Introduction -
Normal rain is slightly acidic with an average pH of 5.6 but when the precipitation with a pH less than 5.6, that is more acidic than normal rain, we call it acid rain or acid precipitation.
When atmospheric water droplets combine with a range of man-made chemical air pollutants such as nitrogen oxides (NOx) and sulphur dioxide (SO₂), acidic rain is formed. It could also be in the form of acidic mist or snow. Although these oxides are the main contributors to acid rain, other acids like hydrochloric acid may also be involved.
The acid rain ultimately falls on the ground, sometimes hundreds of kilometres from the area in which it formed and generally one to four days later. The effects are quite damaging.
Forms of Acid Precipitation-
Acid precipitation with a pH less than 5.6, is more acidic than normal rain. Even normal rain is slightly acidic with an average pH of 5.6, as water combines with naturally occurring carbon dioxide in the air thus forming weak carbonic acid. It includes both: Wet Acidic depositions and Dry Acidic depositions
- Wet Acidic Depositions - This can be in the form of acid rain or acid for or even acid snow can occur in colder areas.
- Dry Acidic Depositions - Acidic particles and gases can also deposit from the atmosphere in the absence of moisture as dry deposition. The acidic particles matter and gases may deposit to the surface quickly or may react during atmospheric water droplet transport to form larger particles that can be harmful to human health.
Causes of Acid Rain -
Emissions of the two air pollutants, nitrogen oxides (NOx) and Sulphur dioxide (SO₂) are the main reasons for acid rain formation. Nitrogen oxides and sulphur dioxides are emitted during fossil fuel combustion and then undergo reactions with water in the air to form the nitric acid (HNO₃) and the sulphuric acid (H₂SO₄) found in acid rain.
The major sources of SO₂ and NOx in the atmosphere are -
In nature, volcanoes, fires, and decomposing matter emit these substances in small amounts. However, since the advent of the Industrial Revolution, human activities have been releasing such pollutants in large quantities. Such emissions are very high in the major industrial centres and have been increasing rapidly since the mid-twentieth century.
Automobiles and coal and oil-fired power stations are major sources of acid-forming compounds. Any burning of coal, oil, and (to a lesser extent) natural gas produces them.
Acid rain affects all elements of the environment, surface and groundwater, soils and vegetation. It negatively affects food chains, reduces biodiversity and damages our world as discussed below -
The Effect of Acid Rain -
- When soil becomes acidified, essential nutrients such as calcium(Ca) and magnesium(Mg) are leached out before the trees and plants can use them to grow. His reduces the soil’s fertility. In addition, acidification may release aluminium from the soil. At high concentrations, aluminium is toxic and damages plant roots. This reduces the plant's ability to take up nutrients such as phosphorus, eventually leading to death.
- It leads to acidification of water bodies, Some 14,000 Swedish lakes, located in acidic crystalline rocks, have been affected by acidification with widespread damage to plant and animal life as a consequence.
- Acid precipitation does not usually kill trees directly. Acid deposition destroys the surfaces of the leaves of trees and plants. This damage causes uncontrolled water loss and slows photosynthesis. It reduces the rate at which leaf litter decomposes, causes the death of useful microorganisms present in tree roots and reduces the rate at which soil organisms (including bacteria) respire.
- Soil acidification releases metals that can harm microorganisms in the soil as well as birds and mammals higher up in the food chain. The most sensitive groups include fish, lichens, mosses, certain fungi and small aquatic organisms. Some organisms may be eliminated, reducing biodiversity.
- A side effect of acid rain is the leaching of aluminium out of the soil into water bodies. Aluminium is very toxic to fish and the birds that prey on the fish. Sometimes, acidification leads to the leaching of cadmium and this can affect animals.
- Acid rain also disturbs the natural cycles of sulphur and nitrogen.
Some Examples of Acid Rain affecting the environment -
- When soil is acidified, it leads to a loss of productivity. The acidification damages plant roots and they are not able to draw in enough nutrients to survive and grow.
- When trees, particularly conifers, are exposed to acid rain for several years, they lose their leaves and die. This is one of the several causes of the decline of forests in Europe, North America, and Japan. Plants like orchids, lichen, and moss are also very sensitive to acid.
- Acid rain falling on lakes and rivers leaves them clear and lifeless. Thousands of lakes in Sweden, Norway, and Canada, for example, have been permanently affected by acid. Fish populations have died and so have species like otters, amphibians, and birds that depend on fish for their food.
- Old buildings are also threatened by acid rain, and the famous St Paul’s Cathedral in London has decayed more in the last 50 years than in the previous two centuries. Some famous statues, such as the Lincoln Memorial and Michaelangelo’s statue of Marcus Aurelius, have started deteriorating because of acid rain. The same is true of many historic buildings in Europe.
- The Taj Mahal was also threatened by acid rain caused by factories in Agra. Thanks to the orders of the Supreme Court, some of these industries have been shifted or closed down.
What can be done about Acid Rain?
Acid rain occurring in a place could be caused by emissions from far away. In most cases, acid rain formed in one country falls on some other country. It is also difficult to prove that a particular country or a factory was responsible or to quantify the amount of contributing pollution from a source.
Pouring powdered limestone into water bodies is a rapid, but short-lived method of reducing acidity. A more permanent, but slow and expensive, method is the liming of surrounding soils.
Some technologies for reducing emissions are flue gas desulphurization in power stations and catalytic converters and engine modifications in automobiles. The best way, of course, is to reduce emissions.
Frequently Asked Questions (FAQs)
Question - What is Acid Rain?
Answer - Clean rain is slightly acidic naturally but when the pH of rain falls below 5.6, we call it acid rain. Acid rain usually has a pH value between 4.2 to 4.4.
Question - What are the effects of Acid Rain?
Answer - Acid rain causes damage to the biological environment of forests, lakes etc. and also physically damages buildings, causes paint discoloration, metal corrosion etc. It affects surface and groundwater, soils and vegetation. It negatively affects food chains and reduces biodiversity.
Question - What is the cause of acid Rain?
Answer - It is formed majorly by sulphur dioxide or nitrogen oxides present in air which combine with water to form much stronger sulphuric acid or nitric acid respectively. Although these oxides are the main contributors to acid rain, other acids like hydrochloric acid may also be involved.
Question - How does Acid Rain affect Plants?
Answer - When soil is acidified, it leads to a loss of productivity. The acidification damages plant roots and they are not able to draw in enough nutrients to survive and grow.
When trees, particularly conifers, are exposed to acid rain for several years, they lose their leaves and die. This is one of the several causes of the decline of forests in Europe.
Question - How does Acid rain harm human life?
Answer - Acid rain harms people directly when they breathe in the acidic air. Acid rain can also harm people indirectly when they eat fish caught in affected lakes or rivers.
Question - How is our world heritage threatened by Acid Rain?
Answer - Old buildings are also threatened by acid rain, and the famous St Paul’s Cathedral in London has decayed more in the last 50 years than in the previous two centuries. Some famous statues, such as the Lincoln Memorial and Michaelangelo’s statue of Marcus Aurelius, have started deteriorating because of acid rain.
The Taj Mahal was also threatened by acid rain caused by factories in Agra.
Question - Write some ways to reduce Acid Rain.
Answer - Pouring powdered limestone into water bodies is a rapid, but short-lived method of reducing acidity. A more permanent, but slow and expensive, method is the liming of surrounding soils.
Some technologies for reducing emissions are flue gas desulphurization in power stations and catalytic converters and engine modifications in automobiles. The best way, of course, is to reduce emissions.
Question -What is the pH value of Acid rain?
Answer - Acid rain usually has a pH value between 4.2 to 4.4.