Which one of the following gases, present in the air near the surface of the Earth, has maximum concentration?

1. Structure of the Atmosphere: The Five Layers (basic)

To understand the chemistry of our world, we must first look at the vessel that holds it: the atmosphere. Think of the atmosphere not as a single uniform block of air, but as a multi-layered cake. Each layer has its own unique chemical composition, density, and temperature behavior. These layers are held close to Earth by gravity, which is why the air is thickest (most dense) at the bottom and becomes incredibly thin as you travel toward space FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.65.

The atmosphere is divided into five distinct layers based primarily on how temperature changes with altitude. At the surface, the air is a rich mixture of gases—predominantly Nitrogen (78.08%) and Oxygen (20.95%), with trace amounts of Argon, CO₂, and Methane Physical Geography by PMF IAS, Composition of The Earth's Atmosphere, p.270-271. As we move upward, the physical and chemical properties of these layers dictate everything from our weather to our ability to communicate via radio waves.

Here is a breakdown of the five layers from the ground up:

Layer	Key Characteristics
Troposphere	The lowermost layer where all weather occurs. Temperature decreases as you go higher. It is thickest at the equator (18km) due to strong convection currents FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), p.65.
Stratosphere	Contains the Ozone Layer, which absorbs UV radiation. Because of this absorption, temperature actually increases with height here.
Mesosphere	The coldest layer. This is where most meteorites burn up upon entry. Temperature starts decreasing again.
Thermosphere	Contains the Ionosphere, which is vital for radio communication as it reflects radio waves back to Earth Physical Geography by PMF IAS, Earths Atmosphere, p.278.
Exosphere	The outermost fringe where the atmosphere gradually fades into the vacuum of space.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.65; Physical Geography by PMF IAS, Earths Atmosphere, p.270-271, 278

2. Permanent vs. Variable Gases (basic)

When we look at the Earth's atmosphere, it is tempting to think of it as a uniform "blanket" of air. In reality, it is a sophisticated chemical cocktail where gases are categorized based on their stability. Permanent gases are those that remain in a fixed proportion to the total volume of the atmosphere, regardless of where you are on the planet. The most dominant of these is Nitrogen (N₂), making up about 78.08% of dry air, followed by Oxygen (O₂) at approximately 20.95% Physical Geography by PMF IAS, Earths Atmosphere, p.271. Because these gases are relatively heavy, they tend to stay concentrated in the lower layers of the atmosphere, though the concentration of Oxygen becomes almost negligible once you reach a height of about 120 km FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Composition and Structure of Atmosphere, p.64.

On the other hand, Variable gases (and other constituents) change in quantity from place to place and time to time. The most famous variable gas is Water Vapour. Its concentration depends heavily on temperature—warm air can hold much more moisture than cold air—which is why it is more abundant over tropical oceans than over frozen poles Physical Geography by PMF IAS, Hydrological Cycle (Water Cycle), p.326. Other variable constituents include Carbon Dioxide (CO₂), which is transparent to incoming solar radiation but traps outgoing heat, and trace gases like Methane (CH₄) and Ozone (O₃) FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Composition and Structure of Atmosphere, p.66.

To help you distinguish between them, here is a quick comparison of how they behave in our atmosphere:

Feature	Permanent Gases	Variable Gases
Concentration	Constant/Fixed proportions.	Changes by location and time.
Examples	Nitrogen (N₂), Oxygen (O₂), Argon (Ar).	Water Vapour, CO₂, Methane (CH₄), Ozone (O₃).
Key Role	Bulk of atmospheric pressure and life support.	Climate regulation and weather patterns.

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.271; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Composition and Structure of Atmosphere, p.64; Physical Geography by PMF IAS, Hydrological Cycle (Water Cycle), p.326; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Composition and Structure of Atmosphere, p.66

3. The Greenhouse Effect and Trace Gases (intermediate)

To understand the Greenhouse Effect (GHE), we must first look at the atmospheric 'budget.' While Nitrogen (78.08%) and Oxygen (20.95%) make up the vast majority of our air, they are effectively 'transparent' to the heat radiating from Earth’s surface Physical Geography by PMF IAS, Earths Atmosphere, p.270. The real work of temperature regulation is done by trace gases—gases that exist in minute concentrations but possess a unique molecular structure that allows them to absorb long-wave terrestrial radiation. Without this natural 'blanket,' Earth's average temperature would be a freezing -18°C instead of the comfortable 15°C we enjoy today.

The impact of a greenhouse gas (GHG) depends on two critical factors: its absorptive capacity (how much heat it can trap) and its atmospheric lifetime (how long it stays in the air before being removed by natural processes) Environment, Shankar IAS Academy, Climate Change, p.260. For instance, Methane (CH₄) is produced largely through anaerobic processes, such as bacterial action in rice fields or the digestive tracts of livestock Environment and Ecology, Majid Hussain, Climate Change, p.11. While it persists in the atmosphere for a much shorter time than Carbon Dioxide, it is far more efficient at trapping heat during its stay.

To compare these different gases, scientists use Global Warming Potential (GWP). GWP measures how much energy the emissions of 1 ton of a gas will absorb over a given period (usually 100 years), relative to the emissions of 1 ton of Carbon Dioxide (CO₂). This allows us to calculate CO₂ equivalents, providing a common scale to measure the climate impact of different emissions Environment, Shankar IAS Academy, Environment Issues and Health Effects, p.425.

Gas	GWP (100-year)	Atmospheric Lifetime	Primary Sources
Carbon Dioxide (CO₂)	1 (Baseline)	Variable (approx. 100 yrs)	Fossil fuel combustion, respiration
Methane (CH₄)	21	~12 years	Wetlands, livestock, rice fields
Nitrous Oxide (N₂O)	310	~120 years	Fertilizers, industrial processes
HFCs / PFCs	1,400 – 11,000+	Up to 50,000 years	Refrigerants, electronics manufacturing

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.270; Environment, Shankar IAS Academy, Climate Change, p.260; Environment and Ecology, Majid Hussain, Climate Change, p.11; Environment, Shankar IAS Academy, Environment Issues and Health Effects, p.425

4. The Nitrogen Cycle and Biological Importance (intermediate)

While nitrogen (N₂) is the most abundant gas in our atmosphere—making up approximately 78.08% of the air we breathe—it presents a biological paradox. It is an absolute requirement for life because it is a fundamental building block of proteins (constituting about 16% of their weight) and nucleic acids like DNA. However, most living organisms cannot use atmospheric nitrogen directly because its two atoms are held together by an incredibly strong triple bond that plants and animals lack the enzymes to break. Physical Geography by PMF IAS, Earth's Atmosphere, p.270; Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19

To become "bio-available," nitrogen must undergo Nitrogen Fixation, where it is converted into usable forms like ammonia (NH₃) or nitrates (NO₃⁻). This happens in three main ways:

• Biological Fixation: Specialized microorganisms like Rhizobium (found in the root nodules of legumes like beans and pulses) and free-living bacteria like Azotobacter perform this heavy lifting. Environment and Ecology, Majid Hussain, Basic Concepts of Environment and Ecology, p.20
• Atmospheric Fixation: The immense energy from lightning can break the nitrogen bonds, allowing it to combine with oxygen to form nitrogen oxides that dissolve in rain.
• Industrial Fixation: Humans create nitrogen fertilizers through chemical processes, which has revolutionized modern agriculture.

Once fixed as ammonia, the nitrogen moves through a process called Nitrification. Here, specific bacteria act as nature’s chemists: Nitrosomonas converts ammonia into nitrites (NO₂⁻), and then Nitrobacter converts those nitrites into nitrates (NO₃⁻), which plants can easily absorb. In agriculture, we often use "Neem-coated urea" because the neem oil acts as a natural inhibitor, slowing down the rate at which urea dissolves and converts in the soil, ensuring that plants have a steady supply of nitrogen rather than losing it to the atmosphere or groundwater. Environment, Shankar IAS Academy, Functions of an Ecosystem, p.20; Indian Economy, Nitin Singhania, Agriculture, p.361

Process Step	Key Agent	Chemical Transformation
Fixation	Rhizobium / Blue-green algae	N₂ → NH₃ / NH₄⁺
Nitrification (Step 1)	Nitrosomonas	NH₃ → NO₂⁻ (Nitrite)
Nitrification (Step 2)	Nitrobacter	NO₂⁻ → NO₃⁻ (Nitrate)

Sources: Physical Geography by PMF IAS, Earth's Atmosphere, p.270; Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19-20; Environment and Ecology, Majid Hussain, Basic Concepts of Environment and Ecology, p.20; Indian Economy, Nitin Singhania, Agriculture, p.361

5. Vertical Distribution: Why Gases Settle Near the Surface (intermediate)

To understand why our atmosphere isn't just a uniform 'fog,' we must look at the tug-of-war between gravity and molecular weight. Near the Earth's surface, the atmosphere is a dense mix where Nitrogen (N₂) and Oxygen (O₂) dominate, accounting for approximately 78.08% and 20.95% of dry air respectively Physical Geography by PMF IAS, Chapter 20, p.270. These gases are relatively 'heavy.' Gravity pulls these molecules toward the center of the Earth, keeping them concentrated in the Troposphere (the lowest 0-12 km). Because of this gravitational pull, the density and pressure of the air are highest at sea level and drop significantly as you climb higher Physical Geography by PMF IAS, Chapter 20, p.274.

As we move into the upper reaches of the atmosphere, specifically the Heterosphere (starting around 80 km), the air becomes so thin that gases no longer stay mixed by turbulence. Instead, they begin to 'settle' into distinct layers based on their atomic weight Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.6. In this region, gravity acts like a cosmic sorter: heavier elements like Oxygen and Nitrogen stay at the bottom of the stack, while the lightest gases—Hydrogen (H₂) and Helium (He)—float to the very top, at the margin of outer space.

In the outermost layer, the Exosphere, the concentration of these light gases is incredibly low. Here, individual atoms of Hydrogen and Helium are only weakly bonded by gravity. If these light molecules gain enough energy from the sun or solar wind, they can reach escape velocity and leak into space—a process known as atmospheric stripping Physical Geography by PMF IAS, Chapter 20, p.280. We are fortunate that Earth’s magnetic field acts as a shield, preventing solar winds from stripping away our atmosphere even faster.

Sources: Physical Geography by PMF IAS, Chapter 20: Earth's Atmosphere, p.270; Physical Geography by PMF IAS, Chapter 20: Earth's Atmosphere, p.271; Physical Geography by PMF IAS, Chapter 20: Earth's Atmosphere, p.274; Physical Geography by PMF IAS, Chapter 20: Earth's Atmosphere, p.280; Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.6

6. Precise Volumetric Concentration of Air (exam-level)

To understand the chemistry of our environment, we must first look at air not as a single substance, but as a uniform mixture of distinct gases. While we often think of the atmosphere as a vast, changing entity, the proportions of its major constituents—known as permanent gases—remain remarkably fixed near the Earth's surface. Nitrogen (N₂) is the undisputed heavyweight, making up approximately 78.08% of dry air by volume, followed by Oxygen (O₂) at roughly 20.95% Physical Geography by PMF IAS, Earths Atmosphere, p.271. These two gases alone account for about 99% of the atmosphere's volume, but they behave very differently: while Oxygen is essential for life and combustion, Nitrogen is chemically relatively inert and does not support combustion Science, Class VIII NCERT, Nature of Matter, p.118. Beyond the 'Big Two,' the remaining 1% contains gases that, despite their small volumes, play massive roles in Earth's climate and chemistry. The third most abundant gas is Argon (Ar) at 0.93%, followed by Carbon dioxide (CO₂), which is considered a trace gas at approximately 0.036% Physical Geography by PMF IAS, Earths Atmosphere, p.270. Below these are even smaller concentrations of noble gases and hydrocarbons. It is a common misconception that Carbon dioxide is more abundant than Argon; in reality, Argon is nearly 25 times more prevalent than CO₂.

Gas Name	Chemical Formula	% Concentration (by Volume)
Nitrogen	N₂	78.08%
Oxygen	O₂	20.95%
Argon	Ar	0.93%
Carbon Dioxide	CO₂	0.036%
Methane	CH₄	0.000179%
Hydrogen	H₂	0.00005%

Gravity plays a crucial role in maintaining this specific concentration profile. Heavier gases, specifically Nitrogen and Oxygen, tend to remain concentrated in the lower layers of the atmosphere (the troposphere) due to their molecular weight Physical Geography by PMF IAS, Earths Atmosphere, p.271. As you ascend, the air becomes 'thinner,' and by a height of about 120 km, the presence of Oxygen becomes almost negligible FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI NCERT, Composition and Structure of Atmosphere, p.66. This stratification is why the precise volumetric concentrations we study are generally measured 'near the surface' in dry air.

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.270-271; Science, Class VIII NCERT, Nature of Matter, p.118; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI NCERT, Composition and Structure of Atmosphere, p.66

7. Solving the Original PYQ (exam-level)

Now that you have mastered the classification of the Earth's atmosphere into permanent gases and variable gases, this question tests your ability to apply those relative proportions. You've learned that gravity keeps the heavier, more stable gases concentrated in the troposphere, which is the layer closest to the Earth's surface. According to Physical Geography by PMF IAS, the atmosphere is a mechanical mixture where a few specific gases dominate the volume of dry air, and this question asks you to identify the leader of that hierarchy.

To arrive at the correct answer, you must recall the specific percentages of the atmospheric constituents. While Oxygen is vital for life, it is not the most abundant; Nitrogen (N2) occupies the top spot with a concentration of approximately 78.08%. Oxygen follows at roughly 21%. When you see the phrase "near the surface of the Earth," remember that the atmosphere is most dense here, and the proportions of these major gases remain consistent due to constant mixing. Therefore, Nitrogen (N2) is the only logically sound choice for the maximum concentration.

A common trap in UPSC is to confuse qualitative importance with quantitative abundance. Students often get distracted by Oxygen (O2) due to its role in respiration, or Methane (CH4) because of its prominence in environmental news. However, as highlighted in Physical Geography by PMF IAS, Methane and Hydrogen (H2) are classified as trace gases, existing in minuscule amounts (0.000179% and 0.00005% respectively). By distinguishing between the major structural components of the air and the minor trace elements, you can quickly eliminate the distractors.