Acid Rain : Causes and Harmful effects

What is an acid rain 

Acid rain is mainly a mixture of sulphuric and nitric acids depending upon the relative quantities of oxides of sulfur and nitrogen emissions.

The formation of nitric acid and sulphuric acid which are secondary pollutants in the atmosphere lead to such kind of pollution known as acid rain.

Also naturally present carbon dioxide in the atmosphere dissolves in rain. The formation of carbonic acid takes place,

 CO2 + H2O → H2CO3 

Due to the above reaction, carbon dioxide dissolves in water until the solution becomes saturated. This results in the rainwater attaining an acidic pH of 5.6. The purest form of rainwater reaches the earth as an acidic solution of pH 5.6. So we can say, acid rain is a rain whose pH is lower than 5.6

Causes of acid rain

Due to the presence of oxides of nitrogen

Atmospheric nitrogen and oxygen combine during thunderstorms and lightning to form nitric oxide,

N2 + O2 → 2NO

which then get converted into nitrogen dioxide

 2NO + O2→ 2NO2 

Further nitrogen dioxide reacts with water to form nitric acid. It reaches the earth in the form of acid rain

 3NO2 + H2O→ 2HNO3 NO 

Due to the presence of sulfur dioxide

Both oil and coal contain Sulphur as an impurity and on burning release sulfur dioxide in the atmosphere. Hydrogen sulfide discharged into the air by natural processes is converted into Sulphur Dioxide by oxidation in the atmosphere

In the atmosphere, Sulphur Dioxide reacts with atmospheric oxygen to form sulfur trioxide by photolytic and catalytic oxidation processes.

 2SO2 + O2→ 2SO3 

Then the sulfur trioxide reacts with water forming Sulphuric Acid which comes down on the earth as acid rain

  SO3 + H2O → H2SO4 

Harmful effects of acid rain

Aquatic life

Acid rain causes the river and even ocean water acidic and adversely affects marine life. Acidic lakes have been found in Belgium, Denmark, West Germany, and The Netherland. In Southern Norway, during the 1970s about 20 % of lakes have lost their fishes.

At low pH, many species of amphibians including frogs, toads, and salamander are particularly sensitive. For example, below pH 4.0, stocks of all fish species decline rapidly because embryos fail to mature at this level of acidity. But some species can grow in the adverse condition of acid rain.

Sphagnum moss and filamentous algae are some examples of such species. They grow very fast and become very large in acid. They can form impenetrable mats that seal off oxygen. Hence the change in pH of freshwater affects the reproduction and survival of many species of fish.

Fertility of soil

The activity of symbiotic nitrogen-fixing bacteria present in the nodules of the leguminous family is inhibited. this is responsible for destroying and reducing the fertility of the soil.

Forests

The acid rain makes the soil acidic. This adversely affects the plant and animal. The acid contains H+ ion sulfate ion and nitrate ions which when added to soil leaches the nutrients from the soil.

Trees that are located in mountainous regions at higher elevations, such as spruce or fir trees, are at greater risk because they are exposed to acidic clouds and fog, which contain greater amounts of acid than rain or snow.

The acid rain also damages the leaves of plants and trees. This is responsible for retarding forests and other vegetation.

Building and monuments

Acid rain causes extensive damage to buildings and monuments made from marble, limestone slate, mortar. Limestone, which is a very common building material is attacked readily.

CaCO3 + H2SO4 → CaSO4 + H2O + CO2 ↑

Most of the monuments are made from Marble which is mostly calcium carbonate and so is attacked by acid rain. The damage caused to the buildings and monuments by acid rain is irreversible.

Human health

Acid rain does not have a direct effect on human health. Its pH is not enough acidic to burn human skin. According to the United States Environmental Protection Agency, Swimming in an acidic lake or walking in an acidic puddle is no more harmful to people than swimming or walking in clean water.

Acid rain causes respiratory problems such as a cough, asthma, headache, eye irritation due to the emission of sulfur dioxide and nitrogen trioxide.

Acidic conditions can play havoc within the human nervous system, respiratory nervous system and digestive system by making people an easy prey to various neurological diseases. It affects human life severely, which causes brain damage, kidney problems, etc.