Which of the following statements regarding ozone layer within the atmos- phere is/are correct ? 1. It absorbs most of the ultraviolet radiation found in the Sun’s rays. 2. Chlorofluorocarbons are serious threat to the ozone layer. Select the correct answer using the code given below :

1. Structure and Layers of the Atmosphere (basic)

Welcome! Before we dive into the specifics of how we protect the ozone layer, we must first understand the stage where this environmental drama unfolds: the Earth's atmosphere. Think of our atmosphere not as a single, uniform blanket of air, but as a five-layered "onion" held in place by gravity. As you move away from the surface, the density of air decreases rapidly—in fact, about 90% of the atmosphere's total mass is squeezed into the very first layer Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.7.

The atmosphere is divided into layers based primarily on temperature variations. In some layers, it gets colder as you go up; in others, it gets warmer. The lowermost layer is the Troposphere, which is the "weather zone" where clouds form and we breathe. Interestingly, its thickness isn't uniform—it's about 8 km high at the poles but stretches to 18 km at the equator because intense heat creates strong convectional currents that push the air upward FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.65.

Just above the troposphere lies the Stratosphere (extending up to 50 km). This layer is crucial for our study because it contains the Ozone Layer. Unlike the troposphere, the stratosphere is very stable with almost no water vapor or vertical winds, which is why commercial jet pilots prefer flying in its lower reaches to avoid turbulence Physical Geography by PMF IAS, Earths Atmosphere, p.276. Above this, we find the Mesosphere (where meteors burn up), the Thermosphere (where temperatures soar), and finally the Exosphere, which gradually fades into the vacuum of space.

Sources: Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.7; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.65; Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Earths Atmosphere, p.276

2. Understanding Ultraviolet (UV) Radiation Types (basic)

To understand why the ozone layer is so vital, we first need to look at the energy it filters: Ultraviolet (UV) radiation. UV radiation is a form of electromagnetic energy from the sun that has a shorter wavelength and higher energy than visible light. Scientists categorize UV radiation into three main types based on their wavelength (measured in nanometers) and their ability to penetrate the atmosphere.

The general rule in physics is that shorter wavelengths carry more energy and are therefore more biologically damaging. However, the atmosphere (specifically the ozone layer and oxygen) acts as a selective filter, stopping the most dangerous rays before they reach your skin.

While UV-C is the most energetic, UV-B is the primary concern for environmentalists. Because it is only partially filtered, any thinning of the ozone layer leads to a direct increase in UV-B levels at the surface. This radiation doesn't just affect humans; it penetrates the ocean surface, harming phytoplankton—the foundation of the marine food chain—and affecting the growth and reproduction of fish, amphibians, and terrestrial plants Environment, Shankar IAS Academy, Ozone Depletion, p.271. By damaging the DNA of cells, UV-B can lead to mutations and suppress the immune systems of mammals, making them more susceptible to infectious diseases Environment, Shankar IAS Academy, Ozone Depletion, p.267.

Sources: Environment and Ecology, Majid Hussain, Chapter 6: Environmental Degradation and Management, p.12; Environment, Shankar IAS Academy, Ozone Depletion, p.267; Environment, Shankar IAS Academy, Ozone Depletion, p.271

3. Good Ozone vs. Bad Ozone (intermediate)

Welcome to Hop 3! To understand ozone protection, we first need to clear up a common confusion: Is ozone a protector or a pollutant? The answer is both. In the world of atmospheric science, we use the phrase "Good up high, bad nearby." Ozone (O₃) is an allotrope of oxygen consisting of three oxygen atoms. Whether it helps or hurts life depends entirely on its altitude in the atmosphere Shankar IAS Academy, Ozone Depletion, p.267.

Good Ozone resides in the stratosphere (roughly 10 km to 50 km above Earth). This is the "ozone layer" we want to protect. It acts like a global sunscreen, absorbing the majority of the sun’s harmful UV-B radiation. Without this shield, life on Earth would face severe DNA damage, skin cancers, and failing ecosystems. Interestingly, the absorption of UV rays by ozone is what causes the temperature to rise in the stratosphere, creating a unique thermal profile PMF IAS, Earths Atmosphere, p.275.

Bad Ozone, on the other hand, is found in the troposphere (the air we breathe). It isn't emitted directly by cars or factories; instead, it is a secondary pollutant. It forms when Nitrogen Oxides (NOₓ) and Volatile Organic Compounds (VOCs) react chemically in the presence of sunlight. This is why ground-level ozone is a primary component of photochemical smog, especially on hot, sunny days with heavy traffic Shankar IAS Academy, Environmental Pollution, p.65. It is highly toxic, causing respiratory issues like pneumonia, watering eyes, and damaging plant tissues Shankar IAS Academy, Environmental Pollution, p.64.

Sources: Environment, Shankar IAS Academy (10th ed.), Ozone Depletion, p.267; Physical Geography by PMF IAS (1st ed.), Earths Atmosphere, p.275; Environment, Shankar IAS Academy (10th ed.), Environmental Pollution, p.64-65

4. International Legal Framework: Vienna and Montreal (intermediate)

When the scientific community realized that human-made chemicals were eating away at the stratosphere's protective shield, the world didn't just stand by; it created one of the most successful examples of international cooperation in history. This response is built on two pillars: the Vienna Convention and the Montreal Protocol. While they are often mentioned together, they serve very different roles. Think of the Vienna Convention as the blueprint or framework, and the Montreal Protocol as the action plan or the legal muscle.

The Vienna Convention for the Protection of the Ozone Layer (1985) was the first major step. It was an agreement to cooperate on research and monitoring of the ozone layer. However, it is crucial to remember that it did not include legally binding goals for the reduction of Ozone Depleting Substances (ODS) like CFCs Environment, Shankar IAS Acedemy, International Organisation and Conventions, p.409. It essentially established that ozone depletion was a global problem requiring a global solution, paving the way for more specific regulations. Today, it stands as one of the most widely ratified treaties in UN history, with 197 parties Environment and Ecology, Majid Hussain, Biodiversity and Legislations, p.12.

The real "teeth" of the framework came with the Montreal Protocol (1987). This treaty moved beyond mere cooperation to mandate the phase-out of substances responsible for ozone depletion, such as Chlorofluorocarbons (CFCs) and Halons Environment and Ecology, Majid Hussain, Biodiversity and Legislations, p.7. It was designed to be flexible, allowing for periodic revisions (like the London, Copenhagen, and Beijing amendments) to accelerate phase-out schedules as better technology became available Environment, Shankar IAS Acedemy, International Organisation and Conventions, p.409. This protocol is the reason why scientists believe the ozone layer is finally on a path to recovery.

Sources: Environment, Shankar IAS Academy, International Organisation and Conventions, p.409; Environment and Ecology, Majid Hussain, Biodiversity and Legislations, p.12; Environment and Ecology, Majid Hussain, Biodiversity and Legislations, p.7

5. Modern Policy: The Kigali Amendment (exam-level)

6. The Chemistry of Ozone Depletion (exam-level)

To understand ozone depletion, we must first look at the natural ozone-oxygen cycle. In the stratosphere, ozone (O₃) is constantly being formed and destroyed by solar UV radiation in a delicate balance. However, human-made chemicals—specifically Chlorofluorocarbons (CFCs)—disrupt this equilibrium. CFCs are remarkably stable and non-toxic in the lower atmosphere (troposphere), which allowed them to stay there for decades. But once they drift into the stratosphere, they encounter intense, high-energy UV radiation. This radiation is strong enough to break the chemical bonds of CFCs, releasing a highly reactive chlorine atom Environment and Ecology, Majid Hussain, Chapter 6, p.13.

The real danger lies in the catalytic nature of the reaction that follows. A free chlorine atom (Cl) attacks an ozone molecule (O₃), stripping away one oxygen atom to form chlorine monoxide (ClO) and leaving behind an ordinary oxygen molecule (O₂). The cycle doesn't end there: the ClO molecule then reacts with a free-roaming oxygen atom (O), which releases the chlorine atom back into its original state while forming another O₂ molecule. Because the chlorine atom is regenerated at the end of the process, it acts as a catalyst. A single chlorine atom can destroy over 100,000 ozone molecules before it is eventually removed from the stratosphere through other chemical pathways Physical Geography by PMF IAS, Earths Atmosphere, p.276.

This process is further accelerated in unique conditions like those found over Antarctica. During the dark winter, Polar Stratospheric Clouds (PSCs) form. These clouds provide a solid surface that facilitates chemical reactions which convert "reservoir" chlorine (stable forms) into highly reactive molecular chlorine. When the sun returns in the spring, UV light splits this molecular chlorine into atomic chlorine, triggering a massive, rapid "hole" in the ozone layer Environment, Shankar IAS Academy, Ozone Depletion, p.270. Even if we stop all CFC emissions today, the layer will continue to be damaged for a long time because these gases have long residence times, often lasting hundreds of years in the atmosphere Environment and Ecology, Majid Hussain, Chapter 6, p.13.

Sources: Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.13; Physical Geography by PMF IAS, Earths Atmosphere, p.276; Environment, Shankar IAS Academy, Ozone Depletion, p.270; Environment, Shankar IAS Academy, Ozone Depletion, p.268

7. Solving the Original PYQ (exam-level)

Now that you have mastered the vertical structure of the atmosphere and the chemical properties of pollutants, you can see how these building blocks converge in this question. As detailed in Physical Geography by PMF IAS, the Ozonosphere—found primarily within the stratosphere—serves as Earth’s vital biological shield. Statement 1 directly tests your understanding of this protective role. It is correct because the ozone layer filters out the majority of high-energy ultraviolet (UV) radiation, particularly the UV-B spectrum. Without this absorption, life on Earth would face significant risks from DNA damage and increased rates of skin cancer, as noted in Science, class X (NCERT 2025 ed.).

To evaluate Statement 2, we look at the anthropogenic impact on this layer. According to Environment and Ecology by Majid Hussain, Chlorofluorocarbons (CFCs) are uniquely dangerous because they remain stable until they reach the stratosphere. Once there, solar radiation breaks them down to release chlorine atoms, which act as catalysts in a chain reaction that destroys ozone molecules at an alarming rate. Since both the protective function (Statement 1) and the specific threat (Statement 2) are scientifically accurate, the correct answer is (C) Both 1 and 2. Reasoning through the chemical mechanism helps you confirm that Statement 2 isn't just a general environmental concern, but a specific stratospheric threat.

When tackling UPSC questions like this, be wary of partial truths. Options (A) and (B) are classic traps designed to catch students who stop reading after identifying the first correct fact. A common UPSC distractor might also involve swapping "ultraviolet" for "infrared" radiation or claiming CFCs are the only threat. By recognizing that Statement 1 describes the natural function and Statement 2 describes the human-induced threat, you can confidently conclude that both must be included for a complete assessment of the ozone layer's current state.