What are the main causes of acid rain in environmental chemistry?

When some gaseous oxides and acid such as NO₂, SO₂ and HCl etc are mixed with rain water and come down from the atmosphere on the earth, then this phenomenon is called acid rain. There are two main causes of acid rain in environment. These are,

· Acid rain due to natural causes and

· Man-made causes of acid rain.

What are the main causes of acid rain in environmental chemistry?

There are some natural events such as, ignition of volcanoes, forest fire etc through which a tiny part of gaseous SO₂, NO₂ etc entered into the air.

Besides, NO₂ oxide is formed by the reaction of atmospheric N₂ and O₂ at the time of lightning. NO₂ is also formed due to the dissociation of nitrogenous compounds of soil by soil bacteria and entered into the atmosphere.

Although, the above said natural causes enhance a little portion of such type of non metallic acidic oxide into the atmosphere.

Volcanic eruption

On the other hand, a huge extent of SO₂, NO₂ and HCl gas entered into the atmospheric air due to man-made causes.

The main man-made sources of SO₂ and NO₂ are thermal power station, acid production industry, motor vehicles, oil refineries and many other sectors.

In the United States, about 70% of sulfur dioxide pollution comes from power plants and about 40% of NO₂ enter the atmosphere from motor vehicles emitting with many other sources.

In Canada, about 60% of sulfur dioxide pollution comes from industrial activities including oil refining and metal smelting.

Again, about 55%-60% of NO₂ gas enters into the air from motor vehicles emitting and many other sources.

From the above contaminants sources, the gaseous substances which are main causes of acid rain, enter the air.

These oxides directly combined with rain water and produced different mineral acids such as sulfuric acid, nitric acid,hydrochloric acid etc and fall on the earth surface in the form of acid rain.

Again, these oxides are further oxidized by air O₂, O₃ and other oxidizing substance, resulting in the formation of more strong acid that creates acid rain.

Besides, from hydrogen chloride manufacturing industry, a lot hydrogen chloride gas entered into the air. Now, HCl gas is a volatile and highly water soluble substance.

So, it is readily dissolved in rain water and form hydrochloricacid that falls on the earth surface as acid rain.

What is the contribution of different acids in acid rain?

In the creation of acid rain, the contribution of sulfuric acid is 62-65% and that of for nitric acid is 32-35%.

The other acids those are responsible for acid rainsare HCl, H₂SO₃ and HNO₂ etc. The contribution of HCl, H₂SO₃ and HNO₂ acids are approximately 2%, 1-2% and 0-2% respectively.