Which one of the following is the correct order in which the gases from the atmosphere disappear as one moves away from the surface of the earth?

1. Composition of the Atmosphere (basic)

Welcome to your first step in mastering the Earth's atmosphere! To understand how our planet breathes, we must first look at what the atmosphere is made of. The atmosphere is a mixture of many different gases, water vapour, and tiny solid particles. While we often think of the air as a uniform mix, it is actually quite layered. In the lower layers (the homosphere, up to about 80 km), the proportions of major gases like Nitrogen (78.08%) and Oxygen (20.95%) remain relatively constant Physical Geography by PMF IAS, Chapter 20, p.271. However, as we move vertically away from the surface, the composition shifts significantly due to the molecular weight of the gases and the pull of gravity. Because heavier gases are more affected by gravity, they tend to cluster closer to the Earth's surface. As you ascend, certain gases become negligible at specific heights. For instance, Carbon dioxide (CO₂) and water vapour—both crucial for regulating temperature—are found only up to an altitude of approximately 90 km. If you go higher, you'll find that Oxygen (O₂), though abundant at sea level, becomes almost negligible at a height of about 120 km Physical Geography by PMF IAS, Chapter 20, p.272. Nitrogen (N₂) is unique because it is both highly abundant and chemically stable. While it also thins out, it persists in significant proportions to higher altitudes compared to Carbon dioxide and Oxygen. This vertical sorting eventually leads us into the heterosphere, where gases are no longer mixed but are arranged in distinct layers based on their weight (with lighter gases like Helium and Hydrogen at the very top). Understanding this "order of disappearance" is vital for grasping how the atmosphere protects us and where its various functions—like the greenhouse effect or aerobic life—actually take place.

Comparison of Altitudinal Limits:

Gas/Component	Approximate Limit of Presence	Role/Nature
Water Vapour & CO₂	Up to 90 km	Heavier; critical for weather and heat retention.
Oxygen (O₂)	Up to 120 km	Essential for life; becomes negligible above this height.
Nitrogen (N₂)	Beyond 120 km	Most abundant; persists longer than O₂ and CO₂.

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.271; Physical Geography by PMF IAS, Earths Atmosphere, p.272; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.64

2. Vertical Structure: The Five Layers (basic)

To understand our atmosphere, imagine it as a multi-storey building where each floor has a different 'climate control' setting. These settings are primarily determined by how temperature changes with height, a concept we call the Lapse Rate. While gravity keeps the atmosphere densest near the surface, the way energy is absorbed at different altitudes creates five distinct layers. Physical Geography by PMF IAS, Chapter 20, p.295. Depending on whether temperature decreases (positive lapse rate) or increases (temperature inversion) as you go up, we define these specific zones.

The first three layers—the Troposphere (0–12 km), Stratosphere (12–50 km), and Mesosphere (50–80 km)—form the lower atmosphere. In the Troposphere, where all weather occurs, the temperature drops as you climb. However, this trend reverses in the Stratosphere because the Ozone layer absorbs solar UV radiation, acting like a giant heater for that middle section. NCERT Class XI, Chapter 7, p.65. Above this, the Mesosphere sees temperature plunging again to the coldest levels in the atmosphere (nearly -100°C), before the Thermosphere and Exosphere take over, where temperatures skyrocket due to direct solar intensity on sparse gas molecules.

Layer	Approx. Height	Temperature Trend	Key Feature
Troposphere	0 – 12 km	Decreases	Weather & Water Vapour
Stratosphere	12 – 50 km	Increases	Ozone Layer (UV Protection)
Mesosphere	50 – 80 km	Decreases	Coldest layer; Meteors burn up
Thermosphere	80 – 700 km	Increases	Ionosphere (Radio waves)
Exosphere	> 700 km	Increases	Merges with Outer Space

Sources: Physical Geography by PMF IAS, Chapter 20: Earths Atmosphere, p.274; Physical Geography by PMF IAS, Vertical Distribution of Temperature, p.295; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7: Composition and Structure of Atmosphere, p.65

3. Greenhouse Gases and Heat Budget (intermediate)

To understand the Earth's temperature, we must look at the planet as a giant energy accountant. The Heat Budget is the balance sheet that tracks incoming energy from the Sun and outgoing energy from the Earth. The Sun radiates energy in short-wave forms (UV and visible light), which easily pass through our atmosphere to heat the surface. However, once the Earth warms up, it becomes a radiating body itself, releasing energy back into space as long-wave terrestrial radiation (infrared) FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7, p.69.

This is where Greenhouse Gases (GHGs) play their critical role. Gases like Carbon dioxide (CO₂), Methane (CH₄), and Water Vapour are transparent to incoming short-wave solar radiation but opaque to outgoing long-wave terrestrial radiation. They act like a thermal blanket, absorbing the heat trying to escape and radiating some of it back to the surface. This process, known as the Greenhouse Effect, is what keeps our planet warm enough to support life FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7, p.64. While CO₂ is the most discussed GHG due to fossil fuel combustion, others like Chlorofluorocarbons (CFCs) and Nitrous oxide (N₂O) are also highly effective at trapping heat FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7, p.96.

To maintain a constant temperature, the Earth must return exactly the same amount of heat it receives. If the Sun sends 100 units of energy, the Earth reflects about 35 units immediately (called Albedo) and absorbs 65 units. Eventually, through various atmospheric processes, these 65 units are radiated back into space. This perfect "give and take" is the Heat Budget Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Chapter 20, p.293. If GHG concentrations increase, they trap more of the outgoing 65 units for a longer period, causing the "account" to bloat and the planet to warm up.

Radiation Type	Source	Wavelength	GHG Interaction
Insolation	Sun	Short-wave	Gases are mostly transparent; they let it pass.
Terrestrial	Earth	Long-wave	Gases are opaque; they absorb and re-radiate it.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7: Solar Radiation, Heat Balance and Temperature, p.69; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7: Composition and Structure of Atmosphere, p.64; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7: World Climate and Climate Change, p.96; Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Chapter 20: Horizontal Distribution of Temperature, p.293

4. Atmospheric Pressure and Density Gradients (intermediate)

To understand the atmosphere, imagine yourself at the bottom of a deep sea, but instead of water, you are surrounded by a 'sea of air.' Atmospheric pressure is simply the weight of the column of air resting above a specific point. Because air has mass and gravity pulls it toward the Earth, the atmosphere is densest at sea level and thins out rapidly as you move upward. In the lower atmosphere, this decrease is quite sharp—roughly 1 millibar (mb) for every 10 meters of increase in elevation FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7, p. 76. However, this rate isn't perfectly constant because factors like temperature and water vapor influence air density; for example, at the peak of Mt. Everest, the air pressure is nearly two-thirds less than at sea level Physical Geography by PMF IAS, Chapter 20, p. 305.

An interesting puzzle arises here: if the vertical pressure gradient (the change in pressure with height) is so strong, why doesn't the air just rush upward into space? This is due to a delicate hydrostatic balance. While the pressure gradient force pushes air upward, it is almost perfectly countered by the downward pull of gravitational force Physical Geography by PMF IAS, Chapter 20, p. 306. This balance keeps our atmosphere 'stuck' to the planet rather than drifting away.

As we ascend further, the atmosphere stops being a well-mixed 'soup' of gases. Because different gases have different molecular weights, they begin to settle or disappear at different altitudes. Heavier components like Carbon dioxide (CO₂) and water vapor are found only up to about 90 km. Oxygen (O₂), while vital, becomes almost negligible by 120 km. In contrast, Nitrogen (N₂), being more abundant and stable, persists much higher Environment and Ecology, Majid Hussain, Chapter 1, p. 7. This transition marks the boundary between the homosphere (where gases are mixed) and the heterosphere (where they layer by weight).

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7: Composition and Structure of Atmosphere, p.76; Physical Geography by PMF IAS, Chapter 20: Earth's Atmosphere, p.305-306; Environment and Ecology, Majid Hussain, Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.7

5. The Homosphere vs. Heterosphere (exam-level)

While we often categorize the atmosphere based on temperature (like the Troposphere or Stratosphere), scientists also classify it based on its chemical composition. This gives us two distinct realms: the Homosphere and the Heterosphere. The boundary between them is known as the turbopause, located at an altitude of approximately 80 km.

The Homosphere extends from the Earth's surface up to about 80 km. In this region, despite the rapid decrease in air density, the proportion of major gases remains remarkably uniform due to constant turbulent mixing. If you took a bottle of air at sea level and another at 50 km, the ratio of Nitrogen to Oxygen would be nearly identical. There are only a few exceptions to this uniformity: the concentration of Ozone (O₃) in the stratosphere, and the concentration of water vapor and pollutants which are concentrated near the surface. Environment and Ecology, Majid Hussain, Chapter 1, p.7. This stable chemical mix was achieved roughly 600 million years ago.

Above 80 km, we enter the Heterosphere. Here, the air is so thin that molecules rarely collide, meaning they no longer 'mix' well. Instead, gravity becomes the primary organizer. Gases settle into distinct layers based on their molecular weight—the 'heavy' gases stay low, and the 'light' gases float to the top. Environment and Ecology, Majid Hussain, Chapter 1, p.6. Because of this sorting and chemical stability, different gases 'disappear' or become negligible at different heights as you move upward:

• Carbon Dioxide (CO₂) & Water Vapor: Found only up to ~90 km.
• Oxygen (O₂): Becomes almost negligible at ~120 km.
• Nitrogen (N₂): Persists the longest because it is both abundant and highly stable.

In the Heterosphere, the layering follows a specific sequence from heaviest to lightest: Molecular Nitrogen (N₂), Atomic Oxygen (O), Helium (He), and finally Hydrogen (H) at the outermost fringes where the atmosphere transitions into the exosphere. Environment and Ecology, Majid Hussain, Chapter 1, p.6.

Feature	Homosphere	Heterosphere
Altitude	Surface to 80 km	80 km to ~10,000 km
Gas Distribution	Uniform mixture (well-mixed)	Layered by molecular weight
Primary Driver	Convection and Turbulence	Gravity and Diffusion

Sources: Environment and Ecology, Majid Hussain, Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.6-7; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7: Composition and Structure of Atmosphere, p.64

6. Vertical Limits of Specific Gases (exam-level)

While we often think of the atmosphere as a uniform blanket of air, it is actually a stratified structure where the concentration of different gases changes drastically with altitude. This distribution is primarily governed by molecular weight and chemical stability. In the lower part of the atmosphere, known as the Homosphere (extending up to about 80–90 km), gases are generally well-mixed due to atmospheric turbulence. However, even within this zone, certain gases reach their "vertical limits" and effectively disappear as we move higher.

The first gases to become negligible are the heavier and more variable ones. Carbon dioxide (CO₂) and water vapour, which are crucial for maintaining the Earth's temperature through the greenhouse effect, are found only up to an altitude of approximately 90 km from the surface FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7, p. 64. Beyond this height, their presence is so minute that they no longer play a significant role in atmospheric processes.

As we climb higher into the reaches of the Thermosphere, even life-sustaining Oxygen (O₂) begins to thin out. By the time we reach a height of 120 km, oxygen becomes almost negligible in quantity Physical Geography by PMF IAS, Chapter 20, p. 272. In contrast, Nitrogen (N₂), which is the most abundant gas in our atmosphere and relatively stable, persists in significant proportions to much higher altitudes than oxygen or carbon dioxide.

Understanding these limits is vital because it explains the transition from the Homosphere to the Heterosphere. In the Heterosphere (above 90 km), the atmosphere is no longer well-mixed; instead, gases begin to settle into distinct layers based on their atomic weight, with lighter gases like Helium and Hydrogen dominating the outermost fringes of our planet.

Gas / Component	Approximate Vertical Limit	Significance
Carbon Dioxide & Water Vapour	90 km	Limit of major greenhouse effect contributors.
Oxygen (O₂)	120 km	Becomes negligible beyond this point.
Nitrogen (N₂)	Higher than 120 km	Most persistent of the major heavy gases.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7: Composition and Structure of Atmosphere, p.64; Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Chapter 20: Earths Atmosphere, p.272

7. Solving the Original PYQ (exam-level)

This question is a classic application of the vertical distribution of atmospheric gases you just studied. To solve it, you must recall the specific altitude limits where key gases become negligible. According to the FUNDAMENTALS OF PHYSICAL GEOGRAPHY (NCERT Class XI), while the atmosphere is well-mixed in the lower layers, individual gases reach their functional limits at different heights. You’ve learned that Carbon dioxide and water vapor are confined to the lowest layers, essentially disappearing after 90 km. Oxygen follows next, becoming almost non-existent at approximately 120 km. Nitrogen, being the most abundant and chemically stable, persists the longest in significant proportions as you move toward the outer reaches of the atmosphere.

Think of it as a molecular ladder: the gases with the lowest altitude thresholds drop off first. Since Carbon dioxide has the lowest ceiling (90 km), it must be the first gas in our "disappearance" sequence. Oxygen follows at 120 km, and Nitrogen remains the dominant gas far beyond those heights. This logical progression leads us directly to the correct answer: (A) Carbon dioxide- Oxygen Nitrogen. As a coach, I suggest you visualize these limits as physical boundaries; once you cross 90 km, CO2 is effectively gone, but you still have Oxygen for another 30 km.

A common trap UPSC uses here—seen in options (B), (C), and (D)—is to confuse the percentage of volume at sea level with the altitude limit. Students often incorrectly choose (D) because they see Nitrogen first and think of its 78% dominance at the surface. However, the question asks for the order of disappearance as one moves away from the surface. Options that place Nitrogen before Oxygen or CO2 are scientifically incorrect because they ignore the molecular stability and abundance that allow Nitrogen to persist at much higher altitudes than its counterparts, as explained in Physical Geography by PMF IAS.