Acid Rain

There is romance in the rain. The fall of tiny wet drops of precipitation can make even the dullest of evenings magical. There is magic heading home on a summer night in the rain, flip flop rhythms walking with the drops through puddles while the July heat falling off you. An open window with the sound of drops hitting a puddled ground or a rusty roof rattling with raindrops offers comfort while the cool showers of spring are a sign of summer giving hope after a dark cold winter.

But not everything contained in those little droplets of water is pure perfection. High levels of acid in rain has created the term unimaginatively coined in the 1950's: Acid Rain. Although acid rain was first discovered in the 17th century after poor conditions was noted in marble, since the Industrial Revolution high levels of acid has been documented by scientists in rain.

Distilled or pure rain normally has a pH of 7, and is considered neutral. The rain or precipitation that has a pH lower than 5.6 is referred to as Acid rain. The literary meaning of acid rain is rain with an excess of salts.

Acid rain has an extremely harmful effect on plants, aquatic life, buildings and architecture due to the deposition of salts.

Cause of Acid Rain

Acid rain is caused by the emission of sulfur dioxide (SO2) and nitrogen oxides (NOX) in the atmosphere. These are then transferred by wind or air and react with water, oxygen and other atmospheric chemicals to form sulfuric and nitric acids. These acids mix with the molecules in water before raining to the ground.

Although a very small portion of the offending acids in acid rain are naturally present (i.e. volcanic eruptions), the major sources of SO2 and NOX in the atmosphere are derived from the burning of fossil fuels to generate electricity, run machines, automobiles, heavy equipment and in manufacturing, industry and oil refinery.

Acid Deposition

Acid rain is the deposition of acids in the air. Acid deposition falls into two categories.

• Wet Deposition

Wet deposition of acid occurs when any form of precipitation (rain, snow, etc.) removes acids from the atmosphere and delivers it to the Earth's surface. This is essentially what we think of when we think of acid rain. Sulfuric and nitric acids are delivered to the Earth's surface through precipitation.

• Dry Deposition

Acid deposition also occurs in the absence of precipitation and is known as a dry deposition. Dry deposition is caused by tiny particles (or particulates) in combustion emissions. This event can be responsible for as much as 20-60% of total acid deposition witnessed. A dry deposition occurs when particles and gases stick to the ground, to plants or other surfaces.

Composition of Acid Rain

The components of acid rain are

• H₂SO₄
• HNO₃
• HCl

Acid rain is usually composed of any of the three above mentioned acids. H₂SO₄ is the major contributor to acid precipitation with 60-70% while HNO₃ contributes about 30-40% to acid rain. HCl is a minor component of acid rain.

Chemical Process

The combustion of fossil fuels creates SO₂ and NO_x. They are converted to sulphuric acid and nitric acid.

Sulphuric acid formation:

SO2 + H2O → H2SO3
H2SO3 + [O] → H2SO4

Nitric Acid Formation Oxides of nitrogen react with H₂O and convert it into nitric acid.

2NO + O2 → 2NO2
2NO2 + H2O + O → 2HNO3

A combination of nitric acid and sulphuric acid form acid rain.

Sources

There are two types of sources emitting different oxides of sulphur and nitrogen, which react with H2O to form nitric acid and sulphuric acid.

• Anthropogenic sources
• Natural sources

Anthropogenic Sources

The principal cause of acid rain is sulfur and nitrogen compounds from human sources. Millions of tons of nitrous oxide (NOx) is added by anthropogenic sources into Earth's atmosphere every year.

• Fossil fuels
  • Coal power plants: Power generating plants contribute 60% to 70% of Sulphur.
  • Automobile exhaust
  • Domestic fire or House fuel
• Nuclear explosions
• Wastes of fertilizer plants
• Livestock production
• Industries and factories

Natural Sources

The principal natural phenomenon that contributes acid producing gases to the atmosphere are emissions from:

• Volcanic eruptions create extremely high amounts of acid rain and fog with acidity which clears an area of any vegetation and frequently causes irritation to the eyes and lungs of inhabitants in nearby settlements.

• Biological Processes: Acid producing gasses are also created by processes that occur in the oceans, on land, and in wetlands. The major biological source of Sulphur containing compounds is dimethyl sulphide.

• Nitric acid in rainwater is an important source for plant life and is also produced by electric activity in the atmosphere such as lightning.

• Acids are released into water bodies by organisms conducting anaerobic activities.

H2S + H2SO3→ H2SO4

SO2 + H2O →SO2H2O

SO2 + H2O ⇌ H+ HSO-3

HSO-3 ⇌ H+ + SO2-3

Adverse Effects

Acid rain has the following adverse effects:

• Corrosive action

The corrosive action of these rains with acid content damages buildings, wood, steel as well as cement and concrete structures. It also damages fabric and other fibrous materials.

CaCO3 + H2SO4 ⇌ CaSO4 + CO2 + H2O

• Decomposition of parent rocks

A low pH causes the release of many toxic metals and the elements in excess amounts as the rate of decay and decomposition of parent rocks is enhanced. However, in small traces, these elements do not pose any problems but in higher concentration, many are highly toxic.

• Soil fertility

Soil biology and chemistry can be seriously damaged by acid rain. Some microbes are unable to tolerate changes to low pH and are killed. The enzymes of these microbes are denatured by the acid. The hydronium ions of acid rain mobilize toxins such as aluminum and leach away essential nutrients and minerals such as magnesium.

2H+(aq) + Mg2(clay)+ ⇌ 2H+(clay) + Mg2+(aq)

• Effects on forests and vegetation

Acid rain reduces the rate of photosynthesis due to the destruction of chlorophyll molecules. It also reduces the growth of plants and decreases their sensitivity to diseases while enzymes are disturbed due to the change in pH. Acid rain also damages the roots of trees and weakens them. The permeability of plant membrane for up-taking is also decreased due to acid rain.

• Effects on lichens

Acid rain damages lichens and prevents them from growing. Lichens are sensitive bioindicators of air pollution and have been seen to be completely absent in places with high air pollution.

• Effect on aquatic systems

Acid precipitation affects aquatic life drastically due to low mineral contents. Both, a lower pH and a higher aluminum concentration in surface water due to acid rain can kill fish by clogging its gills and reducing its O₂ intake. As lakes and rivers become more acidic, biodiversity is reduced. Many types of bacteria and blue-green algae are killed due to acidified H₂O thus disturbing the ecological balance.

• Human health

Heavy metals like cadmium, aluminum, and palladium which enter the blood stream of aquatic animals are then passed on in the food chain to other animals and man. These heavy metals affect the stability of cell membranes, hence prove injurious to health. The presence of heavy metals affects a human's nervous system, digestive system, and respiratory system by causing or exacerbating ailments like asthma and bronchitis.

• Effects on animal health

Amphibian animals have been impacted by high levels of water acidity due to acid rain. Some species of frogs and toads are currently on the verge of extinction due to the inability of their eggs and larvae to survive in a high-acid environment. Acidification of water bodies has affected the food availability for birds that feast on fish. Rabbits that have lived in forests that have witnessed acid rain are being studied as high levels of acidity are causing bile salt induced injury in their digestive systems. High aluminum levels are reportedly responsible for malnutrition and starvation in some forest animals.

Measurement of Acid Rain

Acid rain is measured by using a scale called pH scale. The lower pH scale of substances the more acidic it is.

• Normal rain: 7.0pH
• Slightly acidic: 5.6pH
• Acid rain: less than 5.6pH

Prevention Methods

Technical Solutions

• Energy Conservation:

Energy conservation will result in lesser fuel consumption and slow the emission of sulphuric and nitrous oxides.

Desulphurization and Denitrification

• FGD:

Many coal power plants use Fossil Gas Desulphurization (FGD) to remove sulphur containing gases from their stack gases. FGD removes up to 95% of SO₂ from fossil gases.

• Denitrification:

Denitrification is the use of technology that removes or reduces the nitrogen oxide (NO)_x emission into the atmosphere.

Substitution of Fossil Fuels

Substitution for fossil fuels by other alternative energy forms may offer future solutions.

Reduction of SO2

The release of sulphur dioxide can be reduced by removing sulphur contents before the fuel is burnt, by a process called coal cleaning gasification. Sulphur can also be removed after combustion by fuel gas desulphurization.

Reduction of (NO)x

Nitrous oxide emissions into the atmosphere can be reduced by modifying furnace designs and burner designs. Lowering the temperature during combustion by the use of catalytic activity can also help reduce (NO)_x.

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