Acid Rain Definition, Characteristics, Reasons, Effects, Solutions

When the pH of rain water below 5.6, the rain name as acid rain. Emissions of sulfur dioxide, sulfur trioxide, nitrogen dioxide gases cause the acid rain. There are huge environmental impacts due to acid rains. We have to find sustainable solutions to prevent acid rains. In this tutorial, we will learn about characteristics, reasons, Effects of acid rains and how to find solutions to prevent acid rain.

Written by: Eng (Ms). K. Chathurika Jayanthi, Bsc. Chemical Process Engineering, University of Peradeniya


Definition, What is acid rain?

When the pH of rainwater below 5.6, the rain name as acid rain.

The pure water (which does not contain any impurities) pH is 7. But in atmospheric carbon dioxide ( CO2) dissolved in rainwater and make carbonic acid (equation 1).


Equation 1

CO2(g) + H2O(l) → H2CO3(aq)



Carbonic acid (H2CO3) is a week acid and contributes to increase H+ in the water through equilibrium reactions as shown in equations 2 and 3.


Equation 2

H2CO3(aq) ⇌ HCO3- (aq) + H+(aq)


Equation 3

HCO3- (aq) ⇌ CO32-(aq) + H+(aq)


These equilibrium reactions caused to reduced pH of rainwater. Because of dissolving CO2 in rainwater, the normal rainwater pH becomes 5.6. Rains whose pH is below 5.6 are considered as acid rains. Therefore, CO2 gas alone cannot give a considerable acidity for acid rain formation.


Which gases caused Acid rains?

Mainly oxides of nitrogen (NOx) and oxides of sulphur (SOx) are the gases caused for acid rains.


How oxides of Nitrogen (NOx) are caused for acid rains

Nitrogen dioxide (NO2) and nitric oxide (NO) are the most common oxides of nitrogen caused for acid rains. The atmospheric nitrogen dioxide are dissolved in rainwater and make acids.


Nitrogen dioxide dissolves in the water as equation 4 and produces nitrous oxide and Nitric oxide.

Equation 4

3NO2 (g) + H2O(l) → HNO3(aq) + 2NO(g)



With the presence of Oxygen in the atmosphere, Nitric oxide is oxidized to nitrogen dioxide as shown in equation 5.

Equation 5

2NO (g) + O2(g) → 2NO2(g)


Nitric acid is a strong acid that contributes to increase H+ ions concentration in rainwater as shown in equation 7. This can be caused to reduce pH level of water below 5.6 and caused acid rains.

Equation 6

HNO3(aq) → NO3-(aq) + H+(aq)




How oxides of Sulphur (SOx) cause acid rains?

Sulfur dioxide (SO2) and sulfur trioxide (SO3) are the oxides of Sulphur caused for acid rains


SO2 dissolves in water and makes sulfurous acid (H2SO3) (equation 7).

Equation 7

SO2(g) + H2O(l) → H2SO3(aq)


And also, sulfur dioxide can react with oxygen gas and form sulfur trioxide (SO3) as shown in equation 8. Formed sulfur trioxide react with water and produce sulfuric acid (H2SO4) (Equation 9).

Equation 8

SO2(g) + O2(g) → SO3(g)


Equation 9

SO3(g) + H2O(l) → H2SO4(aq)


H2SO4 acid is strong acid it contributes to increase H+ ion concentration of water through equation 11. This caused to occur acid rain.

Equation 10

H2SO4(aq) → SO42-(aq) + 2H+(aq)



How these acidic gases are added to the atmosphere?

The emission of NOx and SOx to the atmosphere can happen in both natural and anthropogenic ways.

  1. Volcanic eruption - Volcanic eruption is a natural way SO2 emits into the air.

  2. Thundering - Nitrogen gas (N2) and oxygen gas contain in the atmosphere. Nitrogen gas does not react with oxygen gas under normal conditions. When thundering happens a high amount of heat releases and temperature increases in a short period of time where thundering happens. This high temperature and high heat leads nitrogen gas to react with oxygen gas and form NO2 as shown in equation 11.

    Equation 11

    N2(g) + O2(g) → NO2(g)




  3. Burning of fossil fuel - Fossil fuel is the most used energy source in the world. To produce electricity, heat in various industries and for vehicles as well as for some of the domestic works also use fossil fuel. Fossil fuel contains Sulphur as an impurity. Therefore, when burning fossil fuel sulphur reacts with oxygen gas and form SO2 as shown in equation 12. SO2 emits into the atmosphere. If this burning temperature is too high, nitrogen gas can react with oxygen gas and can be form NO2 too.

    Equation 11

    S(s) + O2(g) → SO2(g)



What are the effects of acid rain?

Acid rain can affect plants, animals and the soil. It has an impact on ancient buildings like the Taj Mahal.

  • When acid rains fall on trees, the trees can dry out or die.
  • When acid rains fall, soil pH decreases and it cause to dissolve minerals of soil in the water and will lose its natural fertility or ability to produce natural fertility. Soil-dwelling animals will die due to change in pH.
  • When acidic water falls onto the water bodies pH will decrease. The preferable pH level of aquatic lives is 6-6.5. If pH of the water becomes lower than 5 aquatic lives cannot survive.
  • Buildings made of marble can be eroded by acid rains.

Solutions to prevent acid rains

The solution is reducing the NOx and SOx emission to the atmosphere. We cannot control natural incidents like volcanic eruptions or thundering. But we can control emissions from fossil fuel burning.


Reduce fossil fuel burning

  • Save energy: To save energy we can use public transport, bicycles instead of fuel-driven vehicles, etc.
  • Use regenerative energy: We can solar power, wind power, hydropower and biomass, etc.
  • Use high-efficiency equipment

Reduce SO2 emission from fossil fuel burning

  • Use fossil fuel that does not contain Sulphur or low Sulphur contain fuel.
  • Use desulphurization units to remove SO2 from flue gas before it exits to the atmosphere

Reduce NOx emission from fossil fuel burning

  • Controlling the temperature of a furnace can control the NOx formation in the furnace
  • Control the air to fuel ratio



What are the major industries which emit acidic gases to the atmosphere?

Cement industry: Cement industry emits sulfur dioxide gas. But, they use flue gas treatment system to control the emission of toxic and acidic gases. However after flue gas system, those gases are releases in low concentrations according the environmental regulations.

As cement industry, coal power plant is another industry which emit sulfur dioxide gas. According to the coal type, sulfur dioxide generation varies. If a low quality coal type is used, large amount of sulfur dioxide is released.

As above two industries, automobiles generate both sulfur dioxide and nitrogen dioxide which can produce acid rains.






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