Elements of the atmosphere

Option A is correct because water vapour is a variable component of the atmosphere. Its concentration changes with location, season, temperature, evaporation, and weather, unlike the major permanent gases whose proportions are comparatively stable. “Dry air” calculations therefore exclude water vapour so that the percentages of gases such as nitrogen and oxygen can be stated consistently. The other options contradict observation.

Option B is correct. Nitrogen makes up most of the atmosphere and, under ordinary conditions, does not support combustion. Mixed with oxygen, it lowers the oxygen concentration compared with pure oxygen, so combustion is generally less vigorous than it would be in pure oxygen. Nitrogen does not instantly stop every fire or turn other gases into water vapour.

The correct answer is C. Aerosols are tiny solid particles or liquid droplets suspended in air. They can scatter or absorb light, which may reduce visibility, and some provide surfaces for water vapour to condense on, influencing cloud formation. Their effects vary by type; they do not create mountains or oceans or move the Sun.

The atmosphere is a mixture of permanent and variable gases, water vapour, and suspended particles called aerosols. Gases form its main body, water vapour varies with place and weather, and aerosols include dust, smoke, and other fine particles. Thus option A includes atmospheric components. The other options list landforms, rocks, or internal-Earth materials, not components of the atmosphere.

The correct answer is B. Stratospheric ozone absorbs much harmful ultraviolet radiation, while carbon dioxide absorbs some of Earth’s outgoing infrared radiation and contributes to the natural greenhouse effect. Their roles differ, but both can have important effects even at relatively low concentrations. Nitrogen and oxygen, not these two gases, are the most abundant gases in dry air.

The correct answer is A. In approximate dry-air composition by volume, argon is about 0.93%, while carbon dioxide is about 0.04% (its amount varies over time and by location). Since 0.93% is greater than about 0.04%, argon is more abundant. Oxygen is about 21% of dry air, so argon is not more abundant than oxygen.

The correct answer is A, oxygen. Organisms use oxygen in aerobic respiration to help release usable energy from food. In photosynthesis, plants mainly take in carbon dioxide and water to make sugars using light energy; oxygen is released as a product. Plants also respire, so they use oxygen as well.

A balanced description recognises that the atmosphere is a mixture, not a single substance. Permanent and variable gases form its major gaseous component, while water vapour and aerosols, including fine dust and droplets, influence clouds, weather, radiation, and climate. Thus option C is correct. Nitrogen is only one major gas, dust is only a minor component, and water vapour is definitely present even though its amount varies.

Correct answer: A. Aerosols are tiny solid or liquid particles suspended in air. They can scatter or absorb light, reducing visibility, and some provide surfaces on which water vapour can condense. These particles can therefore contribute to cloud formation.

The correct answer is A. Aerosols are tiny solid particles or liquid droplets suspended in air. Sea spray can carry salt-containing droplets into the atmosphere; after the water evaporates, small salt particles may remain airborne.

The correct answer is A. Stratospheric ozone absorbs much of the Sun’s harmful ultraviolet radiation. Carbon dioxide absorbs some outgoing infrared radiation and is therefore involved in Earth’s heat balance and the greenhouse effect. Nitrogen and oxygen, not these two gases, are the most abundant atmospheric gases.

The correct answer is A. Volcanic eruptions can naturally release ash particles into the air. Industrial activity and fuel burning can produce human-made smoke and fine particles. Nitrogen, argon, ozone and oxygen are gases, not aerosol particles in this context.

The correct answer is A. Stratospheric ozone absorbs much of the Sun’s harmful ultraviolet radiation, helping protect living organisms. Oxygen is used in aerobic respiration, through which cells release energy from food. The other options assign these roles to gases that do not perform them.

The composition of the atmosphere should be studied quantitatively and functionally. Permanent gases such as nitrogen and oxygen have approximate proportions, whereas water vapour and aerosols vary with place, season and weather. Therefore, option C is correct because it combines amount, role and variability. Memorising percentages alone is incomplete, and oxygen alone does not represent the whole atmosphere; dust particles are aerosols, not gases.

The correct answer is B. Nitrogen makes up most of dry air and dilutes oxygen. As a result, combustion is generally less vigorous than it would be in pure oxygen. Clouds form from condensed water, ozone absorbs much harmful ultraviolet radiation, and plants use carbon dioxide as their carbon source in photosynthesis.

The correct answer is B. Earth emits thermal energy as infrared radiation. Water vapour absorbs some of this outgoing radiation and re-emits energy, including back toward the surface, helping the atmosphere retain heat.

The correct answer is B, cloud-droplet formation. Water vapour supplies the water molecules, while aerosol particles can provide surfaces called condensation nuclei. As moist air cools, water vapour can condense on these particles to form tiny cloud droplets.

The correct answer is A. The proportions of permanent gases such as nitrogen, oxygen and argon remain relatively stable over ordinary timescales. Water vapour is variable: its amount changes with location, temperature and weather. Permanent describes its relatively stable proportion, not a solid state.

Answer: A. “Trace” describes a gas’s small concentration, not its importance. Even in small amounts, some gases interact with particular wavelengths of radiation or take part in chemical processes. For example, ozone absorbs ultraviolet radiation, while carbon dioxide and water vapour absorb some outgoing infrared radiation.

Answer: A. Aerobic respiration uses oxygen to release energy from food, and oxygen supports ordinary combustion. In photosynthesis, green plants use carbon dioxide and water, with light energy, to make food. The other options assign these processes to gases that do not have the stated roles.

The governing concept is atmospheric composition. The atmosphere is a mixture of permanent and variable gases, water vapour, and suspended solid or liquid particles called aerosols. These components influence pressure, weather, radiation and visibility. Option A is therefore the complete group. Rocks, soil and lava belong mainly to the lithosphere, rivers and oceans to the hydrosphere, and mountains, plateaus and plains to landforms; hence B, C and D are not correct.

The governing concept is the water cycle in the atmosphere. Humidity measures the amount of water vapour present in air, and clouds form when moist air cools until water vapour condenses into tiny droplets or ice crystals. Argon, nitrogen and helium do not normally condense to form ordinary clouds and are not the substance measured by humidity. Therefore option C is the only correct answer.

The correct answer is B, aerosols. Smoke commonly contains fine solid particles and liquid droplets suspended in air. An increase in smoke and fine particles therefore indicates an increase in atmospheric aerosol particles, which can reduce visibility and worsen air quality.

A is correct. Quantitative importance concerns how much of a gas is present; qualitative importance concerns its role or effect. Carbon dioxide is only a small fraction of the atmosphere, yet it is used in photosynthesis and influences climate by absorbing some outgoing infrared radiation. This shows how a gas can be small in amount but important in function.

A is correct. Most atmospheric water vapour is in the troposphere, the lowest layer, where evaporation and most weather occur. Ozone occurs at different heights, but its greatest concentration is in the stratosphere, where it absorbs much of the Sun’s harmful ultraviolet radiation.