Which Gases Cause Acid Rain?

Acid rain is mostly caused by gaseous sulfur dioxide and gaseous nitrogen dioxide. These gases are produced as a byproduct from the combustion of fuels. In the atmosphere, sulfur dioxide is converted into sulfuric acid via an oxidation reaction. Nitrogen dioxide is transformed into nitric acid due to similar reactions in the atmosphere. Both these products (sulfuric acid and nitric acid) are responsible for the lowering of the rainwater pH, resulting in the environmental concern known as acid rain.

Chemical Processes Undergone by the Gases that Cause Acid Rain

A large amount of fuel is subjected to combustion on a daily basis in order to facilitate the day-to-day activities of human beings. Many of these combustion processes result in the liberation of large quantities of nitrogen dioxide and sulfur dioxide into the atmosphere.

Sulfur Dioxide

Gaseous sulfur dioxide undergoes oxidation when it participates in an intermolecular reaction with the hydroxyl free radicals present in the atmosphere. This chemical reaction results in the formation of a hydrogen sulfite (a chemical compound with the formula HSO3) free radical. This HSO3 radical goes on to react with atmospheric oxygen, forming sulfur trioxide (a chemical compound with the formula SO3) as one of the products. Now, when reacted with the water vapour present in the atmosphere, sulfur trioxide forms sulfuric acid, which is a highly corrosive compound. The formation of sulfuric acid at this level of the atmosphere can greatly increase the acidity of the rainwater by forming a dilute solution with the rainwater and, therefore, lowering its pH value.

An alternate chemical pathway for the formation of sulfuric acid from atmospheric sulfur dioxide does exist. In this alternate pathway, the sulfur dioxide gas is initially subjected to hydrolysis by the atmospheric water vapour. Post the hydrolysis reaction, the product formed undergoes a series of equilibrium reactions to finally afford sulfuric acid as the product.

Nitrogen Dioxide

The formation of nitric acid in the atmosphere from the presence of excess nitrogen dioxide is fairly straightforward. The nitrogen dioxide gas participates in a chemical reaction with the hydroxide free radicals in the atmosphere to form nitric acid as the product. However, it can be noted that nitric acid can also be produced in the atmosphere due to certain natural phenomena such as lightning. The electric discharge into the atmosphere as a result of lightning can facilitate the formation of nitric acid from oxides of nitrogen and water.

Thus, the presence of gases like nitrogen dioxide and sulfur dioxide (in excess) in the atmosphere can contribute towards acid rain. Acid rain is known to have a negative impact on soil, aquatic life, and forests. Acid rain is also known to severely damage entire ecosystems like aquatic ecosystems and forest ecosystems. Man-made structures are also known to be affected by this phenomenon. For example, acid rain can contribute towards the corrosion of many steel structures (such as bridges) and also cause visible degradation to marble and stone constructs. Furthermore, acid rain is also known to have a negative impact on human health. Therefore, the emission of sulfur dioxide and nitrogen dioxide into the atmosphere must be controlled via the regulation of the human activities that cause this.

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