Which one of the following is a cause of acid rains?

Question

Prepp Team · Accepted Answer

Understanding the Causes of Acid Rain

Acid rain is a form of precipitation that is unusually acidic, meaning it has elevated levels of hydrogen ions (low pH). It is primarily caused by atmospheric pollutants, mainly sulfur dioxide (\(\text{SO}_2\)) and nitrogen oxides (\(\text{NO}_x\)). These pollutants react in the atmosphere with water, oxygen, and other chemicals to form acidic compounds like sulfuric acid (\(\text{H}_2\text{SO}_4\)) and nitric acid (\(\text{HNO}_3\)). These acids then fall to the ground as rain, snow, fog, or even dry particles, causing environmental damage.

Primary Pollutants Causing Acid Rain

The main culprits behind acid rain are gases released into the atmosphere from various sources, predominantly human activities:

Sulphur Dioxide (\(\text{SO}_2\)): Released mainly from the burning of fossil fuels (coal and oil) in power plants, industrial boilers, and vehicles. Volcanic eruptions are a natural source.
Nitrogen Oxides (\(\text{NO}_x\)): Primarily released from the combustion of fossil fuels in vehicles, power plants, and industrial processes. Lightning strikes are a natural source.

Once in the atmosphere, these gases undergo complex chemical transformations:

For Sulphur Dioxide:

\(\text{SO}_2 \text{(gas)} + \text{H}_2\text{O} \text{(liquid)} \rightarrow \text{H}_2\text{SO}_3 \text{(aqueous sulphurous acid)}\)

\(2\text{SO}_2 \text{(gas)} + \text{O}_2 \text{(gas)} \rightarrow 2\text{SO}_3 \text{(gas)}\)

\(\text{SO}_3 \text{(gas)} + \text{H}_2\text{O} \text{(liquid)} \rightarrow \text{H}_2\text{SO}_4 \text{(aqueous sulfuric acid)}\)

For Nitrogen Oxides (specifically nitrogen dioxide, \(\text{NO}_2\), which \(\text{NO}\) oxidizes to):

\(3\text{NO}_2 \text{(gas)} + \text{H}_2\text{O} \text{(liquid)} \rightarrow 2\text{HNO}_3 \text{(aqueous nitric acid)} + \text{NO} \text{(gas)}\)

or

\(2\text{NO}_2 \text{(gas)} + \text{H}_2\text{O} \text{(liquid)} + 0.5\text{O}_2 \text{(gas)} \rightarrow 2\text{HNO}_3 \text{(aqueous nitric acid)}\)

Analysing the Given Options

Let's look at the options provided in the question:

Ozone: Ozone (\(\text{O}_3\)) is a gas found in the atmosphere. While it is a pollutant at ground level and contributes to smog and respiratory problems, it is not considered a primary direct cause of acid rain. It can participate in reactions that form acids, but \(\text{SO}_2\) and \(\text{NO}_x\) are the main precursors.
Ammonia: Ammonia (\(\text{NH}_3\)) is an alkaline gas. While it is a significant air pollutant and can contribute to secondary particulate formation (like ammonium sulfate and ammonium nitrate) which are components of acid deposition, it generally acts to neutralize acidity rather than cause it directly.
Sulphur dioxide: As discussed, sulphur dioxide (\(\text{SO}_2\)) is one of the two major gaseous pollutants that react in the atmosphere to form sulfuric acid, a key component of acid rain.
Carbon monoxide: Carbon monoxide (\(\text{CO}\)) is a toxic gas produced by incomplete combustion. It is a significant air pollutant but is not a direct cause of acid rain. It does not readily form acidic compounds like sulfuric or nitric acid in the atmosphere.

Conclusion based on Analysis

Based on the analysis of how acid rain forms and the role of different atmospheric gases, Sulphur dioxide is a well-established primary cause of acid rain.

Role of Different Gases in Acid Rain
Gas	Role in Acid Rain
Sulphur dioxide (\(\text{SO}_2\))	Primary cause (forms sulfuric acid)
Nitrogen oxides (\(\text{NO}_x\))	Primary cause (forms nitric acid)
Ozone (\(\text{O}_3\))	Not a primary direct cause; contributes to other air pollution issues like smog; can be involved in oxidation processes.
Ammonia (\(\text{NH}_3\))	Generally neutralizes acidity; contributes to particulate formation (ammonium salts which are part of acid deposition).
Carbon monoxide (\(\text{CO}\))	Air pollutant but not a direct cause of acid rain.

Revision Table: Acid Rain Causes and Effects

Aspect	Details
Definition	Precipitation (rain, snow, fog) with a pH lower than normal (typically below 5.6) due to atmospheric pollution.
Main Pollutants	Sulphur Dioxide (\(\text{SO}_2\)) and Nitrogen Oxides (\(\text{NO}_x\))
Sources	Burning of fossil fuels (power plants, vehicles, industry), volcanic eruptions.
Acid Formation	\(\text{SO}_2\) forms sulfuric acid (\(\text{H}_2\text{SO}_4\)). \(\text{NO}_x\) forms nitric acid (\(\text{HNO}_3\)).
Environmental Impacts	Acidification of lakes and streams, damage to forests, damage to buildings and statues, effects on human health (indirectly via polluted water/food or directly via respiratory issues linked to precursor pollutants).

Additional Information on Acid Rain Formation and Impacts

Acid rain is a significant environmental problem. The acids can be deposited in two ways: wet deposition (rain, snow, fog) and dry deposition (acidic particles and gases). Both forms can cause harm. Dry deposited particles and gases can be washed from surfaces by rain, becoming acid precipitation.

The environmental effects are widespread:

Aquatic Ecosystems: Acidification of lakes and streams makes them uninhabitable for fish and other aquatic organisms.
Forests and Vegetation: Acid rain can damage leaves, stunt growth, and make trees more susceptible to disease and pests. It also leaches nutrients from the soil.
Buildings and Materials: Acids react with materials like limestone and marble, causing corrosion and damage to historical buildings, statues, and other structures.
Human Health: While not directly harmful to touch, the pollutants that cause acid rain (\(\text{SO}_2\) and \(\text{NO}_x\)) can contribute to respiratory problems like asthma and bronchitis.

Efforts to reduce acid rain focus on reducing emissions of \(\text{SO}_2\) and \(\text{NO}_x\) from power plants and vehicles through cleaner fuels, scrubbers, and catalytic converters.