Statement I : Oxides of sulfur and nitrogen present in high concentration in air are dissolved in rain drops. Statement II : Oxyacids of sulfur and nitrogen make rain water acidic.

1. Air Pollutants: Primary vs. Secondary (basic)

To understand the complex problem of acid rain, we must first distinguish between how pollutants enter our atmosphere. Air pollutants are broadly classified into two categories based on their origin: Primary and Secondary pollutants. Understanding this distinction is fundamental because it determines how we monitor, regulate, and eventually mitigate environmental damage. Environment, Shankar IAS Academy, Environmental Pollution, p. 64

Primary pollutants are those that are emitted directly from a source into the atmosphere in the form in which they were produced. Common examples include Carbon Monoxide (CO) from vehicle exhaust, Sulfur Dioxide (SO₂) from coal-burning power plants, and Nitrogen Oxides (NOₓ) from industrial furnaces. These are the pollutants we can often trace back to a specific chimney or tailpipe. In India, the Air Act of 1981 provides the legal framework to monitor these emissions through various Pollution Control Boards. Environment and Ecology, Majid Hussain, Biodiversity and Legislations, p. 15

Secondary pollutants, on the other hand, are not emitted directly. Instead, they form in the atmosphere when primary pollutants undergo chemical reactions with each other or with natural components like water vapor and sunlight. A classic example is Ground-level Ozone (O₃), which forms when NOₓ and hydrocarbons react in the presence of sunlight. Most importantly for our topic, when primary pollutants like SO₂ and NOₓ react with moisture in the air, they transform into Sulfuric Acid (H₂SO₄) and Nitric Acid (HNO₃). These acids are secondary pollutants and are the primary drivers of acid rain. Environment, Shankar IAS Academy, Environmental Pollution, p. 70-72

Sources: Environment, Shankar IAS Academy, Environmental Pollution, p.64, 70, 72; Environment and Ecology, Majid Hussain, Biodiversity and Legislations, p.15

2. Major Atmospheric Pollutants: SOₓ and NOₓ (basic)

To understand acid rain, we must first meet its two primary chemical parents: Oxides of Sulfur (SOₓ) and Oxides of Nitrogen (NOₓ). These are not just simple gases; in the world of environmental chemistry, they are known as precursors. This means they are the raw materials that, when released into the atmosphere, undergo chemical transformations to become something much more potent.

These pollutants enter our air through two distinct pathways: natural and anthropogenic (human-made). While nature contributes through volcanic eruptions and biological decay (releasing SO₂) or lightning and soil microbes (releasing NOₓ), human activity has tilted the scales. The burning of fossil fuels in thermal power plants is a massive source of sulfur dioxide, while transport vehicles and internal combustion engines are the primary culprits for nitrogen oxides Environment, Shankar IAS Academy, India and Climate Change, p.315. In fact, SO₂ and NO₂ are considered such critical indicators of air quality that they are major components of the Air Quality Index (AQI) Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.119.

The transition from "pollutant gas" to "acid rain" involves a process called reactive dissolution. When these gases rise into the atmosphere, they encounter moisture (water vapor) and oxidizing agents.

• Sulfur Dioxide (SO₂): Reacts with water to form sulfurous acid (H₂SO₃), or it can be further oxidized to sulfur trioxide (SO₃), which then reacts with water to form the much stronger sulfuric acid (H₂SO₄).
• Nitrogen Dioxide (NO₂): Interacts with water or hydroxyl radicals to produce nitric acid (HNO₃).

It is also worth noting that these gases have "side jobs" in environmental destruction. For instance, while we focus on their role in acid rain, Nitric Oxide (NO) also acts as a catalyst that destroys the ozone layer in the stratosphere Environment, Shankar IAS Academy, Ozone Depletion, p.269. This makes SOₓ and NOₓ multi-threat pollutants that affect the earth from the ground level all the way up to the upper atmosphere.

Sources: Environment, Shankar IAS Academy, India and Climate Change, p.315; Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.119; Environment, Shankar IAS Academy, Ozone Depletion, p.269; Physical Geography by PMF IAS, Earths Atmosphere, p.270

3. Connected Concept: Photochemical and Classical Smog (intermediate)

Welcome back! Now that we’ve explored how pollutants enter the atmosphere, we must distinguish between the two primary ways these pollutants manifest as "smog." The term smog was originally coined by Dr. H.A. Des Voeux in 1905 to describe a mixture of smoke and fog Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.40. However, modern environmental science categorizes smog into two distinct types based on their chemistry and the climatic conditions in which they form: Classical Smog and Photochemical Smog.

Classical Smog (also known as London Smog) typically occurs in cool, humid climates. It is a mixture of smoke, fog, and Sulfur Dioxide (SO₂). Because of the high concentration of SO₂ and particulate matter, it acts as a reducing mixture. In contrast, Photochemical Smog (or Los Angeles Smog) occurs in warm, dry, and sunny climates. This type of smog is formed through the interaction of sunlight with Nitrogen Oxides (NOx) and Volatile Organic Compounds (VOCs), primarily from vehicular exhaust and industrial emissions Environment, Shankar IAS Academy, Environmental Pollution, p.64-65. Because it contains high concentrations of oxidizing agents like Ozone (O₃), it is known as an oxidizing smog.

The formation of ground-level ozone is a hallmark of photochemical smog. While ozone in the stratosphere is a protector, ozone at the ground level is a secondary pollutant that causes intense eye irritation and respiratory distress Environment, Shankar IAS Academy, Environmental Pollution, p.65. Understanding these differences is crucial because the same NOx and SO₂ that create these smogs are also the primary drivers of acid rain, which we will explore in the next step.

Sources: Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.40; Environment, Shankar IAS Academy, Environmental Pollution, p.64; Environment, Shankar IAS Academy, Environmental Pollution, p.65

4. Connected Concept: Nitrogen Deposition and Eutrophication (intermediate)

In our previous steps, we looked at how nitrogen oxides (NOₓ) contribute to acid rain. However, the impact of these pollutants doesn't stop with lowering the pH of rainwater. When nitrogen compounds fall from the atmosphere—a process known as nitrogen deposition—they act as a potent, unintended fertilizer for both land and water. In many ecosystems, nitrogen is a "limiting nutrient," meaning its natural scarcity keeps the growth of plants and algae in check. When acid rain or dry particles deliver an excess of nitrogen into lakes, ponds, or coastal waters, it disrupts this delicate balance, triggering a chain reaction called eutrophication.

The process of eutrophication unfolds in a specific, destructive sequence. First, the influx of nitrogen (and often phosphorus from runoff) leads to a population explosion of microscopic algae and phytoplankton, known as an algal bloom Shankar IAS Academy, Aquatic Ecosystem, p.39. While a green or red pond might look vibrant, the aftermath is deadly. As these massive quantities of algae complete their short life cycles and die, they sink to the bottom. Here, aerobic bacteria begin the task of decomposition. These bacteria consume vast amounts of dissolved oxygen (DO) from the water to break down the organic matter. This leads to hypoxia (low oxygen) or even "dead zones" where fish and other aquatic organisms cannot survive Shankar IAS Academy, Ocean Acidification, p.264.

Furthermore, this process has a hidden chemical sting. As bacteria decompose the algae, they release significant amounts of CO₂. When this CO₂ dissolves in the water, it forms carbonic acid, further lowering the pH of the aquatic environment. This means that nitrogen deposition causes a "double blow": it contributes to acidification both directly (as nitric acid in rain) and indirectly (through the biological decay of algal blooms) Shankar IAS Academy, Ocean Acidification, p.264. This phenomenon is particularly severe in coastal waters where agricultural runoff and atmospheric deposition meet Majid Hussain, Environmental Degradation and Management, p.18.

Sources: Shankar IAS Academy, Aquatic Ecosystem, p.39; Shankar IAS Academy, Ocean Acidification, p.264; Majid Hussain, Environmental Degradation and Management, p.18; Shankar IAS Academy, Functions of an Ecosystem, p.20

5. The Chemistry of Acid Rain: Formation of Oxyacids (exam-level)

To understand acid rain, we must look at the atmospheric chemistry where primary pollutants transform into potent secondary pollutants. The atmosphere receives Oxides of Sulfur (SO₂) and Nitrogen (NOₓ) from both industrial activities and natural sources like volcanic eruptions. These gases do not stay as gases for long; they undergo a process of reactive dissolution and oxidation in the presence of atmospheric moisture and sunlight to form oxyacids—acids that contain oxygen atoms in their structure Environment, Shankar IAS Academy, Chapter 5, p.103.

The transformation of Sulfur Dioxide (SO₂) follows two main pathways. First, SO₂ can react directly with water droplets to form Sulfurous acid (H₂SO₃), a relatively weak acid. However, a more significant transformation occurs when SO₂ is oxidized (often aided by catalysts like floating particulate matter or sunlight) to form Sulfur Trioxide (SO₃). When SO₃ reacts with water vapor, it produces Sulfuric acid (H₂SO₄), which is a strong mineral acid and a major contributor to the acidity of rainfall Environment and Ecology, Majid Hussain, Chapter 6, p.8.

Similarly, Nitrogen oxides (NOₓ), such as Nitrogen Dioxide (NO₂), undergo complex reactions. In the presence of sunlight and atmospheric moisture, NO₂ reacts with hydroxyl radicals (OH) or water to form Nitric acid (HNO₃) and sometimes Nitrous acid (HNO₂). These chemical reactions can be summarized as follows:

The accumulation of these oxyacids in clouds significantly lowers the pH of precipitation. While normal rainwater has a pH of about 5.6 (due to dissolved CO₂ forming weak carbonic acid), the presence of H₂SO₄ and HNO₃ can push the pH down to 4 or even lower, creating the phenomenon we identify as acid rain Environment, Shankar IAS Academy, Chapter 5, p.101.

Sources: Environment, Shankar IAS Academy, Chapter 5: Environmental Pollution, p.101, 103; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Chapter 6: Environmental Degradation and Management, p.8

6. Environmental Impact: pH Levels and Stone Leprosy (intermediate)

To understand the environmental impact of acid rain, we must first look at the pH scale, which measures how acidic or basic a substance is on a scale of 0 to 14. While pure water has a neutral pH of 7.0, "normal" rainwater is slightly acidic, with a pH of approximately 5.6. This is because it naturally dissolves atmospheric Carbon Dioxide (CO₂) to form a weak carbonic acid. However, when the pH of precipitation falls below 5.6, it is officially classified as Acid Rain Shankar IAS Academy, Environmental Pollution, p.101. This drop in pH occurs when oxides of sulfur (SO₂) and nitrogen (NOₓ) from industrial emissions and vehicular exhaust react with atmospheric moisture to form potent mineral acids like Sulfuric Acid (H₂SO₄) and Nitric Acid (HNO₃) Majid Hussain, Environmental Degradation and Management, p.7.

One of the most visible and tragic consequences of this acidification is a phenomenon known as Stone Leprosy. This term describes the gradual degradation, pitting, and "skin-like" peeling of historical monuments and buildings made of marble or limestone. The chemistry is straightforward but devastating: marble is composed of Calcium Carbonate (CaCO₃). When sulfuric acid in the rain reacts with marble, it converts the stone into Calcium Sulfate (Gypsum). Because gypsum is more soluble in water and physically weaker than marble, it eventually washes away or flakes off, leaving the structure corroded and discolored Shankar IAS Academy, Environmental Pollution, p.105.

The Taj Mahal in India is the classic example of this impact. The white marble has suffered from yellowing and surface erosion due to the high concentration of sulfur oxides from nearby industries like the Mathura Refinery. This isn't just a cosmetic issue; it represents the literal dissolving of heritage. Beyond stone, acid rain also corrodes metals and damages the protective coatings on ceramics and glass Majid Hussain, Environmental Degradation and Management, p.10.

Sources: Shankar IAS Academy, Environmental Pollution, p.101, 105; Majid Hussain, Environmental Degradation and Management, p.7, 10

7. Solving the Original PYQ (exam-level)

Now that you have mastered the basics of atmospheric chemistry, you can see how the building blocks of environmental pollution converge in this question. Statement I addresses the physical process where oxides of sulfur (SO₂) and nitrogen (NOₓ)—the primary pollutants you studied—interact with atmospheric moisture. As you learned in the concept modules, these gases are highly soluble; when they encounter water vapor or raindrops, they don't just stay in a gaseous state—they dissolve. This represents the critical first step in the formation of acid rain, where industrial and natural emissions are integrated into the hydrological cycle.

To reach the correct answer, a candidate must evaluate the causal link between the dissolution and the resulting chemical change. Statement II identifies the specific chemical products: oxyacids like sulfuric acid (H₂SO₄) and nitric acid (HNO₃). When these acids form through the reaction of dissolved oxides with water, they release hydrogen ions, which significantly lowers the pH of the rainwater. Therefore, Statement II serves as the functional explanation for the chemical transition occurring in Statement I. This logical progression confirms that (A) Both the statements are individually true and statement II is the correct explanation of statement I is the only choice that captures the full chemical narrative.

UPSC often uses options (B), (C), and (D) as traps for students who may understand isolated facts but miss their interconnectivity. Option (B) is a common pitfall; a student might recognize both facts as true but fail to see that the formation of oxyacids is the fundamental reason why the dissolution of these specific gases results in acidity. Options (C) and (D) are easily eliminated if you recall from Environment, Shankar IAS Academy and Environment and Ecology, Majid Hussain that these oxides are the primary precursors to acid rain, making both statements scientifically accurate and inherently linked.