+ ions to increase dramatically, and so the pH of the rainwater drops to a harmful level. 

H2SO4 → HSO-4 + H+

HSO-4 → SO-24 + H+

Gases Responsible for Acid Rain

The major gases responsible for acid rain are nitric oxide, nitrogen dioxide, sulphur dioxide, and sulphur trioxide. The gases responsible for acid rain when precipitates down causes the easing of nutrients from plants, and thus it damages the leaves of plants and trees.

Human Activity Responsible for Acid Rain

It is known that sulphur and nitrogen compounds are the main causes of acid rain. But, these compounds can also be produced as a result of human activity such as the generation of electricity, factories, animal agriculture, and motor vehicles. And the ever-increasing population and industrial growth not only leads to an increase in the issue of acid rain that we already tend to face, but it has also become widespread on a much larger scale. 

Countries like Russia and China continue to face the serious problem of industrial acid rain which majorly arises due to the burning of coal that contains sulphur for the generation of heat as well as electricity. 

Environmental Effects of Acid Rain

Acid rain enhances the number of inorganic reactions and biochemical reactions with deleterious environmental effects, leading to a growing environmental problem worldwide.

• There are many large lakes that have become so acidic (low ph lake), that fish cannot live in them anymore.

• Metal ions are produced by the degradation of the many naturally occurring soil minerals. These metal ions are then washed away in the runoff and causes several effects:

1. Due to the acidic condition, the mobility of the toxic ions, such as Al+3 increases into the water supply.

2. The loss of important minerals such as Ca⁺², from the soil in neutralising the sulphuric acid which causes a deficiency of Ca⁺², creates killing trees and damaging crops.

3. Acid rain causes damage to the forest.

Acid Rain causes Damage to the Forest

• The soil biology and overall chemistry can get severely damaged due to acid rain along with noticeable impacts of soil acidification and acidic water on various plant species. 

• Although on a global scale, acidic rain has a relatively less harmful impact on oceans, it tends to have a serious impact in the shallower waters of coastal areas. Oceanic acidification (a sharp fall in the ocean’s pH) makes it extremely difficult for coastal species to survive. 

• Acid rain does not have any direct damaging effects on human health. But the particulates that are responsible for acid rain are considered to be harmful. If this fine particulate matter increases in the air, then it could lead to health problems like heart and/or lung problems including bronchitis and asthma. 

• Besides that, acid rain also holds the potential to damage various statues, historic monuments, and buildings, especially those which are made of rocks like marble and limestone. This is because these rocks contain quite a large amount of calcium carbonate and the acid present in the rain tends to react with these calcium compounds present in such stones to create gypsum, which gradually flakes off. 

How Do We Stop/Prevent Acid Rain? 

There are certain steps that can be taken to stop acid rain that is caused by the contribution of human factors and these are as follows: 

• Regulate the emissions produced from vehicles. 

• Restrict or limit the usage of fossil fuels for the generation/production of energy and power. Instead, shift to more sustainable sources of energy such as wind power, solar power, etc., as these tend to cause much less pollution as compared to non-renewable sources of energy. 

• Decrease the overall consumption of energy by learning to conserve more and more of it. 

Did You Know?

Acid rain causes the corrosion of metals. For example, the formation of rust on iron gates and iron frames increases due to acid rain.

Acid rain causes a very huge deterioration of the buildings and sculptural materials. The Taj Mahal in India faces this problem. Acid rain is known to occur most commonly across Eastern Europe and the North Eastern United States.