This MCQ module is based on: Atmosphere — Composition, Gases & Ozone
TOPIC 14 OF 29
Atmosphere — Composition, Gases & Ozone
🎓 Class 11
Social Science
CBSE
Theory
Ch 7 — Composition and Structure of Atmosphere
⏱ ~25 min
🌐 Language: [gtranslate]
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7.1 Why Study the Atmosphere?
Can a person live without air? We eat food two or three times a day; we drink water somewhat more often; but we breathe every few seconds. Air is essential to the survival of every living organism. A human being can manage for some hours without food and water, yet cannot last even a few minutes without breathing. That single fact tells us why understanding the atmosphere in detail is a basic part of physical geography.
📖 Definition — Atmosphere
The atmosphere is a mixture of different gases that envelopes the earth on all sides. It contains the life-giving gases — oxygen for humans and animals and carbon dioxide for plants. The air is colourless and odourless and can be felt only when it blows as wind. Roughly 99 per cent of the total mass of the atmosphere is confined to a height of about 32 km above the earth's surface, even though the atmosphere extends much farther up before merging with outer space.
The atmosphere is not just a passive envelope of gas. It is an integral part of the earth's mass: it shields life from harmful solar radiation, traps heat to keep the planet warm enough for life, transports water from the oceans onto the land, and is the very stage on which all weather and climate phenomena unfold. Whatever happens in the layer of air closest to us — the troposphere — affects every aspect of our daily lives, from the food we grow to the storms we shelter from.
THINK ABOUT IT — Life Without Ozone
L3 Apply
Can you imagine what would happen to us in the absence of ozone in the atmosphere?
If ozone? were not present in the stratosphere, the high-energy ultra-violet (UV) radiation streaming in from the Sun would reach the earth's surface unfiltered. The immediate consequences would be: (i) sharp rise in skin cancers, eye cataracts and sunburn in humans and animals; (ii) damage to plant DNA, slashing crop productivity and harming forest growth; (iii) destruction of microscopic phytoplankton at the ocean surface — the base of the marine food chain; and (iv) faster degradation of plastics, paints and rubber. Higher UV doses would also raise mutation rates across all life. Without the ozone shield, life as we know it on land could not have evolved in the first place — that is why ozone is rightly called the "earth's sunscreen".
7.2 Composition of the Atmosphere
The atmosphere is composed of three main ingredients: gases, water vapour and dust particles. The proportion of gases changes only slightly through the lower layers but shifts dramatically with altitude. So much so that oxygen will be almost in negligible quantity at the height of 120 km; carbon dioxide and water vapour are found only up to about 90 km from the surface of the earth. This unequal distribution is one of the reasons life is concentrated in the lowest few kilometres of the atmosphere.
Gases — The Permanent Mixture by Volume
By volume of dry air, the atmosphere is overwhelmingly made up of just two gases — nitrogen and oxygen — together accounting for roughly 99 per cent of the lower atmosphere. The remaining one per cent is shared by argon, carbon dioxide and a long tail of trace gases. The exact percentages by volume of the principal gases are remarkably stable in the lower atmosphere and form the table below.
| Gas | Per Cent by Volume | Quick Note |
|---|---|---|
| Nitrogen (N₂) | 78.08 | Most abundant; relatively inert; vital for proteins via the nitrogen cycle. |
| Oxygen (O₂) | 20.95 | Sustains animal respiration and combustion; produced by photosynthesis. |
| Argon (Ar) | 0.93 | Noble gas; chemically unreactive; used in light bulbs and welding. |
| Carbon dioxide (CO₂) | 0.039 | Tiny by volume but massive in climate effect — the chief greenhouse gas. |
| Neon (Ne) | 0.0018 | Trace noble gas — gives "neon" advertising signs their red-orange glow. |
| Helium (He) | 0.0005 | Lightest after hydrogen; escapes upward and into space. |
| Krypton (Kr) | 0.0001 | Trace noble gas; used in some specialty lights and lasers. |
| Xenon (Xe) | 0.000009 | Rarest stable noble gas in the air. |
| Hydrogen (H₂) | 0.00005 | Lightest element; escapes from the upper atmosphere into space. |
| Methane (CH₄) | 0.00017 | Trace but powerful greenhouse gas; from wetlands, livestock, fossil fuels. |
Pie chart — Major gases of the lower atmosphere by volume
Nitrogen plus oxygen alone accounts for about 99 per cent of dry air; trace gases — though tiny in volume — control the radiation balance of the planet.
Carbon Dioxide — Tiny by Volume, Mighty by Effect
Carbon dioxide is meteorologically a very important gas. Although it makes up only about 0.039 per cent by volume, its behaviour towards radiation is asymmetric: it is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation. In simple terms, sunlight passes through CO₂ to reach and warm the earth, but the heat that the warmed earth then radiates back is trapped by CO₂ and partly returned downward. This selective trapping is the heart of the greenhouse effect?.
The volume of all the other major gases is more or less constant over time, but the volume of carbon dioxide has been rising over the past few decades, mainly because of the burning of fossil fuels — coal, petroleum, natural gas — and large-scale deforestation. The accumulation of additional CO₂ has, in turn, raised the temperature of the lower atmosphere, contributing to global warming.
🔑 Why CO₂ Matters Out of Proportion to Its Volume
Even at 0.039 per cent of the atmosphere, CO₂ is the chief lever of the planet's thermostat. Its molecules selectively absorb infrared wavelengths exactly where the warmed earth radiates most strongly — so a small percentage change in CO₂ produces a large change in trapped heat. This is why doubling the pre-industrial CO₂ level alarms climate scientists.
Ozone — The High-Altitude Shield
Ozone (O₃) is another important component of the atmosphere. It is found at heights between roughly 10 and 50 km above the earth's surface. Ozone acts as a filter: it absorbs the ultra-violet rays radiating from the Sun and prevents them from reaching the surface of the earth. By blocking this dangerous part of the solar spectrum, ozone makes the earth habitable for surface-dwelling life.
Ozone Layer — How It Filters Ultra-violet Rays
Water Vapour — The Variable Gas
Water vapour is also a variable gas in the atmosphere — variable because its proportion changes sharply from one place and one altitude to another. By volume it can range from almost zero to about 4 per cent of the air. In the warm and wet tropics, water vapour may account for as much as 4 per cent of the air; in the dry and cold areas of desert and polar regions it may be less than 1 per cent of the air. Water vapour also decreases with altitude and decreases from the equator towards the poles.
Water vapour does several remarkable things at once. It absorbs parts of the insolation streaming in from the Sun and also preserves the earth's radiated heat. Acting like a blanket, it prevents the earth from becoming too hot during the day and from cooling too much at night. Water vapour also contributes to the stability and instability of the air — when water vapour condenses, it releases latent heat, fuelling clouds, thunderstorms and tropical cyclones.
LET'S EXPLORE — Why Does the Sahara Get So Cold at Night?
L4 Analyse
Tropical deserts like the Sahara record very high day temperatures — sometimes above 45°C — but the same nights can drop close to 0°C. Use the role of water vapour? in radiation balance to explain.
Desert air contains very little water vapour — usually less than 1 per cent by volume — so the natural "blanket" that normally keeps surface heat trapped after sunset is missing. During the day, the cloudless dry sky lets nearly all the solar energy reach the surface, raising temperatures sharply. At night, the same dry sky lets terrestrial radiation escape almost unobstructed back into space, so the surface cools rapidly. In a humid coastal place like Mumbai, abundant water vapour absorbs the outgoing radiation and re-radiates it downward, making nights warm and stuffy. Water vapour, in short, is the "thermal damper" of the atmosphere — too little of it produces extreme diurnal swings.
Dust Particles — The Hidden Half of the Atmosphere
The atmosphere has a sufficient capacity to keep small solid particles suspended in it. These particles originate from many different sources and include sea salts, fine soil, smoke-soot, ash, pollen, dust and even disintegrated particles of meteors burning up in the upper air. Dust particles are generally concentrated in the lower layers of the atmosphere, but convectional air currents may transport them to great heights.
The higher concentration of dust particles is found in subtropical and temperate regions due to dry winds — compared with the lower concentration over equatorial and polar regions, where rainfall and ice cover keep the air relatively clean. Dust and salt particles act as hygroscopic nuclei? — that is, they are condensation nuclei around which water vapour condenses. Without dust, clouds and rainfall would be far less efficient. The same particles also scatter sunlight, producing the red and orange colours of dawn and sunset, and they affect visibility and air quality close to the ground.
🌫️ Did You Know — Saharan Dust Crosses Oceans
Each year, hundreds of millions of tonnes of Saharan dust are carried by trade winds across the Atlantic and fertilise the soils of the Amazon rainforest with phosphorus. The same dust supplies condensation nuclei for clouds in the Caribbean. The atmosphere — even in its dust component — is a planet-scale conveyor belt.
Sea Salt
Lifted by breaking waves; major nucleus for marine clouds and rainfall.
Fine Soil & Dust
From dry continental interiors; carried far by trade winds.
Smoke & Ash
From forest fires, volcanoes and combustion; can dim the sky regionally.
Pollen & Spores
Released by plants; trigger seasonal allergies; act as nuclei.
Meteor Particles
Shed by meteors burning up in mesosphere; settle slowly downward.
7.3 The Atmosphere Above 90 km — A Quiet Reminder
Although the table on page above lists carbon dioxide and water vapour among the main gases, both of them are in fact found only up to about 90 km from the surface of the earth. Above that height, the atmosphere is dominated by the lighter and more reactive species — atomic oxygen, nitrogen, helium and hydrogen — and the air becomes so thin that it is best described as a "near-vacuum". Yet even this rarefied air is part of the earth-atmosphere system, deflecting incoming meteors, generating auroras and reflecting radio waves. We will study these layers in detail in the next part of this chapter.
SOURCE — Why Is Carbon Dioxide Called "Meteorologically Important"?
L2 Understand
The NCERT text says: "Carbon dioxide is meteorologically a very important gas as it is transparent to the incoming solar radiation but opaque to the outgoing terrestrial radiation." Explain in your own words what this sentence means and why it matters for climate.
"Transparent to incoming solar radiation" means CO₂ does not block sunlight on its way down — short-wave solar energy passes through and warms the earth's surface. "Opaque to outgoing terrestrial radiation" means CO₂ does block the long-wave heat that the warmed earth then radiates back. The trapped heat re-emits in all directions, including downward, raising the surface temperature beyond what direct sunlight alone would deliver. This selective gate-keeping is the essence of the greenhouse effect, and the reason why a small change in CO₂ has a large effect on global temperature.
7.4 Snapshot — All Three Constituents at a Glance
| Constituent | Typical Range | Main Role | Vertical Limit |
|---|---|---|---|
| Permanent gases (N₂, O₂, Ar, CO₂, trace) | ≈ 99% by volume | Respiration, photosynthesis, greenhouse effect | O₂ negligible above 120 km; CO₂ up to ~90 km |
| Water vapour | 0–4% by volume; max in tropics | Cloud, precipitation, latent heat, "thermal blanket" | Up to about 90 km; mostly in lower troposphere |
| Dust particles (aerosols) | Variable; high in dry mid-latitudes | Condensation nuclei; scatter light; affect visibility | Mainly in troposphere; convection lifts them higher |
| Ozone (O₃) | Trace; concentrated in stratosphere | Absorbs UV; shields surface life | 10–50 km |
🌍 Big Picture
Without nitrogen, plants would have no protein. Without oxygen, animals could not breathe. Without carbon dioxide, plants could not photosynthesise. Without ozone, surface life would be sterilised by ultra-violet radiation. Without water vapour, there would be no clouds, no rain and no temperature buffering. Without dust, condensation would struggle to begin. Every constituent plays a non-negotiable part in keeping the planet habitable.
🎯 Competency-Based Questions — Composition of the Atmosphere
Case Stem. A weather balloon released over Jodhpur in May records: surface air temperature 44 °C, water-vapour content less than 1%, very high dust load and high CO₂ near busy roads. The same balloon reports CO₂ steadily declining above 80 km and almost no water vapour beyond 90 km. The next morning, the night-time minimum at the same station is 18 °C — a drop of 26 °C in twelve hours. Use this scenario to answer Q1–Q4.
Q1. The very large diurnal range over Jodhpur (44 °C → 18 °C) is best explained by —
L3 ApplyAnswer: (b). The desert atmosphere has very little water vapour, so the natural "thermal blanket" is missing. By day, the cloudless dry sky allows full solar input; by night, terrestrial radiation escapes unblocked and the surface cools rapidly — giving the very large diurnal range.
Q2. The fact that the balloon detects almost no carbon dioxide above 90 km tells us that —
L4 AnalyseCO₂ is a "lower-atmosphere" gas. NCERT specifies that carbon dioxide and water vapour are found only up to about 90 km from the earth's surface. The greenhouse effect therefore operates almost entirely within the troposphere and stratosphere; the upper atmosphere is shaped by other gases.
Q3. The high dust load over Jodhpur in summer would directly increase —
L5 Evaluate(i) The number of condensation nuclei available for cloud formation, (ii) scattering of sunlight (giving red sunsets) and (iii) hazy visibility. Subtropical dry-wind regions naturally have higher concentrations of dust, which acts as hygroscopic nuclei when monsoonal moisture finally arrives — explaining why pre-monsoon dust storms often precede heavy rains.
HOT Q. A school in Delhi wants to monitor air quality. Which two atmospheric constituents — apart from oxygen and nitrogen — would you advise them to track daily, and why? Write a 4–5 sentence advisory.
L6 CreateHint: Track (i) particulate matter (dust) — PM 2.5 and PM 10 — because Delhi sits in the subtropical dry-wind belt where dust is naturally high, plus vehicular soot adds urban load; high PM directly affects health and visibility. (ii) Carbon dioxide — because rising CO₂ near roads indicates fuel-burning intensity and longer-term climate stress. Both can be measured with low-cost school sensors. Reporting daily values in a noticeboard or website will help students connect classroom theory of atmospheric composition to lived urban experience and feed local environmental campaigns.
⚖️ Assertion–Reason Questions — Composition of the Atmosphere
Options:
(A) Both A and R are true, and R is the correct explanation of A.
(B) Both A and R are true, but R is NOT the correct explanation of A.
(C) A is true, but R is false.
(D) A is false, but R is true.
(A) Both A and R are true, and R is the correct explanation of A.
(B) Both A and R are true, but R is NOT the correct explanation of A.
(C) A is true, but R is false.
(D) A is false, but R is true.
Assertion (A): Although carbon dioxide makes up only about 0.039 per cent of the atmosphere, it is meteorologically very important.
Reason (R): Carbon dioxide is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation, and is largely responsible for the greenhouse effect.
Answer: (A) — Both are true and R precisely explains A. CO₂ is a small but extraordinarily efficient regulator of the planet's heat budget, exactly because of this asymmetric radiation behaviour.
Assertion (A): The percentage of water vapour in the air is much higher in the wet tropics than over deserts and polar regions.
Reason (R): Water vapour decreases with altitude and decreases from the equator towards the poles, ranging from up to 4 per cent in warm and wet tropics to less than 1 per cent in dry and cold regions.
Answer: (A) — Both true and R is the correct explanation. Warm air can hold more vapour than cold air, and evaporation rates from the warm tropical oceans are far higher than from cold polar surfaces.
Assertion (A): Dust particles act as hygroscopic nuclei in the atmosphere.
Reason (R): Water vapour condenses around dust and salt particles to produce clouds, and without such nuclei cloud and rainfall formation would be far less efficient.
Answer: (A) — Both true and R explains A. Hygroscopic ("water-attracting") nuclei give vapour a surface to condense on; their absence would slow cloud formation dramatically.
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