Acid Rain; Definition, Characteristics, Causes, Effects and Solutions

Rain or precipitation is a stage of the water cycle in our ecosystems. Usually, water is believed to be neutral in pH when it rains. But whenever the pH of rainwater is below 5.6, the rain is called Acid Rain. There are different causative agents for acid rain, having numerous effects on the entire ecosystem. What we need is to minimize the causative factors/reasons for acid rain and find what are sustainable solutions to prevent acid rain. As well as the ways of effectively managing the current impacts of acid rain is also essential.

Here is our tutorial to explore the characteristics of acid rain, the reasons of occurring acid rain, the effects of acid rain on our earth, and the sustainable ways to prevent acid rain.

What is Acid Rain?

If the pH of rainwater is below 5.6, the rain becomes acid rain. When that acid rain touches the substrates, reactions can occur degrading such surfaces of substrates.

Usually, when it rains, Atmospheric Carbon dioxide reacts with rainwater and forms Carbonic acid (H₂CO₃).

The formed carbonic acid maintains an equilibrium with water while releasing H⁺ into the medium. Because it is a weak acid.

These reactions make the water acidic (less pH) compared to pure rainwater. Due to the dissolving of carbon dioxide in rainwater, the pH becomes 5.6.

But the most important fact is that carbon dioxide itself can't form acid rain. Because the pH of acid rain is less than 5.6.

What are the Causative Agents of Acid Rain?

Which Gases Cause Acid Rain

Mainly two types of oxides in the gaseous state cause acid rain.

Oxides of nitrogen (NO_x)
Oxides of sulfur (SO_x)

How do oxides of Nitrogen (NO_x) cause acid rain?

The most common oxides of nitrogen that cause acid rain are

Nitrogen dioxide (NO2)
Nitric Oxide (NO)

The atmospheric Nitrogen dioxide dissolves in rainwater as follows. Then it makes Nitrous oxide and Nitric acid.

Then Nitric oxide is oxidized to Nitrogen dioxide in the presence of atmospheric oxygen.

Since Nitric acid is a strong acid, it can increase H⁺ ions concentration in rainwater while reducing the pH of water below 5.6. That's how oxides of Nitrogen cause acid rain.

How do oxides of Sulfur (SO_x) cause acid rain?

Mainly the following oxides of sulfur cause acid rain.

Sulfur dioxide (SO₂)
Sulfur dioxide (SO₃)

When SO₂ dissolves in rainwater it produces Sulfurous acid (H₂SO₃).

Further sulfur dioxide (SO₂) can react with oxygen gas and form sulfur trioxide (SO₃).

This produced sulfur trioxide reacts with rainwater and creates Sulfuric acid (H₂SO₄)

The produced strong acid; H₂SO₄ can release H⁺ ions into the rainwater and increase H⁺ ion concentration. It converts rainwater into acid rain.

How Acidic Gases are added to the Atmosphere?

As discussed above the emission of NO_x and SO_x to the atmosphere cause for acid rain. It can happen naturally or in anthropogenic (with human involvement) ways.

Natural emissions of NOx and SOx to the atmosphere

Volcanic eruptions naturally emit SOx into the atmosphere
Trough thundering

Even though both Nitrogen gas (N₂) and Oxygen gas (O₂) are available in the atmosphere, these gases don't react under normal conditions. When it thunders, a high amount of heat is released into the atmosphere resulting in a temperature increase for a short time period. This condition leads to the reaction of nitrogen gas and oxygen gas, resulting in the formation of nitrogen dioxide (NO₂).

Anthropogenic emissions of NOx and SOx to the atmosphere

Mainly it is due to the burning of fossil fuels. Fossil fuel is used for different purposes from domestic purposes to industrial activities, becoming the most used energy source in the world. Whenever we burn fossil fuel, we burn sulfur which was in fossil fuel as an impurity.

As a result,

It emits Sulfur dioxide.

It makes a temperature rise, letting nitrogen gas react with oxygen and make Nitrogen dioxide as shown above.

The Major Industries Emit Acidic Gase to Atmosphere

The cement industry emits sulfur dioxide gas.

But now the industry uses flue gas treatment systems to control the emission of acidic gases as well as toxic gases. By following the environmental regulations, those gases are released in low concentrations into the atmosphere after the flue gas system.

The coal power plant is another industry that emits sulfur dioxide gas.

If a low-quality coal type is used, a large amount of sulfur dioxide is released because sulfur dioxide generation varies according to the coal type.

Automobiles generate both sulfur dioxide and nitrogen dioxide

What are the Effects of Acid Rain?

The serious effects of acid rain impact entire ecosystems including plants, animals, and soil.

Plants can dry out or die when they are touched by acid rain due to burning. In the long run, it reduces photosynthetic efficiency making a great threat to food security. When trees die, global warming enhances making it more difficult to survive on Earth.
The pH of the soil is reduced when acid rain falls on the soil. The water that comes from acid rain can be either absorbed by the soil or flowed on the surface till it meets a surface water body. When absorbed by the soil, the reduced pH impacts the dissolution of available minerals in the soil in water. The loss of soil fertility affects food production. As well as the organisms that live in the soil can be affected by the changed pH level. So this is a threat to their existence.
When acid rain falls onto the water bodies, the pH of water bodies will decrease. The aquatic lives prefer 6-6.5 as the pH level to survive. So in this situation, their survival is impossible. This can result in the loss of existence of many aquatic organisms.
Even the constructed ancient buildings like the Tajmahal can be damaged due to the eroding of marble due to acid rain.
When humans are exposed to acid rain, it can burn our skin, and cause skin diseases even cancers.

How Can We Prevent Acid Rain?

Since the emission of NO_x and SO_x to the atmosphere causes acid rain, we can prevent acid rain by preventing such gaseous emissions to the environment. The natural incidents causing acid rain (volcanic eruptions or thunder) are not within our capability of controlling.

Reduction of fossil fuel burning

Use public transport
Use bicycles instead of fuel-driven vehicles whenever possible
Use regenerative power sources like wind power, hydropower, solar power, and biomass for electricity generation
Shift for high-efficiency equipment

Reduction of SO₂ emission from fossil fuel burning

Use fossil fuel that does not contain Sulphur
Use low Sulphur-containing fuel.
Remove SO₂ from flue gas before it exits to the atmosphere by using desulphurization units

Reduction of NO_x emission from fossil fuel burning

Control NO_x formation in the furnace by controlling the temperature of the furnace
Control the air-to-fuel ratio