Elements of the atmosphere

C is correct. During photosynthesis, plants, algae, and some microorganisms use light energy to make organic matter from carbon dioxide and water. This takes carbon dioxide out of the atmosphere. By contrast, combustion, respiration, and volcanic emissions generally release carbon dioxide.

Breaking waves, bubbles bursting at the ocean surface, and strong winds produce sea spray. When droplets evaporate, dissolved salts remain as tiny solid particles that become marine aerosols in the atmosphere. Therefore option C is correct. Polar ice is not the normal direct source, plant respiration does not produce salt particles, and terrestrial combustion is associated with smoke or ash rather than marine salt.

Correct answer: C, particulate matter. Ash consists of tiny solid particles, and smoke commonly contains suspended particles and may also contain small liquid droplets. These airborne particles are classified as particulate matter. Smoke may also include gases, but it is the suspended particles—not those gases—that make C the best answer.

A is correct. Argon is a noble gas and is chemically unreactive under ordinary conditions. It makes up a small but fairly stable proportion of dry air. Rainfall consists of water, oxygen is used in ordinary aerobic respiration, and the ozone layer consists mainly of ozone molecules.

The correct answer is B. In photosynthesis, plants take in carbon dioxide and release oxygen. In respiration, organisms use oxygen and release carbon dioxide. Together, these processes are major parts of the biological exchange of these gases between living things and the atmosphere.

The correct answer is A. Dust and smoke contain particles suspended in the air. These particles can scatter light in different directions, contributing to haze and reduced visibility. They may also absorb some light. Dust and smoke do not change an element’s atomic number or cause biological fixation of helium.

The correct answer is A. Nitrogen makes up about 78% of dry air, mostly as N₂. Most organisms cannot use this atmospheric form directly; nitrogen-fixing microbes and other natural or industrial processes convert it into usable compounds. Thus nitrogen is abundant, but not directly available to most organisms.

The correct answer is A, oxygen. Green plants release oxygen during photosynthesis, while plants, animals, and many microorganisms use it during respiration. This exchange shows how living organisms both affect and depend on an atmospheric component.

The correct answer is A. Ozone in the stratosphere absorbs much of the Sun’s harmful ultraviolet radiation before it reaches Earth’s surface. This reduces UV exposure that can damage living cells. Clouds form mainly through the condensation of water vapour, not because of ozone.

A is correct. Water vapour is a variable gas whose amount changes with weather and location, while dust consists of suspended solid particles. The other pairs are gases, not particulate matter; nitrogen and oxygen are also relatively stable major components of the atmosphere.

The correct answer is B, very far apart. The exosphere is extremely rarefied, so it contains very few gas particles in a given volume compared with the denser lower atmosphere. As a result, the average distance between its particles is large.

B is correct. Stratospheric ozone absorbs much of the Sun’s harmful ultraviolet radiation, especially UV-B and UV-C. If ozone decreases, less of this radiation is absorbed in the stratosphere, weakening this protection and allowing more ultraviolet radiation to reach lower levels and Earth’s surface.