Acid Rain - Definition, Causes, Effects and Measures

By : Neha Dhyani

Updated : Sep 14, 2022, 8:41

Acid rain is caused when Sulfur Dioxide (SO2) and Nitric Oxide (NOx) mix with water and oxygen in the atmosphere, transforming into sulphuric and nitric acids. This harmful chemical composition, called acid rain, then falls on the earth. It is one of the many consequences of air pollution.

The precipitation and deposition of rain with a low pH level (below 5.6) makes it acidic in nature, resulting in acid rain. Acid rain can result in many adverse consequences for the following:

  • Freshwaters
  • Forests
  • Soils
  • Insects
  • Aquatic life-form

What is Acid Rain?

Acid rain refers to the unusually acidic precipitation that falls on earth. This type of rainwater contains low pH levels, between 4-5 on average, and can cause harm to aquatic animals, plants, freshwater, and human infrastructure. Acid deposition on the earth can be of two forms:

  • Wet Deposition: When acid deposition contains water, it is called wet deposition. Acid rain, sleet, snow, and fog are wet depositions.
  • Dry Deposition: When acid deposition is made of dust and gases, it is called dry deposition. The acid chemicals may fall on the earth in the form of dust and stick to vehicles, buildings, and trees.

Acid Rain Definition

Acid rain can be defined as extremely acidic precipitation that falls on the earth and harms the environment. It is caused by polluted and acidic air in the atmosphere. The earliest reports of acid rain can be traced back to 17th century Europe. Specifically, after the Industrial Revolution, the general quality of air decreased due to greater emissions of sulfur dioxide and nitric oxides, resulting in the falling of acidic rain.

Acid Rain Causes

Acid rain can be caused due to both natural and man-made reasons. This type of rain results from polluted air and the main responsible gas for acid rain is sulfur dioxide and nitric oxides. The causes of acid rain are given below:

Natural Causes of Acid Rain

The natural causes of acid rain include the fact that the atmospheric composition of certain pristine environments holds higher quantities of sulfur and nitric oxides. Find more natural causes of acidic rain here:

  • Lightning strikes reach temperatures of around 30,000 Celsius, which causes the nitrogen and oxygen in the environment to react and create acidic chemicals. Lightning also causes wildfires, which increase carbonic and sulphuric acids.
  • Volcanic eruptions release a lot of harmful gases that react with water vapour and cause acid rain.
  • Terrestrial vegetation releases carbon dioxide while rotting, whereas aquatic plants (in oxygen-free environments) release methane. All of these gases increase acid in rainwater.

Human Causes of Acid Rain

Many human activities can result in greater pollution levels in the atmosphere, which, in turn, results in acid rain. Here are some human causes of acidic rain:

  • The burning of fossil fuels to generate electricity causes acid rain. Electricity power generators release huge amounts of Sulfur dioxide and Nitric oxide.
  • The smoke released by vehicles emits nitric oxides and sulfur dioxide, which are the major contributors to acid rain.
  • Oil refineries and heavy industries emit harmful gases into the air, increasing the acidic composition.

Acid Rain Effects

Acid rain effects the environment in a negative way. Since the low pH of the rain can result in it becoming extremely acidic, it can harm wildlife, flora, and even man-made objects. Some of the harmful effects of acid rain are as follows:

  1. The chemicals in acid rain dissolve into the soil and destroy its nutrients, making it infertile. This affects the growth of plants.
  2. Acid rain wears off the waxy coating of leaves, which harms plants' health, hindering the photosynthesis process.
  3. It releases toxic elements like aluminium into the water, harming the fish and disturbing aquatic life.
  4. Statues, buildings, vehicles, pipes, etc., erode faster when they are subjected to acid rain. This is also why the Taj Mahal is turning yellowish in color.
  5. Acid rain doesn't burn human skin but eventually affects our health. It thickens the air, causing visual impairment and worsening asthma, bronchitis, and other lung problems.

How to Prevent Acid Rain?

It is no secret that acid rain adversely affects us and our environment. It is the by-product of industrialization and modern lifestyles. With timely actions and the right measures, the impact of acid rain can be reduced. Here are some ways how it can be done:

  1. To reduce the acid composition in the atmosphere, we should use less energy and burn less fuel. Fossil fuels should be replaced with cleaner fuels (natural gas, coal with less sulfur).
  2. The waste gases can be treated with flue-gas desulphurization to prevent acid rain.
  3. Other sources of energy, like hydroelectricity, should be made more affordable so that they can replace fossil fuels.
  4. Acids can be neutralized by mixing limewater or powdered limestone in soil and water.

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FAQs on Acid Rain

Q1. What is acid rain?

Acid rain is the unusually acidic precipitation that falls on earth and harms aquatic animals, plants, freshwater, human infrastructure, etc, due to its extremely acidic nature. This type of rainwater contains low pH levels, between 4-5 on average.

Q2. Which is the main gas responsible for acid rain?

The main responsible gas for acid rain is sulfur dioxide and nitric oxide. These gases decrease the pH levels of the rainwater, making it extremely acidic. This water then falls in the form of rain and causes harm to the environment.

Q3. What are the causes of acid rain?

The causes of acid rain are mainly the decreased pH levels in the atmosphere. Here are some other causes:

  • Man-made causes - Burning fossil fuels, smoke released by vehicles, oil refineries, and heavy industries.
  • Natural causes - Volcanic eruptions, lightning, etc.

Q4. What contributes the most to acid rain?

Beyond all other natural and man-made causes, air pollution contributes the most to acid rain. Air pollution results in increased levels of sulfur dioxide and nitric oxide in the air, resulting in decreased pH levels of rain.

Q5. What is the pH level of acid rain?

Acid rain has a pH below 5.6, mostly between 4.2 and 4.4. The low pH levels of acid rain are caused by the increased levels of sulfur dioxide and nitric oxide in the atmosphere.