Acid Rain: The Chemical Climatology

Environmental Protection Agency (EPA) has defined Acid Rain or Acid Deposition as a form of precipitation with acidic compounds such as such as sulfuric or nitric acid that fall on the ground from the atmosphere in wet or dry forms. E.g. rain, snow, fog, hail or even dust that is acidic. Normal rain is slightly acidic with a pH of 5.6 whereas pH of acidic rain generally ranges between 4.2-4.4.

The term acid rain was firstly coined by Scottish chemist Robert Angus Smith in 1852, who is also known as the "father of acid rain." Smith decided on the term while investigating rainwater chemistry near industrial cities in England and Scotland (Refer BaselConvention for transboundary movement of hazardous wastes). He described about this phenomenon in 1872 in the book "Air and Rain: The Beginnings of a Chemical Climatology." However, it was not given any attention until the late 1950s when scientists in the United States started studying the phenomenon, and in the 1960s and early 1970s, acid rain became recognized as a regional environmental issue that affected Western Europe and eastern North America.

Forms of Acid Rain or Deposition

Wet Deposition

The acids (mainly sulfuric and nitric acid) made in the atmosphere fall to the ground mixed with rain, snow, fog, or hail.  

Dry Deposition

When acidic particles and gases deposit from the atmosphere in the absence of moisture is termed as dry deposition.it occurs when dust and smoke that have high amounts of sulphur dioxide and nitrogen oxides settle to the ground, or onto buildings, cars and vegetation. It is estimated that half of the acidity in the atmosphere falls back to earth through dry deposition.

Causes of Acid Rain

Acid rain can be caused by both natural and man- made sources which are:

Natural Sources

Volcanoes

Oceans

Biological processes in the soil

Lightening

Man-made Sources

Burning of fossil fuels

Forest Fires

Industrial production of sulfuric acid

Smelting of metal sulphide ores

Chemistry of Acid Rains

Three types of acids contribute in the formation of acid rains which are sulphur oxides (SO_x), nitrogen oxides (NO_x) and oxides of hydrogen such as water and OH radicals.

The sulphur dioxide reacts with the hydroxyl radical via an intermolecular reaction written below:

SO₂ + OH → HOSO₂

HOSO₂ is unstable and then it reacts with atmospheric Oxygen as follows:

HOSO₂ + O ₂ → HO₂ + SO₃

The SO₃ formed so rapidly reacts with the water and forms the Sulphuric acid as follows:

SO₃ (g) + H ₂O (l) → H₂ SO₄ (l)

The Nitrogen Dioxide also reacts with the OH to produce the Nitric Acid

NO₂ + OH → HNO ₃

The above reactions may take in the cloud drops as follows:

SO₂ (g) + H ₂O ↔SO₂·H₂O

SO₂·H ₂O ↔H⁺+ HSO₃−

HSO₃ ⁻ ↔ H⁺+ SO₃^-−

Effects of Acid Rain

Acid rains have various effects on plants, animals, places and environment as a whole. Read more about ozone layer here.

Aquatic life

When acid rain mixes with water it results in proliferation of aluminium in lines and also affects eggs of frogs and marine organisms.

Human Health

Acid rains cause indirect harm to human health as the pollutants forming acid rains such as SO_X and NO_X gases interact to form fine particles that can enter into the respiratory tract and may causes lung diseases. Dry deposition can cause heart and lung disease such as Asthma and Bronchitis.  

Vegetation

Acid rains may have harmful impact on vegetation and can weaken trees by destroying their leaves and limiting the supply of nutrients. Acid rains dissolves nutrients and minerals and carry them away before the plants can use them to grow. Acid rain damage cuticle of plant leaves and interrupts photosynthesis and respiration. Acidic rains also cause leaching of heavy metals like aluminium, lead and mercury. Such metals when percolate into ground water affect soil micro flora/fauna.

Buildings and Monuments

Acid rain is also responsible for the destruction of buildings and monuments made up of marble and limestone. Have you heard of KyotoProtocol? This is because these rocks contain a large amount of Calcium Carbonate, which reacts with the Sulfuric Acid to create Gypsum. Gypsum flakes off easily. This is shown in the following reaction:

CaCO3 (s) + H 2SO4 (aq) ↔CaSO4 (aq) + CO 2 (g) + H2 O (l)

Examples are Taj Mahal, Statue of liberty, etc.

 

Solutions to the Acid Rains

Reduce burning of fossil fuels

Focusing on alternative energy sources such as wind, solar and tidal energy.

Liming: damage to water bodies can be eliminated by adding lime. But it is an expensive method.

Decreasing NO_X emission from power stations and vehicles (fitting catalytic converters in cars)

Using fuel with lower sulphur content.

Plantation of trees

 

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