Acid rain is a form of precipitation (rain, snow, hail, etc) which happens when acidic components in the atmosphere, like pollutants, react with water and oxygen to form acids. These then fall as acid rain. Sulphuric, nitric, and carbonic acids are formed in general. These acids may fall on the ground and have adverse effects on the soil chemistry, and may also pollute the water sources. A more scientific term is Acid Deposition.
Cause of Acid Rain
The term first came up in 1872 in a book Air and Rain: The Beginning of a Chemical Climatology. But it did not gain popularity until the 1970s. Acid rain started happening right after the industrial revolution in parts of Europe ad North America. Later this phenomenon started occurring in Asia, Africa, Australia, and South America as well.
The past few years have witnessed an excessive amount of air pollution due to factory exhausts, vehicular exhausts, etc. These emissions contain S02 (Sulphur Dioxide), and NOx (Generic Nitrogen Oxides). A very small portion of these chemicals is also produced by natural components, such as volcanoes. But by far, the largest source of emission is the burning of fossil fuels. Human activities as well have contributed to global emissions. Garbage burning, vehicle exhausts, stubble burning are a few examples of these.
Fossil fuels are still used across the world as a primary element for the production of electricity. The power plants use coal and oil (most abundant fossil fuels) to produce heat (fire). This heat is then utilized to convert water into steam. The steam is then used to drive the turbines which produce electricity. The image below depicts this process.
Fossil fuel burning produces 2/3rd of the total emission. Manufacturing units, refineries, factories, use fossil fuels to power their production lines and other machinery.
Though the areas where the factories are set up are most prone to acid rains, other distant areas also receive acid rain since these SO2 and NOx are easily blown to distant parts of a locality by the wind.
Determining the Acidity
The basic science behind determining the acidity of any chemical is its pH value (power of hydrogen). A substance that has a pH value less than 7 is considered acidic, while more than 7 is considered basic. Water is neutral having a pH value of 7. The normal rain has a pH value of 5.6, which is slightly acidic. The reason for this is the presence of dissolved CO2. The CO2 forms a weak carbonic acid with water which gives the acidity to the rainwater. The pH of acid rain is between 4.2 and 4.4.
The acids which reach the ground in the form of rain are called wet depositions, which some of these remain as particles in the air, which are called dry depositions. It is much more difficult to monitor and study dry depositions when compared with wet depositions.
Effects of Acid Rain
1. Acidification of Water sources: When the acid rain depositions are washed into the lakes or streams, the water sources are turned acidic. A Long-Term Monitoring (LTM) Network is used to measure the acidity of such water bodies. Some research showed that the pH of water directly affects the aquatic lives and ecosystem. There is a decline in the health of fishes, clams, crayfishes, etc.
2. Acidification of Land and Forests: The acid rains dissolve in the soil and increase their acidity. This weakens the trees. Such trees are more vulnerable to damage from stressors, such as pests, weeds, etc. The acid rain also damages the chemistry of the soil which is necessary for plant growth. Essential chemicals such as calcium, magnesium, nitrogen, etc react with these acids and hence plant growth is slowed.
3. Damages to Metallic Structures: A basic chemistry principle can be used to explain this. When acids react with metals, hydrogen and salts are produced. Hence this leads to corrosion of the metal. Acid rains are extremely dangerous for metallic structures and monuments as it reacts with the surface and degrades the structure.
4. Damages to Stone Structures: When an acid reacts with stones like limestone, marble, it breaks the material down to release CO2. One of the 7 Wonders of the World, the Taj Mahal, fell victim of acid rain. There are many industries in Agra and a large number of sulphur oxides are emitted on daily basis. The acid rain, hence caused, reacted with the marble of the Taj Mahal and formed sulfates and nitrates. Hence the shine of the monument is slowly decreasing. The Indian government has been taking action since 1995 to prevent further damage. Below is the picture of the Taj Mahal I clicked last year when I went on a trip to Agra. The Taj Mahal now has a yellowish hue due to excessive pollution and acid rain.
5. Effects on Humans and Animals: Once acids when dissolves in the soil, the sulphur present in the acid accelerates the conversion of elemental mercury into methyl mercury by bacteria, which is extremely poisonous for humans. It is considered a neurological toxin. Methyl mercury targets the cells in the brain and disrupts neurological processes. This is consumed by animals who eat the plants and it moves up the food chain, a process generally called biological magnification. Methyl mercury takes place in the animal fat cells which later humans consume. This is harmful to not only the animals but to humans as well. Acid rain may also cause skin irritations, skin cancer. Hence without a doubt, one can say, it is extremely dangerous for all of us.
6. Effect on Ecosystems: Not all the components of an ecosystem can tolerate the level of acidity produced by the acid rains. If even one organism of the ecosystem is affected, the balance is lost and repercussions can be immense.
All the above reasons are good enough to make someone understand how serious the problem is. When we talk about environmental issues, everything comes down to just one reason, Emission. This week we also celebrated World Zero Emission Day. Check out the article here- https://www.projecthelpngo.org/post/zero-emission-day-6-best-ways-to-reduce-your-emission
I hope everyone acts responsible and takes the necessary steps needed to avoid such issues.
Author: Subhasri Banerjee
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