Acid rain

Sreelakshmi P Vijayan
Updated on

Acid rain is a comprehensive term that involves the deposition of any form of precipitation containing both dry and wet acidic components having pH in the range of 4-5. This includes rain, snow, fog, hail and even dust that is acidic. It is to be noted that the pH of normal drinking water is 7. Hence acid rain is a rain that is usually acidic in nature than normal rain.

What causes acid rain?

Acid rain occurs when oxides of sulfur (SO2) and oxides of nitrogen (NOX) get released into the atmosphere and are transported by wind and air currents. These oxides (SO2 and NOX) then react with water, oxygen and other chemicals to form corresponding sulfuric acid and nitric acid and get mixed with water and other materials before falling on the earth.

Let’s look into the chemistry behind the formation of acid rain:

chemical reaction

Emission of SO2 and NOX

Major portions of SO2 and NOX emission arises due to the burning of fossil fuels whereas a small portion arises from natural sources such as volcanic eruptions, wildfires etc.

The foremost sources of SO2 and NOX in the atmosphere are:

  • Generation of electricity by burning fossil fuels. 2/3 portion of SO2 and 1/4 of NOX in the atmosphere come from electric power generators.
  • Motor vehicles (automobiles) and heavy equipments.
  • Thermal power plants, industries, manufacturing, oil refineries etc.

Acid rain has been displayed to have adverse impacts on ecosystems, woods, freshwaters and soils. It not only kill insect and aquatic life-forms but also have impacts on human health. It also causes damage to buildings, monuments, infrastructure etc.

How to measure Acid Rain

Acidity and basicity of a substance can be measured using a pH scale. A substance having a pH less than 7 are acidic in nature, a substance having a pH greater than 7 are alkaline and for which having a pH of 7.0 is neutral. Normal rain has a pH of approximately 5.6. It is slightly acidic because carbon dioxide (CO2) in it dissolves in water forming carbonic acid (a weak acid).  Acid rain usually has a pH between 4.2 to 4.4.

The NADP (National Atmospheric Deposition Program) and NTN (National Trends Network) measures acidic wet deposition. They collect acid rain from more than 250 monitoring sites throughout America, Canada, Alaska, Hawaii etc. Unlike wet deposition, dry deposition is very hard and expensive to measure. Dry deposition estimates for N and S- pollutants are delivered by the Clean Air Status and CASTNET.

ph scale

Effect on ecosystems

Acid rain adversely affects the life of aquatic organisms in environments such as streams, lakes, and marshes. At low pH, most of the fish eggs cannot hatch. At lower pH levels, some adult fish may die. Frogs have a critical pH of around 4. But the mayflies are much more sensitive and do not survive in a pH below 5.5.

Acid rain as it flows through the soil, can leach aluminum from clay particles and then flow into streams and lakes. This decreases the pH further and Al is harmful to both plants and animals. Acid rain also removes the necessary minerals and nutrients from the soil that is required for the trees to grow leaving them dead with brown leaves. It corrodes water pipes, making heavy metals to seep into drinking water.

Effect on human health

The emitted SO2 and NOX reacts in the atmosphere leading to minute sulfate and nitrate particles. Scientific studies have revealed that the inhalation of these particles adversely effects heart function, such as fatal heart attacks, increased heart diseases and increased lung infection involving breathing difficulties in people having asthma.

Effect on monuments and building

taj mahal

The Taj Mahal in Agra is one of the seven wonders in the world, a historical monument in pure white marble. The release of pollutants from the neighbouring Mathura Oil Refinery at Agra and many other industries leads to acid rain and progressively corroded the Taj Mahal and leads in the discolouration of marble turning it yellowish. 

Marble cancer is the term used for the deterioration of marble by acid rain.

The chemical reaction of acid rain with marble is:

H2SO4 + CaCO3  ——————>CaSO4 + H2O + CO2


Read more: The Chemical Compositions of Insect Venoms

Check your knowledge

Acid rain is generated when oxides of sulphur and nitrogen released in the atmosphere combines with water, oxygen and other molecules having low pH. Acid rain is a highly acidic pollutant.

Acid rain pollutes water resources, creating threat to aquatic life and perturbs the ecosystem. It also leads to deterioration of building, monuments and structures. It destroys forests and agriculture which adversely affect the existence of both humans and animals.

Combustion of fossil fuels need to be reduced. Use of alternate energy sources (solar, wind, hydra etc.) can be considered. Renewable energy sources are very ecofriendly.

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