Causes and Effects of Acid Rain

The regions that are affected by acid rain the most are eastern part of United States, Eastern Europe and Southeast Canada, China and Taiwan.  Acid rain, or more accurately acid precipitation, is the term used for describing rainfall with a pH level lower than 5.6.

This type of pollution is a matter of great debate currently due to its potential of causing environmental damages all across the world. For the last decade or so, acid rain has caused destruction to hundreds of lakes and streams in many parts of the world, including the US, Canada, and Europe.

Acid rain forms due to the oxides of sulfur and nitrogen combining with the moisture content of the atmosphere, resulting in the formation of sulfuric and nitric acids. These acids can be dispersed far away from their places of origin.

The oxides of nitrogen or NO_x and sulfur dioxide or SO₂ are the two main sources of acid rain.

Sulfur dioxide, which is a colorless gas, is released as a by-product when fossil fuels that contain sulfur are burned.

Formation

• Industrial processes like the processing of crude oil, utility factories, and iron and steel industries.
• Natural means and disaster can also result in sulfur dioxide being released into the atmosphere, such as rotting vegetation, plankton, sea spray, and volcanoes, all of which emit about 10% sulfur dioxide.

• On the whole, industrial combustion is responsible for 69.4% sulfur dioxide emissions into the atmosphere, and vehicular transportation is responsible for about 3.7%.

Chemistry
When sulfur dioxide reacts with the atmospheric moisture, it undergoes oxidation to form sulfate ions.

SO₂ (g) + O₂ (g) = SO₃ (g)

The sulfate ions then combine with hydrogen atoms from the atmosphere to form sulfuric acid in the aqueous state.

SO₃ (g) + H₂O (l) = H₂SO₄ (aq)

Sulfur dioxide affects the breathing capacity of lungs and cause permanent damage to them. Shortness of breath, asthma, recurring cough are some of the major problems related to constant exposure to this gas.

Nitrogen oxide is another major component of acid rain. Nitrogen compounds that contain oxygen atoms, are known as oxides of nitrogen. For example, nitrogen dioxide and nitrogen monoxide are oxides of nitrogen, and they are generically termed as NO_x.

Formation

• These gases are produced in combustion processes, which involve extremely high temperatures. For example, utility plants, automobiles and chemical industries such as in the production of fertilizers.

• Five percent of nitrogen oxide is emitted by natural processes like lightning, volcanic eruptions, forest fires, and action of bacteria in the soil. Industrial processes emit 32% and vehicular transportation is responsible for 43%.

Chemistry
While reacting with atmospheric moisture, nitrogen oxide also undergoes oxidation reaction to give rise to nitric or nitrous acid.

NO₂ (g) + H₂O (l) = HNO₃ (aq) + HNO₂ (aq)

Nitrogen oxide, which is a dangerous gas in itself, causes damage to the respiratory organs by attacking the membranes in them; thus increasing the chances of respiratory diseases.

It also causes smog and plays a critical role in damaging the ozone layer. The nitrogen oxide can be carried far away from the original location of its emission.

Plant Life
Acid rain seeps into the earth and poisons plants and trees by dissolving toxic substances in the soil, such as aluminum, which gets absorbed by the roots. This rain also dissolves the beneficial minerals and nutrients in the soil which are then washed away, before the plants and trees have a chance of using them in order to grow.

When there is frequent acid rain, it corrodes the waxy protective coating of the leaves. When this protective coating on the leaves is lost, it results in making the plant susceptible to disease. Due to the damaged leaves, the plant loses its ability to produce sufficient amounts of nutrition which is need for it to stay healthy.

It results in making the plant vulnerable to the cold weather, insects, and disease, which can in turn lead to its death.

Acid rain also affects aquatic organisms adversely. A high amount of sulfuric acid in the seawater interferes with the ability of fish to take in nutrients, salt, and oxygen. As far as freshwater fish is concerned, in order for them to stay alive, they need to balance the amount of minerals and salts in their tissues.

The molecules of acid result in mucus forming in their gills, which prevents them from absorbing oxygen in adequate amounts. Plus, the acidity, which reduces the pH level, causes the imbalance of salt in the tissues of fish.

This change in the pH level also impairs some of the fish's ability to maintain their calcium levels. It in turn affects the reproduction processes of the fish. Lack of calcium also causes deformed bones and weakened spines in fish.

Other than causing harm to ecosystems, acid rain also damages man-made structures and materials. For example, acid rain dissolves sandstone, limestone, and marble. It also corrodes ceramic, textiles, paints, and metals. Rubber and leather deteriorate if exposed to acid rain. Stone monuments and carvings lose their shine when exposed to this contaminated rain.

Most of all, acid rain affects human health adversely. It can harm us through atmospheric and soil pollution. Acid rain leads to the formation of toxic compounds by reacting with naturally occurring chemical compounds. Once these toxic compounds are formed, they can seep into the drinking water, and also enter the food chain.

This contaminated food can damage the nerves in children, or result in severe brain damage, or even death. Scientists suspect that aluminum, one of the metals affected by acid rain, is associated with Alzheimer's disease.

The emissions of nitrogen oxide and sulfur dioxide cause problems like irritation of throat, nose and the eyes, headache, asthma, and dry cough.