What is acid rain, defining it in terms of pH levels and the substances that contribute to its acidity? Furthermore, explain in detail the chemical processes involved in its formation, starting from the emission of pollutants (such as sulfur dioxide and nitrogen oxides) into the atmosphere and tracing their transformation through various reactions, including oxidation and hydrolysis, culminating in the precipitation of acidic compounds. How do natural sources, like volcanoes and lightning, contribute compared to human-caused sources, such as industrial processes and vehicle emissions?
Answer
Acid rain is a broad term referring to precipitation that is more acidic than normal. Normal rain is slightly acidic, with a pH of around 5.6, due to the presence of naturally occurring carbon dioxide in the atmosphere that dissolves in rainwater to form carbonic acid. Acid rain, however, has a pH lower than 5.6, typically ranging from 4.2 to 4.4, and in some severely polluted areas, even lower. This increased acidity is primarily caused by human-generated emissions of sulfur dioxide (SO₂) and nitrogen oxides (NOx) into the atmosphere.
Formation of Acid Rain:
The formation of acid rain involves a complex series of chemical reactions in the atmosphere:
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Emission of Pollutants: The primary sources of SO₂ and NOx emissions are:
- Burning of Fossil Fuels: Power plants that burn coal or oil, industrial facilities, and vehicles are major emitters of SO₂ and NOx. Coal, in particular, often contains sulfur, which is released as SO₂ during combustion.
- Industrial Processes: Certain industrial processes, such as smelting of metal ores and manufacturing of chemicals, also release SO₂ and NOx.
- Natural Sources: Natural sources such as volcanic eruptions, lightning strikes, and biological decay also contribute to SO₂ and NOx emissions, but these are generally much smaller than human-caused emissions.
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Atmospheric Transport and Transformation: Once released into the atmosphere, SO₂ and NOx can be transported over long distances by winds. During transport, they undergo chemical transformations through oxidation.
- Oxidation of SO₂: Sulfur dioxide is oxidized to sulfur trioxide (SO₃) through reactions with oxidants such as hydroxyl radicals (OH), ozone (O₃), and hydrogen peroxide (H₂O₂). This oxidation can occur in both the gas phase and in cloud droplets.
- Oxidation of NOx: Nitrogen oxides, mainly nitric oxide (NO) and nitrogen dioxide (NO₂), are oxidized to nitric acid (HNO₃) through reactions with oxidants such as OH and O₃.
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Acid Formation: The oxidized products, SO₃ and HNO₃, readily react with water in the atmosphere to form sulfuric acid (H₂SO₄) and nitric acid (HNO₃), respectively.
- SO₃ + H₂O → H₂SO₄
- HNO₃ already exists, it just mixes with water.
- Deposition: The sulfuric and nitric acids formed in the atmosphere can be deposited back to the Earth’s surface in two main ways:
- Wet Deposition: Acidic pollutants are incorporated into raindrops, snow, fog, or hail, which then fall to the ground. This is what is commonly referred to as "acid rain."
- Dry Deposition: Acidic particles and gases directly deposit onto surfaces such as vegetation, soil, and buildings. This dry deposition can be washed off by subsequent rain, further contributing to acid rain.