What is Acid Rain? - WorldOfChemicals

Everything about Acid Rain you need to learn

Acid Rain

Acid rain is the rain or any other form of precipitation which is generally acidic, means which has exalted levels of hydrogen ions. The corrosive effect of acidic or polluted air was noted in the 17th century on limestone and marble by John Evelyn. Robert Angus Smith was the first in 1852, who showcased the relationship between acid rain and pollution in the atmosphere in Manchester. The etymology acid rain was coined by Robert Angus Smith in 1872.

Emissions of chemicals resulting in Acidification:

The most crucial gas leading to acidification is sulfur dioxide.

Natural phenomena:

The dominant natural process that injects acidic gases to the atmosphere are volcanoes emissions. For example, Laguna Caliente crater fumaroles of Poas Volcano develop extremely large amount of acid rain with high acidity of around 2 pH. Gases producing acids are also formed by some biological processes that appears on the land, wetlands and in the oceans. Dimethyl sulfide is the main biological source of sulfur composed compounds.

Important source of fixed nitrogen is nitric acid found in rainwater and it is also formed in the atmosphere because of electrical activities like lightning.

Human Activity:

The principal reason causing acid rain is sulfur and nitrogen produced from natural sources like electricity generation, factories and vehicles. Electrical power generation done with coal is the highest contributor in producing acidic gas responsible for acid rain. Smoke dispersed from factories also contributes to acids in acid rain.

Chemical Processes in Acid Rain:

Sulfur dioxide and nitric oxides is produced through combustion of fuels. Then are converted into sulfuric acid and nitric acid.

Gas Phase:

In this phase sulfur dioxide is oxidized by reacting with hydroxyl radical by means of an intermolecular reaction.

SO2 + OH· → HOSO2

Which is then followed by:

HOSO2· + O2 → HO2· + SO3

Sulfur dioxide is then converted briskly into sulfuric acid in the existence of water.

SO3 (g) + H2O (l) → H2SO4 (aq)

Nitrogen dioxide then reacts with OH forming Nitric Acid:

NO2 + OH· → HNO3

Cloud Droplets Chemistry:

In the presence of clouds, loss rates of SO2 is faster and it is because of the reactions in the water droplets.


Sulfur dioxide get dissolved in water and then series of hydrolyses equilibrium reactions happens:


Large number of aqueous reactions are there that oxidizes sulfur, leading to the production of sulfuric acid. The most important oxidation reactions takes place with ozone, hydrogen peroxide and oxygen.

Acid Deposition:

Wet Deposition:

Wet deposition happens when any form of precipitation like rain, snow etc., removes acid present from the atmosphere and pass it to the surface of the earth. Wet removal of both gases and aerosols present in the atmosphere is important for wet deposition.

Dry Deposition:

In the absence of precipitation, acid deposition takes place via dry deposition. Dry deposition is responsible for about 20-60% of total acid deposition. Dry deposition occurs when gases and other particles get stuck to the earth surface or plants.

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