12.3.4.6. Acid Rain

Oxides of sulfur and nitrogen originating from industrial operations and fossil fuel combustion are the major sources of acid forming gases. Acid forming gases are oxidized over several days by which time they travel several thousand kilometers. In the atmosphere these gases are ultimately converted into sulfuric and nitric acids. Hydrogen chloride emission forms hydrochloric acid. These acids cause acidic rain. Acid rain is only one component of acidic deposition. Acidic decomposition is the total wet acidic deposition (acid rain) and dry deposition.

Acid rain is primarily caused by specific gases in the atmosphere, mainly oxides of sulfur (SO₂) and nitrogen (NOₓ). These gases mainly come from industries and vehicles that burn fossil fuels. When they are released into the atmosphere, they can travel large distances and undergo a chemical reaction over several days. This reaction turns them into acids, specifically sulfuric acid (H₂SO₄) and nitric acid (HNO₃), which fall back to Earth as acid rain. It's important to note that acid rain isn't the only way that acid can enter the environment; dry deposition also occurs when these acids settle on surfaces without precipitation.

Think of acid rain like pouring lemon juice on a clean surface. The lemon juice represents the acids formed in the air, and as it hits the surface, it can start to cause damage, similar to how acid rain can harm buildings, plants, and water bodies. Just like the lemon juice might make a surface sticky or break it down, acid rain can deteriorate materials over time.

Rain water is turned acidic when its pH falls below 5.6. In fact clean or natural rain water has a pH of 5.6 at 20˚ c because of formation of carbonic acid due to dissolution of CO2 in water. In absence of rain, dry deposition of acid may occur. Acid forming gases like oxides of sulphur and nitrogen and acid aerosols get deposited on the surface of water bodies, vegetation, soil and other materials. On moist surfaces or in liquids these acid forming gases can dissolve and form acids similar to that formed in acid rain.

Rainwater is usually slightly acidic due to naturally occurring carbonic acid, resulting from carbon dioxide dissolving in water. This means that pure rainwater has a pH level of about 5.6 at 20°C. When the pH drops below this level, the rainwater becomes acidic, indicating that additional acids have been mixed into it, largely from the sources mentioned. Additionally, acid can also settle directly onto surfaces as dry deposition, which then can dissolve into water bodies or soil, contributing further to acid levels.

Imagine the rainwater being like a sponge soaked in a solution. Normally, it holds a mild, balanced flavor (pH of 5.6). If you were to soak the sponge in something sour, like vinegar (representing the acid), the flavor of the sponge would change, becoming much more sour, just like how rainwater turns acidic when it absorbs harmful gases.

Acid rain causes a number of harmful effects below pH 5.1. The effects are visible in the aquatic even at pH less than 5.5. 1. It causes deterioration of buildings especially made of marble e.g. monuments like Taj Mahal. Crystals of calcium and magnesium sulphate are formed as a result of corrosion caused by acid rain. 2. It damages stone statues. Priceless stone statues in Greece and Italy have been partially dissolved by acid rain. 3. It damages metals and car finishes. 4. Aquatic life especially fish are badly affected by lake acidification. 5. Aquatic animals suffer from toxicity of metals such as aluminium, mercury, manganese, zinc and lead which leak from the surrounding rocks due to acid rain. 6. It results in reproductive failure, and killing of fish. 7. Many lakes of Sweden, Norway, and Canada have become fishless due to acid rain. 8. It damages foliage and weakens trees. 9. It makes trees more susceptible to stresses like cold temperature, drought, etc. Many insects and fungi are more tolerant to acidic conditions and hence they can attack the susceptible trees and cause diseases.

Acid rain has profound effects, particularly when the pH falls below 5.1. One major impact is the deterioration of buildings and monuments, especially those made from marble, like the Taj Mahal, which suffers from loss due to corrosion. Stone statues and metal surfaces are similarly affected and can corrode or dissolve. Furthermore, acid rain is detrimental to aquatic ecosystems, causing fish populations to decline and even leading to lakes becoming devoid of fish. It also releases toxic metals from the surrounding environment, which can further harm aquatic and terrestrial life. Trees, weakened by acid rain, become more vulnerable to various stress factors and pests.

Consider how a soda can rusts or gets damaged when left outside in the rain. The acidic components of the soda slowly erode the metal, similar to how acid rain deteriorates buildings and natural structures. Just as you would want to protect your soda can from damaging weather, we need to protect natural and man-made structures from acid rain's destructive effects.

1. Emission of SO and NO from industries and power plants should be reduced by using pollution control equipment. 2. Liming of lakes and soils should be done to correct the adverse effects of acid rain. 3. A coating of protective layer of inert polymer should be given in the interior of water pipes for drinking water.

To combat acid rain, it is important to reduce the emissions of sulfur dioxide (SO₂) and nitrogen oxides (NOₓ), mostly released from industrial processes. This can be achieved through pollution control technologies that capture or neutralize these emissions before they enter the atmosphere. Another effective measure is liming, which involves adding lime to lakes and soils to neutralize acidity and restore a healthier ecosystem. Protective coatings inside water pipes can also prevent the leaching of harmful substances due to acidic conditions, ensuring that drinking water remains safe.

Think about how using a good quality filter in a fish tank helps keep the water clean and healthy for the fish. Similarly, when industries adopt cleaner technologies to reduce pollution and when lakes are 'treated' with lime, it's like filtering out the acidity to protect aquatic life and ecosystems, allowing them to thrive just like the fish in a well-maintained tank.

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