Elements of the atmosphere

Correct answer: A, carbon dioxide. Plants take in carbon dioxide and use it with water and light energy to make sugars during photosynthesis. Carbon dioxide also absorbs some of the infrared radiation emitted by Earth’s surface, contributing to the greenhouse effect. The same gas therefore has a role in both processes.

The governing concept is that atmospheric importance depends not only on concentration but also on chemical and radiative effects. Trace gases such as ozone, carbon dioxide, and methane occur in small amounts, yet they absorb radiation, influence temperature, participate in chemical cycles, or protect life. Therefore option A is correct. Trace gases are not all more abundant than major gases, are present in the atmosphere, and are not restricted to liquid forms in oceans.

The governing idea is that the atmosphere is a mixture of permanent gases, variable gases, water vapour, and suspended particles. Their interactions influence temperature, clouds, precipitation, visibility, and climate. Thus option B gives the comprehensive conclusion. Nitrogen is abundant but not the only weather-forming component, argon is largely inert, and dust is an atmospheric particulate.

The correct answer is B. Ozone in the stratosphere absorbs much of the Sun’s harmful ultraviolet radiation. Near the ground, ozone is a reactive air pollutant that can form through sunlight-driven reactions involving other pollutants. It can irritate the lungs and harm plants, so its effects depend on where it is found.

The correct answer is A. In the heterosphere, gases are not uniformly mixed as they are in the lower, well-mixed atmosphere. At great altitudes, diffusion and gravity cause gases to become distributed more according to molecular mass, with lighter gases relatively higher than heavier ones.

A is correct. In the homosphere, mixing keeps the main gases relatively well blended. Higher up, in the heterosphere, molecular diffusion increasingly separates gases according to their molecular masses, so lighter gases become more prevalent with altitude.

A is correct. Stratospheric aerosols can scatter or absorb radiation, affecting the atmosphere’s radiative balance. Some aerosols can also influence chemical reactions involving ozone. The other choices describe processes or effects unrelated to stratospheric aerosols.