Which one among the following is not a greenhouse gas?

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. Far from being a simple 'void,' it is a precise mixture of components that makes life possible. Broadly, we can categorize its composition into three parts: permanent gases, variable gases, and solid particles. The 'permanent' gases, such as Nitrogen and Oxygen, remain in a fixed proportion to the total volume in the lower layers of the atmosphere, while 'variable' components like water vapor and carbon dioxide fluctuate based on location and time Physical Geography by PMF IAS, Earths Atmosphere, p.271. Nearly 99% of the atmosphere's clean, dry air is composed of just two gases: Nitrogen (78.08%) and Oxygen (20.95%). The remaining 1% consists of Argon (0.93%) and various trace gases. Although Carbon Dioxide (CO₂) constitutes only about 0.03% to 0.04% of the volume, its role in regulating the Earth's temperature is immense Environment and Ecology, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.6.

Gas	Percentage by Volume (Approx)	Type
Nitrogen (N₂)	78.08%	Permanent
Oxygen (O₂)	20.95%	Permanent
Argon (Ar)	0.93%	Permanent
Carbon Dioxide (CO₂)	0.036%	Variable
Neon, Helium, Methane	Traces	Variable/Trace

It is vital to understand that this composition is not uniform at all heights. Due to gravity, heavier gases like Oxygen and Nitrogen tend to settle in the lower layers. As we move upward, the air becomes 'thinner.' For instance, Oxygen becomes almost negligible at a height of 120 km, while Carbon Dioxide and water vapor are only found up to an altitude of 90 km from the surface FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Composition and Structure of Atmosphere, p.64. This concentration of essential gases near the surface is why the Troposphere (the lowest layer) is the most critical for life and weather phenomena.

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.271; Environment and Ecology (Majid Hussain), BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.6; FUNDAMENTALS OF PHYSICAL GEOGRAPHY (NCERT), Composition and Structure of Atmosphere, p.64

2. The Mechanism of the Greenhouse Effect (basic)

To understand the Greenhouse Effect, we must first look at how energy travels from the Sun to the Earth and back. The Sun emits energy in the form of short-wave solar radiation. Because these waves are short and high-energy, they pass through our atmosphere relatively easily. However, once the Earth’s surface absorbs this energy, it heats up and becomes a radiating body itself. The energy the Earth sends back toward space is different—it is long-wave terrestrial radiation (also known as infrared radiation) FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 9, p.69.

The magic—and the danger—of the greenhouse effect lies in how certain gases react to these two types of radiation. Gases like Carbon Dioxide (CO₂), Methane (CH₄), and water vapor are transparent to incoming short-wave radiation but opaque to outgoing long-wave radiation. This means they allow sunlight to enter but trap the heat trying to escape. This process is very similar to a glass greenhouse used in cold climates: the glass lets in the sunlight but prevents the heat from radiating back out, keeping the plants inside warm FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 11, p.96.

It is crucial to remember that the atmosphere is not heated directly by the Sun; rather, it is heated from below by the Earth’s terrestrial radiation Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Chapter 7, p.7. Without this natural mechanism, Earth’s average temperature would be a frozen -18°C instead of the comfortable 15°C we enjoy today. However, when human activities increase the concentration of these Greenhouse Gases (GHGs), they trap too much heat, leading to global warming Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Chapter 7, p.9.

Feature	Incoming Solar Radiation	Outgoing Terrestrial Radiation
Wave Type	Short-wave radiation	Long-wave (Infrared) radiation
Interaction with GHGs	Passes through (Transparent)	Absorbed and Re-emitted (Opaque)
Primary Effect	Heats the Earth's surface	Heats the atmosphere from below

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 9: Solar Radiation, Heat Balance and Temperature, p.69; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 11: World Climate and Climate Change, p.96; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Chapter 7: Climate Change, p.7, 9

3. Air Pollution: Acid Rain and Sulphur Dioxide (SO₂) (intermediate)

When we discuss global warming, we often lump all "air pollutants" together. However, as an aspiring civil servant, you must distinguish between Greenhouse Gases (GHGs) and Atmospheric Pollutants like Sulphur Dioxide (SO₂). While GHGs like CO₂ trap terrestrial radiation and warm the planet, SO₂ is a criteria pollutant that, ironically, can have a cooling effect on the Earth's surface by forming sulphate aerosols that reflect incoming solar radiation Environment, Shankar IAS Academy, Chapter 29, p.426. Despite this cooling, SO₂ is a major environmental threat because it is the primary precursor to Acid Rain.

The formation of acid rain is a multi-step process. It begins when oxides of sulphur and nitrogen are released from natural sources (like volcanoes) or anthropogenic sources (like coal-fired power plants). In the atmosphere, these oxides undergo oxidation—often stimulated by sunlight and photo-oxidants like ozone—to transform into strong acids like Sulphuric Acid (H₂SO₄) and Nitric Acid (HNO₃) Environment, Shankar IAS Academy, Chapter 5, p.103. These acids then dissolve in cloud water and reach the ground as precipitation with a pH lower than 5.6.

The impact of this "acidic precipitation" is widespread, affecting both the environment and the economy:

• Materials: It causes "marble cancer" or the corrosion of building stones (like the Taj Mahal) and the tarnishing of metals Environment, Shankar IAS Academy, Chapter 5, p.105.
• Ecology: It leaches essential nutrients from the soil and increases the acidity of water bodies, which can be lethal to fish and aquatic flora.
• Socio-economics: In agricultural nations like India, the damage to crops and fisheries directly impacts the Gross National Product (GNP) and the quality of life Environment, Shankar IAS Academy, Chapter 5, p.105.

To combat this, mitigation strategies include shifting to low-sulphur coal, implementing Environmental Impact Assessments (EIA) before setting up industries, and utilizing technologies like Flue-Gas Desulphurization (FGD) to scrub emissions Environment and Ecology, Majid Hussain, Chapter 1, p.10.

Sources: Environment, Shankar IAS Academy, Chapter 29: Environment Issues and Health Effects, p.426; Environment, Shankar IAS Academy, Chapter 5: Environmental Pollution, p.103; Environment, Shankar IAS Academy, Chapter 5: Environmental Pollution, p.105; Environment and Ecology, Majid Hussain, Chapter 1: Environmental Degradation and Management, p.10

4. Ozone Depletion and Montreal Protocol (intermediate)

To understand ozone depletion, we must first distinguish between 'good' ozone and 'bad' ozone. While ozone (O₃) in the troposphere (the air we breathe) acts as a potent greenhouse gas and pollutant, ozone in the stratosphere (the 'ozone layer') is our primary shield against harmful ultraviolet (UV) radiation. Scientific discovery in the 1970s revealed that synthetic chemicals—specifically Chlorofluorocarbons (CFCs) and Halons—were drifting into the stratosphere. Once there, solar radiation breaks them down, releasing chlorine and bromine atoms that catalyze the destruction of ozone molecules at an alarming rate Environment, Shankar IAS Academy, Ozone Depletion, p.272.

The global response to this crisis led to the Montreal Protocol (1987), widely considered the most successful environmental treaty in history. Its primary objective is the phase-out of Ozone Depleting Substances (ODS). It is important to note the synergy between ozone protection and climate change: many ODS are also powerful greenhouse gases with high Global Warming Potential (GWP). However, you must distinguish this from the Montreal Action Plan (2005), which was an intergovernmental meeting focused on extending the Kyoto Protocol for greenhouse gases Environment and Ecology, Majid Hussain, Biodiversity and Legislations, p.7.

India has been a proactive party to the Vienna Convention and the Montreal Protocol since 1991-92. To fulfill its international commitments, the Indian government notified the Ozone Depleting Substances (Regulation and Control) Rules, 2000 under the Environment (Protection) Act, 1986. These rules set strict deadlines for phasing out ODS like CFCs and Halons. More recently, through amendments in 2014 and 2019, India successfully prohibited the use and import of HCFC-141b, a chemical used in foam manufacturing Environment, Shankar IAS Academy, International Organisation and Conventions, p.410.

Substance Group	Primary Use	Status in India
CFCs	Refrigerants, Aerosols	Phased out (except medical use)
Halons	Fire Extinguishers	Phased out (except essential use)
HCFCs	Industrial Solvents/Foams	Aggressive phase-out (e.g., HCFC-141b)

Sources: Environment, Shankar IAS Academy, Ozone Depletion, p.272; Environment and Ecology, Majid Hussain, Biodiversity and Legislations, p.7; Environment, Shankar IAS Academy, International Organisation and Conventions, p.410

5. Ozone: Stratospheric Protection vs. Tropospheric Warming (intermediate)

Ozone (O₃) is an allotrope of oxygen consisting of three atoms bound together. In the world of atmospheric science, ozone is often described as having a "dual personality" depending on where it resides in the atmospheric layers. In the stratosphere (located roughly 20 to 50 kilometers above Earth), ozone is our primary shield. It efficiently absorbs harmful Ultraviolet (UV) radiation from the sun, acting like a global sunscreen that protects life from skin cancer and genetic damage Environment, Shankar IAS Academy, Ozone Depletion, p.267. However, when ozone forms in the troposphere (the layer we breathe), it is considered a pollutant and a potent greenhouse gas (GHG) that contributes to global warming.

The behavior of ozone in these two layers is fundamentally different. In the stratosphere, ozone is naturally created and destroyed in a balanced cycle. However, human-made chemicals like Chlorofluorocarbons (CFCs) and Nitric Oxide (NO) disrupt this balance, leading to ozone depletion or the "ozone hole" Environment, Shankar IAS Academy, Ozone Depletion, p.269. In the troposphere, ozone is not usually emitted directly; instead, it is a secondary pollutant formed when sunlight reacts with Nitrogen Oxides (NOx) and volatile organic compounds (smog) Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.11. While it helps form harmful smog, it also traps terrestrial infrared radiation, making it a significant contributor to the greenhouse effect.

Feature	Stratospheric Ozone ("Good")	Tropospheric Ozone ("Bad")
Primary Role	Absorbs harmful UV radiation.	Acts as a Greenhouse Gas; traps heat.
Formation	Natural action of UV on Oxygen (O₂).	Photochemical reaction (Sunlight + NOx + VOCs).
Impact of CFCs	CFCs destroy this protective layer.	CFCs themselves act as GHGs here.

It is important to understand the role of CFCs as a "double-edged sword." While they migrate to the stratosphere to catalyze the destruction of the ozone layer, they are also incredibly efficient at absorbing infrared radiation in the lower atmosphere—wavelengths that CO₂ and water vapor often miss Environment and Ecology, Majid Hussain, Climate Change, p.11. This means CFCs contribute to global warming both directly (as GHGs) and indirectly (by altering atmospheric chemistry) Fundamentals of Physical Geography, NCERT, World Climate and Climate Change, p.96.

Sources: Environment, Shankar IAS Academy, Ozone Depletion, p.267; Environment, Shankar IAS Academy, Ozone Depletion, p.269; Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.11; Environment and Ecology, Majid Hussain, Climate Change, p.11; Fundamentals of Physical Geography, NCERT, World Climate and Climate Change, p.96

6. Identifying Greenhouse Gases (GHGs) vs. Aerosols (exam-level)

To master climate change, we must distinguish between two players in the atmosphere: Greenhouse Gases (GHGs) and Aerosols. While both affect the Earth's temperature, they do so through very different physical mechanisms. Greenhouse Gases are gaseous substances that are transparent to incoming short-wave solar radiation but opaque to outgoing long-wave terrestrial radiation. In simpler terms, they allow sunlight in but trap the heat trying to escape back into space, acting like a thermal blanket for the planet. Fundamentals of Physical Geography, NCERT Class XI, World Climate and Climate Change, p.96

The primary GHGs include Carbon dioxide (CO₂), Methane (CH₄), Nitrous Oxide (N₂O), and synthetic chemicals like Chlorofluorocarbons (CFCs) and Hydrofluorocarbons (HFCs). Each gas has a different Global Warming Potential (GWP), which measures how much energy the emissions of 1 ton of a gas will absorb over a given period, relative to 1 ton of CO₂. For instance, while CO₂ is the most abundant human-emitted GHG, Methane is significantly more potent pound-for-pound in the short term. Environment, Shankar IAS Academy, Climate Change, p.260

Aerosols, on the other hand, are tiny solid particles or liquid droplets suspended in the air. Unlike GHGs, many aerosols — most notably those formed from Sulphur Dioxide (SO₂) — actually have a cooling effect on the Earth. They reflect incoming sunlight back into space (increasing the Earth's albedo) and act as seeds for cloud formation. This is why SO₂, despite being a major air pollutant from volcanic eruptions and coal combustion, is not classified as a greenhouse gas; it typically works against global warming rather than contributing to it.

Feature	Greenhouse Gases (GHGs)	Aerosols (e.g., Sulphates)
State	Gaseous	Particulate (Solid/Liquid)
Main Mechanism	Absorb outgoing infrared radiation	Reflect incoming solar radiation
Impact on Temp	Warming	Mostly Cooling (except Black Carbon)
Examples	CO₂, CH₄, N₂O, Tropospheric Ozone	Sulphate particles (from SO₂), Sea salt, Dust

It is also vital to understand the dual role of Ozone (O₃). While stratospheric ozone is our "good" shield against UV radiation, tropospheric ozone (ground-level ozone) is a potent greenhouse gas and a harmful pollutant. Environment and Ecology, Majid Hussain, Climate Change, p.9

Sources: Fundamentals of Physical Geography, NCERT Class XI, World Climate and Climate Change, p.96; Environment, Shankar IAS Academy, Climate Change, p.260; Environment and Ecology, Majid Hussain, Climate Change, p.9

7. Solving the Original PYQ (exam-level)

Now that you have mastered the atmospheric composition and the radiative properties of various gases, this question tests your ability to synthesize those building blocks. As you learned in FUNDAMENTALS OF PHYSICAL GEOGRAPHY (NCERT), the defining characteristic of a Greenhouse Gas (GHG) is its capacity to absorb and re-emit long-wave terrestrial radiation. To solve this, you must distinguish between general air pollutants and those specific molecules that create the 'blanket effect' around Earth.

Walking through the options, you can eliminate Methane immediately as a primary GHG. The real challenge lies in Ozone and Chlorofluorocarbons. As noted in Environment and Ecology by Majid Hussain, while these gases have other roles—such as UV protection or ozone depletion—they are also incredibly efficient at trapping heat. The correct answer is Sulphur dioxide because, as explained in Physical Geography by PMF IAS, it does not trap infrared radiation; instead, it often forms sulphate aerosols that reflect incoming sunlight, actually contributing to a cooling effect on the planet's surface.

UPSC frequently uses Ozone as a trap because students often associate it only with the 'Ozone Layer' and protection. However, you must remember its dual nature: it is a protector in the stratosphere but a potent greenhouse gas in the troposphere. Similarly, Chlorofluorocarbons are a common pitfall because they are primarily studied under 'Ozone Depletion,' yet their Global Warming Potential (GWP) is thousands of times higher than CO2. By process of elimination and understanding the cooling nature of aerosols, Sulphur dioxide stands out as the only non-GHG among the choices.