Acid Rain: Definition, Formation, Causes, Effects and Controls

Introduction to Acid Rain-

The term Acid Rain was coined in 1852 by Scottish chemist Robert Angus Smith, according to the Royal Society of Chemistry, which calls him the “father of acid rain.”

Smith decided on the term while examining rainwater chemistry near industrial cities in England and Scotland. He wrote about his findings in 1872 in the book “Air and Rain: The Beginnings of a Chemical Climatology”.

In the 1950s, scientists in the United States started studying the phenomenon, and in the 1960s and the early 1970s, acid rain became recognized as a regional environmental issue that affected Wester.

North-eastern America, North Western Europe, and India are facing an acute problem of acid rain. Acid rain has affected certain rivers, lakes, streams, and forests all over the world.

acid rain

What is Acid Rain?

Acid rain literally means “the presence of excessive acids in rainwater”.

A simple definition of acid rain can be ”A form of rain that is acidic in nature, called acid rain”.

Acid precipitation is a mixture of strong mineral acids sulphuric acid (H2SO4), nitric acid (HNO3), and in some locations, hydrochloric acid (HCI).

Acid rain ph is less than 5.6, and the value of distilled water is in equilibrium with atmospheric carbon dioxide.

Acid rain is also known as Acid precipitation or Acid Deposition. Usually, Air Pollution plays a major role in acid rain.

Acid rain is an environmental hazard that is the transponder in nature.

Formation of Acid Rain

Acid rain is one of the forms of acid deposition which can either be wet or dry, acid rain, snow, dew, fog, frost, and mist is the wet form of deposition, while dust particles containing sulphate and nitrates which settle on the ground is called dry deposition.

Wet Acid Rain Coal, fuel wood, or petroleum products have sulphur and nitrogen.

These elements, when burnt in atmospheric oxygen and converted into their respective oxides (SO2 and NO3), are highly soluble in water.

By anthropogenic and natural sources, oxides of sulphur and nitrogen enter the atmosphere. 

acid rain formation

Acid Rain Reactions-

Reaction with Sulphur

S+O2 = SO22S02 +O2 = 2SO3

Reaction with Nitrogen

NO+O3 = NO2+O2NO3 + NO2 = N2O5

When air is saturated with water droplets (humid conditions), N2O5 invariably reacts with water vapours to form droplets of HNO3

N2O5 + H2O = 2HNO3

Besides some HNO3 is also formed 

N2O3 + H2O = 2HNO3

SO3 in humid conditions forms droplets of H2SO4

SO2 + 1/2O2 + H2O = H2SO4


HNO3 and H2SO4 thus formed combine with HCI to generate precipitation, which is commonly referred to as acid rain.

The primary reason for concern is that acid deposition acidifies streams, and takes on coarse, sandy soils low in lime.

The effect is seen particularly in headwater areas and in wet montane environments, wherever sulphate loading from anthropogenic sources is strong.

The chemical and physical consequences of lake acidification include increased leaching of calcium from terrestrial soils, mobilization of heavy metals such as aluminium, zinc, and manganese and an increase in the transparency of lake waters.

The biological consequences include market changes in communities of aquatic plants and animals, with a progressive lessening of their diversity.

Acid deposition may further impoverish forest soils, developed on sandy substrata poor in lime.

As a consequence of the accelerated leaching of nutrients, such as phosphorous, potassium, magnesium and calcium from these soils, forest productivity would eventually be reduced.

Moreover, the acid sulphate particles that contribute to acid precipitation are in the size range that penetrates deep into the lung, and they may well exacerbate lung diseases and increase mortality rates.

Major Causes of Acid Rain-

Sulphur dioxide (SO2) and oxides of nitrogen and ozone to some extent are the primary causes of acid rain.

It is a natural phenomenon caused by chemicals emitted during volcanic eruptions, the decomposition of plants, and other causes.

However, the majority of acid rain today is caused by human activity such as burning fossil fuels (petroleum and coal) that come from factories, vehicles, power plants, and boilers.

Burning fossil fuels generates nitrogen oxide and dioxide and sulphur trioxide that is released into the atmosphere.

These react in the atmosphere with suspended droplets and other substances and generate acid rain, which can spread across hundreds of miles.

The causes of acid rain can be both natural and man-made. Both volcanoes and decaying vegetation release gases that result in the formation of acid rain.

However, the majority of gases come from man-made sources, such as fossil fuel combustion.

Both natural and man-made sources are known to play a role in the formation of acid rain. But, it is mainly caused by the combustion of fossil fuels which results in emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx).

There are mostly three major causes which contribute to acid rain and they are-

  1. Natural Sources
  2. Man-Made Sources
  3. Sources of Sulfur and Nitrogen Oxides

1. Natural Sources-

The major natural causal agent for acid rain is volcanic emissions.

Volcanoes emit acid-producing gases to create higher-than-normal amounts of acid rain or any other form of precipitation such as fog and snow to an extent of affecting vegetation cover and the health of residents within the surrounding.

Decaying vegetation, wildfires, and biological processes within the environment also generate acid rain-forming gases.

Dimethyl sulfide is a typical example of a major biological contributor to sulfur-containing elements in the atmosphere.

Lighting strikes also naturally produces nitric oxides that react with water molecules via electrical activity to produce nitric acid, thereby forming acid rain.

2. Man-Made Sources-

Human activities leading to chemical gas emissions such as sulfur and nitrogen are the primary contributors to acid rain.

The activities include air pollution sources emitting sulfur and nitrogen gases like factories, power generation facilities, and automobiles.

In particular, the use of coal for electrical power generation is the biggest contributor to gaseous emissions leading to acid rain.

Automobiles and factories also release high scores of gaseous emissions on daily basis into the air, especially in highly industrialized areas and urban regions with large numbers of car traffic.

These gases react in the atmosphere with water, oxygen, and other chemicals to form various acidic compounds such as sulfuric acid, ammonium nitrate, and nitric acid.

As a result, these areas experience exceedingly high amounts of acid rain.

The existing winds blow these acidic compounds over large areas across borders and they fall back to the ground in the form of acid rain or other forms of precipitation.

Upon reaching the earth, it flows across the surface, absorbs into the soil and enters into lakes and rivers, and finally gets mixed up with seawater.

The gases such as sulfur dioxide and nitrogen oxides are primarily gases occurring from electric power generation by burning coal and are responsible for acid rain.

3. Sources of Sulfur and Nitrogen Oxides-

Most of the nitrogen and sulfur oxides result from human activities.

The major sources of sulfur dioxide emissions are electric utilities (60%), industrial combustion (17%), and industrial processes (8%).

Transportation, with internal combustion engines(IC Engine), produced more than 50% of all NOx with additional emissions from electric sources (26%) and industrial combustion (14%).

Agricultural activities, in particular manure handling, are the largest source of ammonia emissions but some ammonia is also given off by industry and by the transportation sector.

Ammonium sulphate, ammonium bisulphate, and ammonium nitrate are part of the atmospheric particulates.

Frequently there are particles of nitric and sulphuric acid mixed with these salts. The aerosols eventually fall out of the atmosphere and are the source of dry deposition of nitrogen and sulphur oxides.

The acids react with aluminium and calcium oxides in the soil, forming water-soluble calcium and aluminium salts of nitrate and sulphate anions.

As rain flows over and through the soil, these soluble materials dissolve and flow into bodies of water.

In regions that have a build-up of snow and ice through the winter, the wet and dry deposition of acids in the snow leads to rapid acidification of lakes and streams in the spring.

Effects of Acid Rain-

These are the places where the effect of acid rain can be clearly seen and they are-

  1. On vegetation
  2. On Humans
  3. On Microorganism
  4. Socio-Economic Impact
  5. Impact on soil
  6. Effect on Public Health
  7. Effect on Forests
  8. Acid Rain Damage on Metals
  9. Effects on Fish and Other Aquatic Life
  10. Effects on Plants and Trees
  11. Effect on Water Resources

1. Effect On Vegetation-

Acid rain affects trees and undergrowth in forests in several ways, causing reduced growth or abnormal growth:

  • Discoloration and loss of foliar mass.
  • Loss of feeder root biomass especially in conifers. 
  • Premature senescence (aging) of older needles in conifers.
  • Increase in susceptibility of damage to the secondary root and foliar pathogen.
  • Death of herbaceous vegetation beneath affected trees.

2. Effect On Humans-

  • Bad smell, reduced visibility, irritation of skin, eyes, and respiratory tract.
  • Diseases include chronic bronchitis, pulmonary emphysema, and cancer.
  • Some indirect effects like food poisoning, and infiltration of drinking water. 
  • An increase in the level of toxic heavy metals like manganese, copper, cadmium, and aluminium also contribute to detrimental effects on humans.

3. Effect On Micro-Organism-

  • Ph determines the proliferation of any microbial species in a particular environment rate at which it can reproduce.
  • The optimum ph of most bacteria and protozoa is near neutrality, most fungi prefer an acidic environment, and most blur-green algae prefer an alkaline environment.
  • Due to Continuous acid rain, microbial species in the soil and water shift from bacteria-bound to fungi-bound and cause an imbalance in the microflora.
  •  This cause a delay in the decomposition of soil organic material, and an increase in fungal disease in aquatic life and forests.

4. Effect On Socio-Economic-

  • The adverse effect of acid on farming and fishing leads to the deterioration of life quality indices like GNP and per capita income, especially in predominantly agricultural and developing countries like INDIA.

5. Effect On Soil-

  • The exchange between hydrogen ions and the nutrients cations like potassium and magnesium in the soil cause leaching of the nutrients, making the soil infertile. . This is accompanied by a decrease in the respiration of soil organisms.
  • An increase in ammonia in the soil due to a decrease in other nutrients decreases the rate of decomposition.

Must Raed- Soil Pollution- Definition, Sources, Effects, Prevention and Control

6. Effect On Public Health-

  • Sulphur dioxide and nitrogen oxide gases and their particulate matter like sulphates and nitrates, degrade visibility and can cause accidents, leading to injuries and deaths.
  • Human health is not directly affected by acid rain because acid rainwater is too dilute to cause serious health problems.
  • Dry depositions can cause serious health problems when inhaled. Intensified levels of acid depositions in dry form in the air can cause lung and heart problems such as bronchitis and asthma.

7. Effect On Forests-

  • It makes trees vulnerable to disease, extreme weather, and insects by destroying their leaves, damaging the bark, and decreasing their growth.
  • Forest damage due to acid rain is most evident in Eastern Europe – especially Germany, Poland, and Switzerland.

8. Effect On Metals-

  • Acid rain of pH value 3 to 5 is known as ‘stone cancer’. It is observed that lots of buildings and historical monuments are harmed worldwide because of acid rain.
  • At present both the railway and airplane industries have to spend a lot of money to repair the corrosive damage done by acid rain.
  • Marble, limestone, sandstone, and ancient monuments are dissolved by acid rain. Metals, paints, textiles, and ceramics can readily be corroded due to acid rain.

9. Effect On Fish and Other Aquatic Life-

  • Acid rain affects ponds, rivers, streams, lakes, gulfs, seas, oceans, etc. by increasing their acidity. As a result fish and other aquatic creatures can no longer live, Acid rain affects fish directly or indirectly.
  • Direct effects are the alteration of blood chemistry, retardation of egg development, etc. Indirect effects are the reduction in the kinds and supply of food available to fish, the creation of toxic to fish, etc.
  • Some lakes have become so acidic that they are no longer able to support fish life. At a pH, lower than 5 most fish eggs will not hatch, and also kill adult fishes.
  • At pH 6, freshwater shrimp cannot live and at pH< 4.5, all fish die. Snails, crayfish, and certain other invertebrate animals are very sensitive to acid and may rapidly disappear if acidity increases.
  • Some microorganisms, plankton, insects, and blue-green algae are the food of fishes; these are affected by acid rain, and ultimately fishes are declined due to the lack of food.

10. Effect On Plants and Trees-

  • The pH value between 5 and 8 is the ideal pH range for the plant’s growth, and out of these ranges in soils, plants face difficulties to germinate or grow.
  • No plants grow if the pH is less than 3.7. Acid rain reduces plant growth and yield due to foliar injury.
  • It decreases vital nutrients of soil that are formed by nitrogen (N), calcium (Ca), magnesium (Mg), potassium (K), etc.
  • Abundant of aluminium (Al), mercury (Hg), manganese (Mn), cadmium (Cd), and lead (Pb) in the soil in the non-toxic form, in the presence of acid rain these become toxic in soil and cause damage or death to plants and trees.
  • When calcium is decreased from the needles of red spruce, trees become less cold tolerant and exhibit winter injury and even may die.
  • Acid rain changes the chemistry of leaf surfaces and decreases pollen germination, fertilization and seed development, and fruit formation.
  • It adds H+ ions to the soil which reacts with compounds of calcium, magnesium, and potassium from soil particles.
  • Useful micro-organisms which release nutrients from decaying organic matter, into the soil are killed off. As a result plants and trees lose nutrients. The roots of plants and trees are affected by acid rain, and hence their life span reduces.

11. Effect On Water Resources-

  • Harmful substances like aluminium, lead, and mercury, are washed away from the soil to neighboring water resources by the acid rain, thus resulting in water pollution.
  • These chemicals and their acids do not allow the flora and fauna to grow and reproduce. This also results in the depletion of oxygen levels in the water, thereby making it difficult for fish to respire.
  • They die due to suffocation and poisoning caused by the presence of these chemicals. Birds that feed on fish that are affected by these harmful chemicals accumulate these elements in their systems.
  • These chemicals are then passed on to animals that feed on such birds. In this manner, chemicals get introduced to each tropic level of the food chain.
  • In each tropic level, the concentration of chemicals gets multiplied. This phenomenon is known as biomagnification.

12. Effect On Oceans

  • Oceans are an important reservoir of CO2 absorbing a significant quantity of it (one-third) produced by anthropogenic activities and effectively buffering climate change.
  • The uptake of atmospheric carbon dioxide is occurring at a rate exceeding the natural buffering capacity of the oceans.
  • Increasing acidity depresses metabolic rates and immune responses in some organisms.

Acid Rain Prevention and Control-

Here are some of the Steps For Minimisation of Acid Rain-

  1. Restoring Damaged Environments
  2. Alternative Energy Sources
  3. Individual, National/State, and International Actions
  4. Policy Interventions
  5. Energy conservation
  6. Transportation
  7. Alternative fuels

1. Restoring Damaged Environments-

  • The use of limestone or lime, a process called liming, is a practice that people can do to repair the damage caused by acid rain to lakes, rivers, and brooks.
  • Adding lime into acidic surface waters balances the acidity. It’s a process that has extensively been used, to keep the water pH at optimum.
  • Even though, liming is an expensive method and has to be done repeatedly. Furthermore, it only offers a short-term solution at the expense of solving the broader challenges of SO2 and NOx emissions and risks to human health.
  • Nevertheless, it helps to restore and allow the survival of aquatic life forms by improving chronically acidified surface waters.

2. Alternative Energy Sources-

  • Besides fossil fuels, there is a wide range of alternative energy sources that can generate electrical power. These include wind energy, geothermal energy, solar energy, hydropower, and nuclear power.
  • Harnessing these energy sources can offer effective electrical power alternatives instead of using fossil fuels. Fuel cells, natural gas, and batteries can also substitute the use of fossil fuels as cleaner energy sources.
  • As of today, all energy sources have environmental and economic costs as well as benefits. The only solution is using sustainable energy that can protect the future.

3. Individual, National/State, and International Actions-

  • Millions of people, directly and indirectly, contribute to SO2 and NOx emissions.
  • Mitigation of this challenge requires individuals to be more informed about energy conservation and ways of reducing emissions such as turning off lights or electrical appliances when not using them, using public transport, using energy-efficient electrical appliances, and use of hybrid vehicles or those with low NOx emissions.
  •  The damage to water bodies can be prevented and eliminated by adding lime. Many chemicals such as caustic soda, sodium carbonate, slacked lime, and limestone is most popular for increasing the pH of acidified water. Liming completely removes only some of the symptoms of acidification and it is expensive and also not a real cure.
  •  Reducing the amount of emission of SO, from power plants by burning less fossil fuels whereby also focusing on alternate energy sources like Tidal, Wind or Hydropower will do great wonders in reducing Acid Rain.

4. Policy Interventions-

  • There have been many policy interventions for the prevention of Acid Rain by the global community well as India. 
  • In the 1980s, Several north-eastern States of the USA and Ontario, Canada, sued the US Environment Protection Agency in 1980 to take action to control acid precursor emissions emanating from the United States, and subsequently, US congress formed the National acid precipitation assessment program (NAPAP) and mandated it to conduct a 10-year scientific, technological and economic study of the acid rain menace under the Acid Precipitation Act of 1980.
  • SO2 emission is reduced by governments making it mandatory for energy producers to clean smoke stacks by using scrubbers that trap pollutants before they are released into the atmosphere.
  • NO2 emission is reduced by the introduction of catalytic converters in cars, where they chemically convert emissions to less harmful substances before the gas leaves the tailpipe of the car.
  • In India, stress is laid down on controlling Acid Rain by two interventions, firstly reducing SO2 emissions and secondly by reducing NOx emissions.
  • Also, India has decided to skip BS-V vehicles altogether as it is directly adopting BS-VI norms from BS-IV norms, being commenced from 2020 all over the country instead of earlier thought 2022.

5. Energy Conservation-

  • The biggest step that can be taken to prevent acid rain is to decrease energy consumption.
  • Close the lights whenever you leave the room and turn off computers and televisions when you’re not using them.
  • Whenever you’re not using an electrical appliance, simply shut it off to conserve energy.

6. Transportation-

  • Because cars are a major contributor to acid rain pollution, it’s important to find alternative modes of transportation in an effort to decrease our dependency on fossil fuels.

6. Alternatives Fuels-

  • An excellent way to prevent acid rain is to stop using nonrenewable fuels and switch over to renewable sources of energy such as solar energy, wind, and water energy.
  • As the technology for these alternative energies increases, they will become more accessible to the public.


The causes and effects of acid rain are well understood now. Fortunately, the effects have been limited, and there is no evidence to suggest that they will worsen in the next few decades with present emissions. 

Acid rain can be greatly reduced by changing transportation habits. Reduction in the amount of time spent in the car by walking, biking, or taking public transportation. Be always fuel efficient and use green vehicles.


Wrapping Up-

This was all about Acid Rain. I believe I have covered every aspect of this. Please leave a note if I missed something, and I’ll try to add it as soon as possible.

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You should never miss the following articles, which are all highly recommended:

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Water Pollution- Definition, Sources, Causes, Effects Prevention and control



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