Which one of the following statements is correct ? Ozone is found in the Earth’s atmosphere mostly between ,

1. Composition of the Earth's Atmosphere (basic)

The Earth's atmosphere is a dynamic envelope of gases, water vapor, and aerosols that surrounds our planet, held in place by gravity. It didn't start out this way; our modern atmosphere evolved through three distinct stages. It began with the loss of the primordial atmosphere (mostly Hydrogen and Helium) due to solar winds. This was followed by degassing from the Earth’s hot interior (releasing water vapor and other gases), and finally, the composition was fundamentally modified by living organisms through photosynthesis, which introduced significant amounts of Oxygen FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), The Origin and Evolution of the Earth, p.15. Today, the dry atmosphere is dominated by Nitrogen (78.08%) and Oxygen (20.95%), which together account for about 99% of its volume. Argon (0.93%) is the third most abundant gas, followed by trace gases like Carbon dioxide (0.036%), Neon, Helium, and Methane Physical Geography by PMF IAS, Earths Atmosphere, p.270. It is crucial to understand that this composition is not uniform at all heights. The atmosphere thins out as we go higher, and different components drop off at different altitudes. For instance, while Nitrogen and Oxygen are the heavy hitters near the surface, Oxygen becomes almost negligible at a height of 120 km, and Carbon dioxide and water vapor are found only up to 90 km from the surface FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.64. Beyond gases, the atmosphere contains water vapor and dust particles (aerosols like sea salt, ash, and smoke), which are vital for weather phenomena. A critical functional aspect of certain gases, particularly CO₂, is their role in the Earth's energy balance: they are transparent to incoming short-wave solar radiation but opaque to outgoing long-wave terrestrial radiation, effectively trapping heat FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.66.

Component	Percentage (Approx)	Key Characteristic
Nitrogen (N₂)	78.08%	Relatively inert; most abundant gas.
Oxygen (O₂)	20.95%	Vital for respiration; negligible above 120 km.
Argon (Ar)	0.93%	Most abundant noble/inert gas.
Carbon Dioxide (CO₂)	0.036%	Greenhouse gas; found up to 90 km altitude.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), The Origin and Evolution of the Earth, p.15; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.64; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.66; Physical Geography by PMF IAS, Earths Atmosphere, p.270

2. The Thermal Structure of the Atmosphere (basic)

To understand the atmosphere, we must look at it as a multi-story building where each floor has a different "climate." This vertical arrangement is primarily determined by how temperature changes with height, a concept known as the Lapse Rate. In simple terms, the lapse rate is the rate at which air temperature decreases as you move upward Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295. If the temperature drops as you go higher, we call it a positive lapse rate; if it gets warmer (which happens in certain layers), it is called a temperature inversion.

The first and most vital layer is the Troposphere. This is where we live, breathe, and where all weather occurs. An interesting feature of the troposphere is that its thickness is not uniform: it extends to about 8 km at the poles but reaches up to 18 km at the equator Fundamentals of Physical Geography, NCERT Class XI, Composition and Structure of Atmosphere, p.65. This happens because the intense heat at the equator creates strong convection currents that push the air much higher. Within this layer, temperature decreases at a standard rate of roughly 6.5°C per kilometer, which is why mountain tops are colder than the plains.

Above the troposphere lies the Stratosphere, separated by a boundary called the tropopause. Unlike the layer below, the stratosphere actually gets warmer as you go higher. This "inversion" happens because of the Ozone Layer (O₃), which resides mainly between 20 km and 50 km Physical Geography by PMF IAS, Earths Atmosphere, p.276. Ozone molecules absorb ultraviolet (UV) radiation from the sun, converting that energy into heat and protecting life on Earth from harmful rays. Most of the ozone is concentrated in the lower part of this layer, specifically between 20-35 km.

Layer	Height Range	Temperature Trend	Key Feature
Troposphere	0 to 8-18 km	Decreases with height	Contains 90% of atmospheric mass and all weather phenomena.
Stratosphere	Up to 50 km	Increases with height	Contains the Ozone Layer; ideal for flying jet aircraft.
Mesosphere	50 to 80 km	Decreases with height	The coldest layer of the atmosphere.

Sources: Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295; Fundamentals of Physical Geography, NCERT Class XI, Composition and Structure of Atmosphere, p.65; Environment and Ecology by Majid Hussain, Basic Concepts of Environment and Ecology, p.7; Physical Geography by PMF IAS, Earths Atmosphere, p.276

3. Aviation and the Stratosphere (intermediate)

In our previous hops, we looked at the chaotic, weather-filled Troposphere. Now, let’s ascend into the Stratosphere, which serves as the primary 'highway' for long-distance commercial and military jet aircraft. Extending from the tropopause up to about 50 km, this layer offers a stark contrast to the air below. While the troposphere is characterized by constant mixing and turbulence, the stratosphere is stratified (layered), meaning there is very little vertical mixing. FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.65.

Why do pilots prefer this layer? There are two main physical reasons:

• Weather Stability: The stratosphere is almost entirely free of clouds and water vapor. Since most weather phenomena like thunderstorms and heavy rain are confined to the troposphere, flying above the tropopause allows aircraft to avoid the 'bumps' caused by convective currents (vertical winds). Physical Geography by PMF IAS, Earths Atmosphere, p.275.
• Temperature Inversion: Unlike the troposphere, where it gets colder as you go higher, the temperature in the stratosphere actually increases with altitude. This is caused by the Ozone layer absorbing ultraviolet (UV) radiation, which heats the surrounding air. Physical Geography by PMF IAS, Earths Atmosphere, p.275. This 'warm-over-cold' arrangement acts as a lid, preventing vertical air movement and ensuring a smooth, stable flight path.

Although generally clear, the lower stratosphere near the poles can occasionally host Polar Stratospheric Clouds (PSCs) during winter, which are crucial to understand for environmental chemistry even if they don't typically hinder aviation. Environment, Shankar IAS Academy (ed 10th), Ozone Depletion, p.270.

Feature	Troposphere	Stratosphere
Temperature Trend	Decreases with height (Normal Lapse Rate)	Increases with height (Negative Lapse Rate)
Air Movement	Strong vertical winds/convection	Horizontal flow/Stability
Visibility	Often obscured by clouds/dust	Clear and high visibility

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.275-276; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.65; Environment, Shankar IAS Academy (ed 10th), Ozone Depletion, p.270

4. Ozone as a Pollutant (Ground-Level Ozone) (intermediate)

In our journey through atmospheric composition, we encounter a fascinating paradox: the molecule Ozone (O₃). While we often celebrate it as a protector in the upper atmosphere, it takes on a villainous role when found near the Earth's surface. In the troposphere, ozone is a potent secondary pollutant. This means it is not emitted directly from a chimney or a tailpipe; instead, it is cooked up in the atmosphere through complex chemical reactions involving precursor gases. Environment, Shankar IAS Academy, Ozone Depletion, p.267

Ground-level ozone is the primary ingredient of photochemical smog. It forms when Nitrogen Oxides (NOₓ) and Volatile Organic Compounds (VOCs)—emitted by cars, power plants, and industrial boilers—react in the presence of sunlight. Because sunlight is the catalyst, ozone levels typically peak during sunny afternoons. Environment, Shankar IAS Academy, Environmental Pollution, p.65. It is important to remember that while stratospheric ozone is long-lived, ground-level ozone is relatively short-lived but highly reactive, making it hazardous to breathe and damaging to vegetation. Physical Geography by PMF IAS, Earths Atmosphere, p.276

Feature	Stratospheric Ozone ("Good")	Ground-Level Ozone ("Bad")
Location	10 km to 50 km above Earth	Troposphere (surface level)
Role	Shields Earth from harmful UV radiation	Major component of smog; health hazard
Formation	Natural UV action on Oxygen (O₂)	Man-made precursors + Sunlight

Beyond being a health irritant, tropospheric ozone also acts as a Greenhouse Gas (GHG). Unlike CO₂, which stays in the atmosphere for decades, ozone’s impact is shorter-term but significant because it traps heat effectively within the lower atmosphere. Fundamentals of Physical Geography, NCERT, World Climate and Climate Change, p.96. Understanding this "dual personality" of ozone is critical for UPSC, as it bridges the gap between atmospheric chemistry and environmental policy.

Sources: Environment, Shankar IAS Academy, Ozone Depletion, p.267; Environment, Shankar IAS Academy, Environmental Pollution, p.64-65; Physical Geography by PMF IAS, Earths Atmosphere, p.276; Fundamentals of Physical Geography, NCERT, World Climate and Climate Change, p.96

5. International Ozone Protection Treaties (intermediate)

When scientists discovered that man-made chemicals were thinning the ozonosphere—the region of the stratosphere between 20 km and 50 km rich in O₃—the global community responded with unprecedented cooperation. This effort is anchored by two major legal pillars: the Vienna Convention and the Montreal Protocol. While the 1985 Vienna Convention acted as a framework for research and cooperation, it was the 1987 Montreal Protocol that gave the world "teeth" to act by setting legally binding targets to phase out Ozone Depleting Substances (ODS) like Chlorofluorocarbons (CFCs) Environment, Shankar IAS Academy, International Organisation and Conventions, p.389.

India’s commitment to this cause began early; the nation became a party to the Vienna Convention and the Montreal Protocol in 1989. To meet these international obligations without crippling its industrial growth, India developed a comprehensive "Country Programme" in 1993 to phase out substances like CFC-11, Halons, and Carbon Tetrachloride (CTC) Environment, Shankar IAS Academy, International Organisation and Conventions, p.409. This transition is supported by the Protocol’s Multilateral Fund, which assists developing nations in adopting ozone-friendly technologies.

The story of ozone protection evolved significantly with the Kigali Amendment (2016). Initially, the world replaced CFCs with Hydrofluorocarbons (HFCs) because HFCs do not deplete the ozone layer. However, HFCs were soon identified as extremely potent greenhouse gases, thousands of times more effective at trapping heat than CO₂. The Kigali Amendment, which entered into force in 2019, mandates a global phase-down of HFCs, effectively turning an ozone treaty into a powerful tool for fighting climate change Indian Economy, Nitin Singhania, Sustainable Development and Climate Change, p.602.

Feature	Vienna Convention	Montreal Protocol
Nature	Framework Convention (Non-binding)	Regulatory Protocol (Legally Binding)
Focus	Cooperation on research and monitoring	Specific phase-out schedules for chemicals
Successor	Led to the Montreal Protocol	Amended by Kigali to include HFCs

Sources: Environment, Shankar IAS Academy, International Organisation and Conventions, p.389; Environment, Shankar IAS Academy, International Organisation and Conventions, p.409; Indian Economy, Nitin Singhania, Sustainable Development and Climate Change, p.602; Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.11

6. Solar Radiation and UV Absorption (exam-level)

To understand how our planet stays habitable, we must look at the Ozonosphere—a specialized region within the stratosphere that acts as Earth’s biological shield. While the atmosphere is generally transparent to short-wave solar radiation, allowing sunlight to reach the surface, it is selectively opaque to the most lethal components of that radiation. Ozone (O₃), though only a trace gas, is the primary agent responsible for absorbing ultraviolet (UV) radiation in the wavelength range of 0.1 to 0.3 microns Environment and Ecology by Majid Hussain, Environmental Degradation and Management, p. 11. This absorption process is not just a filter; it is a chemical reaction that converts UV energy into heat, which is why temperatures in the stratosphere actually increase with altitude. While ozone exists in small amounts throughout the atmosphere, about 90% of it is concentrated in the stratosphere, between 10 km and 50 km above the Earth's surface Physical Geography by PMF IAS, Earths Atmosphere, p. 272. However, for your exams, it is crucial to distinguish between the presence of ozone and its peak concentration. The highest density of this gas—the true 'ozone layer'—is typically found between 20 km and 35 km. Interestingly, in the lower troposphere, ozone and water vapor also absorb some near-infrared radiation, but their most critical role remains the high-altitude absorption of UV rays NCERT Class XI Fundamentals of Physical Geography, Solar Radiation, Heat Balance and Temperature, p. 68. The importance of this absorption cannot be overstated. Without the ozone layer, the increased influx of UV-B radiation would disrupt the fundamental biogeochemical cycles of both terrestrial and aquatic ecosystems Environment by Shankar IAS Academy, Ozone Depletion, p. 271. For humans and animals, this shield prevents DNA damage and serious health conditions like melanoma (a dangerous form of skin cancer) Environment and Ecology by Majid Hussain, Environmental Degradation and Management, p. 14. By intercepting these high-energy waves before they reach the troposphere, the ozonosphere ensures that the energy reaching the surface is safe for life to flourish.

Sources: Environment and Ecology by Majid Hussain, Environmental Degradation and Management, p.11, 14; Physical Geography by PMF IAS, Earths Atmosphere, p.272; NCERT Class XI Fundamentals of Physical Geography, Solar Radiation, Heat Balance and Temperature, p.68; Environment by Shankar IAS Academy, Ozone Depletion, p.271

7. The Ozonosphere: Vertical Distribution and Range (exam-level)

While Ozone (O₃) is a trace gas making up less than 0.00005% of the atmosphere’s volume, its impact on life is monumental. It is not spread evenly throughout the air we breathe; instead, it is vertically distributed with about 90% of its total mass residing in the Stratosphere Physical Geography by PMF IAS, Earths Atmosphere, p.272. Although ozone molecules can be found starting from the ground (where they act as pollutants), the region known as the Ozonosphere typically spans from 20 km to 50 km above the Earth's surface, crossing the upper stratosphere and reaching into the lower mesosphere Physical Geography by PMF IAS, Earths Atmosphere, p.276.

Within this broad range, the concentration of ozone is not uniform. The "heart" of this shield, often called the Ozone Layer, features the highest concentration of O₃ molecules between 20 km and 30 km Physical Geography by PMF IAS, Earths Atmosphere, p.272. This specific altitude is where the atmospheric pressure and the intensity of solar Ultraviolet (UV) radiation are perfectly balanced to facilitate the Ozone-Oxygen Cycle. In this cycle, UV light splits O₂ into individual oxygen atoms, which then bond with other O₂ molecules to create O₃. This constant chemical activity is why this region is sometimes referred to as the Chemosphere Physical Geography by PMF IAS, Earths Atmosphere, p.276.

The vertical distribution of ozone dictates the thermal structure of our atmosphere. Because ozone molecules are incredibly efficient at absorbing harmful UV radiation, they convert this energy into heat. This creates a negative lapse rate (or temperature inversion) in the stratosphere, where temperature actually increases with altitude—rising from about -57 °C at the tropopause to nearly 0 °C at the stratopause (50 km) Physical Geography by PMF IAS, Earths Atmosphere, p.275. Without this vertical concentration of ozone, the stratosphere would not have its characteristic stable temperature profile, which allows for smooth airplane flights above the turbulent weather of the troposphere FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Composition and Structure of Atmosphere, p.65.

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.272; Physical Geography by PMF IAS, Earths Atmosphere, p.276; Physical Geography by PMF IAS, Earths Atmosphere, p.275; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.65

8. Solving the Original PYQ (exam-level)

Now that you have mastered the vertical structure of the atmosphere and its chemical composition, this question requires you to synthesize those building blocks. You have learned that while ozone is a trace gas, its concentration is not uniform. The concept of the Ozonosphere is central here—it refers to the specific region where ozone molecules are dense enough to filter harmful solar radiation. As noted in Physical Geography by PMF IAS, although a small amount of ozone exists in the troposphere as a pollutant, roughly 90% of the Earth's total ozone is concentrated within the stratosphere.

To arrive at the correct answer, you must reason through the altitude boundaries of the atmospheric layers. The stratosphere begins above the tropopause (roughly 10–15 km) and extends up to the stratopause at approximately 50 km. While the absolute peak concentration is often cited by experts as occurring between 20 and 35 km, the question asks where ozone is "mostly" found. Because the entire functional ozone layer is contained within the stratigraphic limits, the range (C) 20 and 50 km is the most accurate and encompassing description of its home. This is supported by Environment and Ecology by Majid Hussain, which identifies the upper limit of the ozone-rich zone at the 50 km mark.

UPSC often uses boundary traps to test your precision. Option (A) 5–10 km is strictly within the troposphere, where ozone is a hazardous component of smog rather than a protective layer. Option (B) 10–20 km only touches the very base of the stratosphere, missing the bulk of the gas. Conversely, option (D) 50–100 km extends into the Mesosphere and Thermosphere, where the air is far too thin to maintain significant ozone levels. By identifying the stratosphere as the primary reservoir, you can logically conclude that only 20 to 50 km fits the scientific profile of the ozonosphere.