Which one, among the following atmospheir gases, filters out most of the ultraviolet radiation of the Sun?

1. Composition of the Atmosphere (basic)

The atmosphere is a dynamic mixture of gases, water vapor, and aerosols that sustains life on Earth. While we often think of 'air' as a single substance, it is actually composed of **Permanent Gases**, which remain in fixed proportions regardless of location, and **Variable Gases**, which fluctuate based on environmental conditions. Interestingly, the atmosphere we breathe today is a 'second atmosphere'; the early Earth was largely devoid of oxygen until photosynthesizing cyanobacteria began producing it billions of years ago Physical Geography by PMF IAS, Earths Atmosphere, p.270. Today, two gases—Nitrogen (N₂) and Oxygen (O₂)—make up roughly 99% of the dry atmosphere by volume, while the remaining 1% consists of Argon and various trace gases.

Major Constituents of the Dry Atmosphere:

Gas	Percentage by Volume	Role/Nature
Nitrogen (N₂)	78.08%	Chemically relatively inert; essential for plant proteins.
Oxygen (O₂)	20.95%	Vital for respiration and combustion.
Argon (Ar)	0.93%	An inert noble gas.
Carbon Dioxide (CO₂)	0.036%	Crucial for photosynthesis; transparent to solar radiation but opaque to terrestrial heat NCERT Class XI Fundamentals of Physical Geography, Composition and Structure of Atmosphere, p.66.

The distribution of these gases is not uniform as we move upward. Heavier gases like Nitrogen and Oxygen tend to concentrate near the surface. In fact, Oxygen becomes almost negligible at a height of about **120 km**, while Carbon Dioxide and water vapor are confined to the lower **90 km** of the atmosphere Physical Geography by PMF IAS, Earths Atmosphere, p.272.

Beyond these bulk gases, **Ozone (O₃)** plays a specialized role. Although it exists in tiny amounts (less than 0.00005%), it is concentrated in the stratosphere between 20 km and 30 km. This 'Ozone Layer' acts as a biological shield by absorbing high-energy ultraviolet (UV) radiation from the Sun, converting it into heat and preventing it from damaging DNA in plants and animals. Additionally, the atmosphere contains solid particles like sea salt, pollen, and soot, which act as 'hygroscopic nuclei' around which water vapor condenses to form clouds NCERT Class XI Fundamentals of Physical Geography, Composition and Structure of Atmosphere, p.66.

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.270-272; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.66

2. Thermal Structure of the Atmosphere (basic)

To understand the atmosphere, we must look at it not as a uniform mass of air, but as a multi-layered sandwich, where each layer is defined by how temperature changes with height. This is known as the thermal structure. The atmosphere is held to Earth by gravity, making it most dense at the surface and increasingly rarefied as we move upward. The transition points between these layers are called "pauses" (like the tropopause), where the temperature trend briefly stabilizes before reversing FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.65.

The lowermost layer, the Troposphere, is the "weather laboratory." It extends roughly 8 km at the poles and up to 18 km at the equator because strong heat currents push the air higher in the tropics. In this layer, temperature decreases as you go higher—a phenomenon called the Normal Lapse Rate—reaching about -57°C at the tropopause Environment and Ecology, Majid Hussain, Structure of the Atmosphere, p.7. This is because the troposphere is primarily heated from below by the Earth's surface radiation, rather than directly by the sun.

Above the troposphere lies the Stratosphere (up to 50 km), which behaves very differently. Here, the temperature actually increases with altitude. This "temperature inversion" happens because of a vital chemical reaction: the Ozone-Oxygen Cycle. Ozone (O₃) molecules absorb high-energy ultraviolet (UV) radiation from the sun. This process breaks chemical bonds and releases kinetic energy as heat, effectively acting as Earth's thermal shield Physical Geography by PMF IAS, Earths Atmosphere, p.275. Beyond this, we find the Mesosphere (the coldest layer), the Thermosphere (where temperatures soar due to ionisation), and finally the Exosphere, where the atmosphere gradually fades into the vacuum of space.

Layer	Height Range	Temperature Trend	Key Feature
Troposphere	0 - 13 km	Decreases with height	Contains 90% of atmospheric mass and all weather.
Stratosphere	13 - 50 km	Increases with height	Contains the Ozone layer; ideal for jet aircraft.
Mesosphere	50 - 80 km	Decreases with height	Coldest layer; meteors burn up here.
Thermosphere	80 - 700 km	Increases with height	Contains the Ionosphere; facilitates radio communication.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.65; Environment and Ecology, Majid Hussain, Structure of the Atmosphere, p.7; Physical Geography by PMF IAS, Earths Atmosphere, p.275

3. The Greenhouse Effect and Heat Budget (intermediate)

To understand why Earth is a hospitable home rather than a frozen wasteland, we must look at the Greenhouse Effect. Think of our atmosphere as a giant, invisible thermal blanket. While the Sun sends us energy in the form of short-wave solar radiation, the atmosphere is largely transparent to it, allowing it to reach and warm the Earth's surface. However, once the Earth heats up, it becomes a radiating body itself, emitting energy back toward space as long-wave terrestrial radiation FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Solar Radiation, Heat Balance and Temperature, p.69.

This is where the "Greenhouse" magic happens. Just as the glass in a gardener's greenhouse allows sunlight in but prevents heat from escaping, certain gases in our atmosphere—like Carbon Dioxide (CO₂), Methane (CH₄), and Water Vapour—are opaque to this outgoing long-wave radiation. They absorb this heat and re-radiate it back toward the Earth's surface, effectively "trapping" it in the lower troposphere Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.7. This natural process keeps our planet about 33°C warmer than it would otherwise be.

Feature	Incoming Solar Radiation	Outgoing Terrestrial Radiation
Wavelength	Short-wave (High Energy)	Long-wave (Lower Energy/Infrared)
Atmospheric Interaction	Passes through easily	Absorbed by Greenhouse Gases (GHGs)
Primary Effect	Heats the Earth's surface	Heats the atmosphere from below

This leads us to the concept of the Heat Budget. For the Earth to maintain a constant temperature over time, there must be a perfect balance: the amount of energy received from the Sun must equal the amount of energy returned to space FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Solar Radiation, Heat Balance and Temperature, p.69. If we increase the concentration of greenhouse gases through human activity, we "thicken" the blanket, causing more heat to be retained than released, which upsets this budget and leads to global warming Environment, Shankar IAS Academy (ed 10th), Climate Change, p.254.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Solar Radiation, Heat Balance and Temperature, p.69; Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.7; Environment, Shankar IAS Academy (ed 10th), Climate Change, p.254; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), World Climate and Climate Change, p.96

4. Atmospheric Pollutants and Air Quality (intermediate)

To understand atmospheric pollutants, we must first distinguish between 'good' and 'bad' ozone. While Ozone (O₃) is a pollutant at the ground level (troposphere), it is our planet's essential sunscreen in the stratosphere. Concentrated between 20 km and 30 km, the ozone layer absorbs nearly all high-energy UV-C radiation and most UV-B radiation, which would otherwise cause skin cancer, cataracts, and damage to marine phytoplankton Physical Geography by PMF IAS, Earths Atmosphere, p.272. This protective shield works through a dynamic Ozone-Oxygen Cycle: UV light splits oxygen molecules (O₂) into individual atoms, which then bond with other O₂ molecules to form O₃, converting harmful radiation into harmless heat energy Physical Geography by PMF IAS, Earths Atmosphere, p.276. However, human activity has introduced 'ozone-depleting substances' like Chlorofluorocarbons (CFCs). When these drift into the stratosphere, UV rays break them down, releasing chlorine and bromine radicals. These radicals act as catalysts—a single chlorine atom can destroy over 100,000 ozone molecules before being removed from the atmosphere FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, World Climate and Climate Change, p.96. This imbalance leads to the thinning of the layer, most famously the 'Ozone Hole' over Antarctica. To manage air quality and pollutants like ground-level ozone, India utilizes the National Air Quality Monitoring Programme (NAMP), executed by the Central Pollution Control Board (CPCB). This program tracks trends in ambient air quality and identifies 'non-attainment cities' that do not meet the National Ambient Air Quality Standards (NAAQS) Environment, Shankar IAS Acedemy, Environmental Pollution, p.69.

Feature	Stratospheric Ozone	Tropospheric Ozone
Nickname	Good Ozone	Bad Ozone (Pollutant)
Function	Blocks harmful UV rays	Causes respiratory issues; smog
Lifespan	Long-lived	Short-lived

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.272, 276; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, World Climate and Climate Change, p.96; Environment, Shankar IAS Acedemy, Environmental Pollution, p.69

5. International Conventions on Atmosphere (exam-level)

To understand international atmospheric conventions, we must first appreciate the stratospheric ozone layer. Located between 20 km and 30 km above Earth, this layer acts as our planet's "sunscreen," filtering out lethal UV-C radiation and the majority of UV-B radiation. As we've seen in our study of atmospheric chemistry, ozone (O₃) is naturally formed and destroyed in a delicate cycle triggered by solar energy PMF IAS, Earths Atmosphere, p.272.

The crisis began when human-made chemicals, specifically Chlorofluorocarbons (CFCs) and Halons, reached the stratosphere. These substances release chlorine and bromine radicals which act as catalysts. A single chlorine radical can initiate a chain reaction that destroys over 100,000 ozone molecules before it is neutralized PMF IAS, Earths Atmosphere, p.276. This scientific realization led to the Montreal Protocol (1987), a landmark treaty designed to phase out these Ozone Depleting Substances (ODS).

While the Montreal Protocol was remarkably successful in phasing out CFCs, the world transitioned to Hydrofluorocarbons (HFCs) as substitutes. HFCs are "ozone-friendly" but are extremely potent greenhouse gases, thousands of times more effective at trapping heat than CO₂. To address this, the Kigali Amendment was adopted in 2016 (entering into force in 2019), expanding the Protocol's mandate to phase down HFCs to combat climate change Nitin Singhania, Sustainable Development and Climate Change, p.602.

Feature	Montreal Protocol (Original)	Kigali Amendment
Primary Target	Ozone Depleting Substances (CFCs, Halons)	Hydrofluorocarbons (HFCs)
Environmental Focus	Ozone Layer Recovery	Mitigating Climate Change/Global Warming
Status	Universally Ratified	Legally binding for ratifying nations

Sources: PMF IAS, Earths Atmosphere, p.272, 276; Shankar IAS Academy, International Organisation and Conventions, p.409; Nitin Singhania, Sustainable Development and Climate Change, p.602; Majid Hussain, Biodiversity and Legislations, p.7

6. The Ozonosphere and UV Radiation Types (intermediate)

The Ozonosphere is a specialized region within the stratosphere, located roughly between 20 km and 30 km above the Earth's surface. Although ozone (O₃) makes up less than 0.00005% of the atmosphere's total volume, it serves as our planet's "atmospheric sunscreen." Without this layer, the high-energy radiation from the Sun would make life on land impossible Physical Geography by PMF IAS, Chapter 20, p.276. The primary role of the ozonosphere is to filter Ultraviolet (UV) radiation, which is classified into three types based on wavelength and energy:

UV Type	Wavelength	Atmospheric Interaction	Biological Impact
UV-A	Longest	Mostly reaches the surface.	Necessary for Vitamin D but causes skin aging.
UV-B	Medium	Mostly absorbed by the ozone layer.	Causes DNA damage, cataracts, and skin cancer.
UV-C	Shortest	Completely absorbed by ozone and oxygen.	Extremely lethal and high-energy.

The shielding effect of the ozonosphere is maintained through a continuous chemical process known as the Ozone-Oxygen Cycle (or Chapman Cycle). When high-energy UV light hits an oxygen molecule (O₂), it splits it into two free oxygen atoms. These reactive atoms quickly combine with other O₂ molecules to form ozone (O₃). When O₃ itself absorbs UV radiation, it splits back into O₂ and an oxygen atom, effectively converting harmful radiation into heat energy Physical Geography by PMF IAS, Chapter 20, p.272. This cycle is what warms the stratosphere, creating the temperature inversion characteristic of that layer.

When this delicate balance is disrupted by pollutants like Chlorofluorocarbons (CFCs) or nitrogen oxides from supersonic aircraft, the ozone layer thins out—a process called ozone depletion Environment and Ecology, Majid Hussain, Chapter 1, p.12. The danger lies in the "catalytic" nature of this destruction: a single chlorine atom released from a CFC molecule can destroy over 100,000 ozone molecules before it is neutralized Environment, Shankar IAS Academy, Ozone Depletion, p.268. Increased levels of UV-B reaching the surface don't just affect humans; they cause mutations in plant DNA, reduce crop yields, and disrupt the base of the marine food chain by harming phytoplankton Environment, Shankar IAS Academy, Ozone Depletion, p.271.

Sources: Physical Geography by PMF IAS, Chapter 20: Earths Atmosphere, p.276; Physical Geography by PMF IAS, Chapter 20: Earths Atmosphere, p.272; Environment and Ecology, Majid Hussain, Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.12; Environment, Shankar IAS Academy, Ozone Depletion, p.268; Environment, Shankar IAS Academy, Ozone Depletion, p.271

7. Solving the Original PYQ (exam-level)

Now that you have mastered the vertical structure of the atmosphere and the chemical composition of air, this question allows you to apply those building blocks to a critical planetary defense mechanism. You’ve learned that the atmosphere isn't just a uniform mix of gases for breathing, but a series of functional layers where trace elements perform heavy lifting. The Ozonosphere, located within the stratosphere, is the specific concept at play here. As explained in Physical Geography by PMF IAS, even though Ozone constitutes less than 0.00005% of the atmosphere, its unique molecular structure allows it to perform a role that the major gases cannot.

To arrive at the correct answer, follow the logic of the oxygen-ozone cycle: the Sun emits high-energy UV-B and UV-C radiation which is lethal to life. When you see "filter," think of a chemical shield. While Oxygen (O2) molecules are involved in the process by splitting apart, it is the resulting Ozone (O3) that effectively absorbs the majority of this radiation, converting it into heat energy. Therefore, the correct answer is (D) Ozone. It acts as an invisible filter between 20 km and 30 km altitude, ensuring that the most damaging wavelengths never reach the Earth's surface.

UPSC frequently uses "major gases" as traps to test if you confuse abundance with function. Nitrogen (B) and Oxygen (A) make up about 99% of the atmosphere, but they are transparent to most UV radiation; they provide the pressure and life-support we need, but not the specific filtering shield. Helium (C) is a light, noble gas found primarily in the exosphere and plays no role in radiation filtering. Always remember: in Geography, a gas's importance is often defined by its utility and location rather than its total volume.