What is Acid Rain?
In simple terms, acid rain is the deposition of acid on the Earth’s surface, in the form of rain. Rainwater is always slightly acidic as it reacts with atmospheric carbon dioxide, forming weak carbonic acid. There are several factors that lead to acid rain, and these can be segregated into natural causes and man-made causes of acid rain. It has many adverse effects on human made structures.
Apart from the rain, acid deposition can take place in the form of snow, hail, fog, or sleet. When the pH of rainwater is lower than 7, it is evident that it has acidic components. The concentration of hydrogen ions in a solution gives the measure of its acidity. Acid rain has adverse impacts on human beings, trees, animals, and human-made structures. For example, acid rain effects are well evident on limestone and marble buildings like the Taj Mahal.
What are the Causes of Acid Rain?
Natural Causes
There are several natural sources of acidification of rainwater. The molecular nitrogen produced due to lightning reacts with atmospheric oxygen forming acidic oxides of nitrogen. These nitrogen oxides react with rainwater and lead to acid rain. When wildfires break out in forest areas, organic nitrogen is released into the air. Hence, wildfires lead to acid rain. In most places, wildfires and lightning are likely to reduce the pH of rainwater to nearly 5.
Man-Made Causes
It can be observed mainly in urban and volcanic areas. In urban areas, acidic gases such as sulphur dioxide, nitrogen oxide, etc., are released into the atmosphere, with the emissions from vehicles and factories. These gases form acids when they come in contact with the rainwater and lead to acid rain. In volcanic areas, acid rain is mainly caused by sulphuric acid, hydrochloric acid, and nitric acid.
Acid rain is caused in areas where fossil fuels like coal, natural gas, and oil are burned. A substantial amount of acidic gases is emitted into the atmosphere from the burning of fuels in vehicles. Smelting metal ores is another man-made factor that leads to the release of harmful acidic gases into the atmosphere and eventually causes acid rain.
The following chemical reactions are involved in the formation of acid rain.
NO2 + H2O → HNO3
HNO3 ↔ H+ + NO3
SO2+ H2O → H2SO4
H2SO4 ↔ H+ + HSO4
HSO4↔ 2H+ + SO42-
In the above-given chemical reactions, the reactants are in an aqueous solution state. They form an acidic coating on the water droplets in clouds.
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Acid Rain Effects on Human Made Structures
Human-made structures like old monuments, buildings, and gravestones that are made of marble or limestone are highly susceptible to decay due to acid rain. One of the main gases that cause acid rain is sulphur dioxide. It reacts with limestone forming gypsum in the presence of water. Gypsum is soluble in water, hence decays easily and gets washed away with acid rain. Similarly, marble also gets decayed when it comes in contact with acid rain.
The corroding effect of acid rain on the Taj Mahal is termed marble cancer. This magnificent historic building is made up of impeccable white marble stone. It is one of the eight wonders of the world. However, the Taj Mahal is getting corroded by acid rain and is now having a yellowish sheen. The chemicals like soot, carbon dioxide, nitrogen dioxide, sulphur dioxide, etc., from the industrial emissions, are deposited on the white marble of the Taj Mahal. This, in addition to acid rain, leads to the corrosion and yellowing of the white marble of the Taj Mahal. Acids from rainwater react with the marble of the Taj Mahal forming a powdery textured substance that dissolves in water and gets washed away with rain.
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The chemical reaction that causes the decay of marble due to acid rain is given below.
CaCO3 + H2SO4 → CaSO4 + H2O + CO2
The chemical formula for marble is CaCO3 and when it comes in contact with sulphuric acid it decomposes into CaSO4, water, and carbon dioxide.
Effects of Acid Rain on Waterbodies
The acid rain effects were observed in the 1960s and 1970s for the first time in western regions of Europe and eastern regions of North America. The life of aquatic animals gets adversely affected by acid rain. The plant and animal life in the aquatic regions exhibited a decline due to acid rain. The soil of the acid-sensitive areas has a low acid-neutralizing capacity. Acidification of the soil of these areas leads to the release of aluminium bound to the soil. Aluminium in its dissolved state can impact both plants and animals. Hence, in areas of acid rain, aluminium released from the soil enters the neighbouring water bodies. This, in turn, hikes up the aluminium content of the water bodies, which impairs the breathing capacity of fishes through gills. Again, a hike in the acidity of water bodies due to sulphur deposition leads to the formation of fatal mercuric compounds. When this mercuric compound enters the food chain or food web, it may lead to many deaths.
FAQs on Effects of Acid Rain on Human Made Structures
1. What is acid rain and how does it form?
Acid rain is any form of precipitation, such as rain, snow, or fog, that is unusually acidic, meaning it has a pH level lower than the normal 5.6. It forms when pollutants like sulphur dioxide (SO₂) and nitrogen oxides (NOx) are released into the atmosphere. These pollutants, primarily from burning fossil fuels in industries and vehicles, react with water, oxygen, and other chemicals to form sulphuric acid and nitric acid, which then fall to the ground with precipitation.
2. How does acid rain damage human-made structures?
Acid rain damages human-made structures through a process of chemical weathering. When acidic rainwater lands on buildings, statues, and monuments, it reacts with the minerals in the materials. For instance, it can dissolve stone, corrode metal, and cause paint to peel. This continuous exposure leads to the weakening and gradual deterioration of the structure's surface and integrity over time.
3. Why are monuments made of marble and limestone especially damaged by acid rain?
Monuments made of marble and limestone are particularly vulnerable because these stones are primarily composed of calcium carbonate (CaCO₃). The sulphuric acid in acid rain reacts with calcium carbonate to form calcium sulphate (gypsum), which is water-soluble. This newly formed substance is flaky and easily washed away by rain, causing the stone to crumble and lose its detail. This phenomenon is often referred to as 'stone leprosy' or 'marble cancer'.
4. What are some real-world examples of structures affected by acid rain?
Several famous structures around the world show visible damage from acid rain. Key examples include:
- The Taj Mahal in India: The white marble has been yellowing and eroding due to acid rain from nearby industrial pollution.
- The Parthenon in Athens, Greece: This ancient monument has suffered significant deterioration of its marble details.
- The Statue of Liberty in New York: The copper sheeting has been corroded by acid deposition, which contributed to its colour change and structural repairs.
- Bridges and railways: Metal structures like steel bridges are susceptible to corrosion and rust, which weakens their structural safety.
5. What is the difference between the impact of 'wet deposition' and 'dry deposition' on buildings?
Both are forms of acid deposition, but they affect structures differently:
- Wet Deposition: This is what we commonly call acid rain, snow, or fog. Its impact is widespread during a precipitation event, washing over surfaces and causing chemical reactions.
- Dry Deposition: This occurs when acidic pollutants (gases and particles) settle on surfaces during dry weather. These particles then react with moisture from dew or humidity to form acids directly on the structure. Dry deposition can sometimes be more concentrated and damaging as the acidic compounds remain on the surface for longer periods.
6. How does the damage to a stone monument differ from damage to a metal bridge?
The type of damage depends on the material. For a stone monument (like one made of marble), the damage is primarily dissolution, where the acid dissolves the material itself, causing it to crumble and lose its shape. For a metal bridge (typically made of steel), the damage is corrosion. The acid accelerates the process of rusting, which weakens the metal's structural integrity and can lead to safety hazards, rather than just aesthetic decay.
7. What are the primary methods for protecting human-made structures from acid rain?
Protecting structures involves two main strategies. Firstly, the most effective method is reducing the pollutants at their source by implementing stricter emission controls on industries and vehicles, and by using cleaner energy sources. Secondly, for existing structures, protective measures can be applied directly. These include applying sealants or protective coatings to stone surfaces to prevent acid contact, regular cleaning to remove acidic particles, and undertaking restoration work to repair existing damage, as done for many historical monuments.
