Elements of the atmosphere

A is correct. Polluted air can contain suspended particles such as smoke and fine aerosols. In humid conditions, water can collect on some particles, making them larger and more effective at scattering light. Haze or fog may then make distant objects harder to see. High humidity alone does not always produce fog.

The governing concept is Earth-system interdependence. Atmospheric gases regulate radiation and temperature, water vapour drives clouds and precipitation, and aerosols influence visibility, cloud formation and health. These components interact with oceans, land, ice and living organisms, so studying only one element gives an incomplete explanation of weather, climate and life-support conditions. Option A is correct; the other options deny these connections or assign weather to rocks alone.

A is correct. Nitrogen makes up about 78% of dry air and is relatively unreactive under ordinary atmospheric conditions because the N₂ molecule has a strong triple bond. Its abundance dilutes oxygen, so the atmosphere is less supportive of intense combustion than an oxygen-rich mixture would be. This is a contribution to chemical stability, not proof that nitrogen is completely inert; lightning, microbes and industrial processes can convert it into nitrogen compounds.

The governing concept is photochemical ozone formation. High-energy ultraviolet radiation can split an O₂ molecule into oxygen atoms; an oxygen atom then combines with another O₂ molecule to form O₃, with a third molecule carrying away excess energy. Therefore A correctly identifies O₃ as triatomic oxygen and explains its chemical link with O₂. B, C and D are chemically false.

Atmospheric composition is linked to pressure, temperature, radiation balance, surface chemistry and possible biological activity. Comparing gases on different planets helps scientists infer how atmospheres evolve and whether conditions might permit liquid water or life as we know it. Option A is correct, though composition alone cannot prove habitability. The other options are false because planets differ greatly and gases strongly influence planetary environments.

Water vapour connects weather and climate because it operates on both short and long time scales. In daily weather, evaporation, condensation and cloud formation produce humidity, clouds and rainfall. Over longer periods, water vapour absorbs outgoing longwave radiation and acts as an important greenhouse gas, influencing temperature and climate feedbacks. Therefore, option A is correct; the other options deny well-established atmospheric roles.

A is correct. Percentages show how much of each gas is present, but not all of its effects. A complete analysis also considers how a gas reacts, interacts with radiation and affects processes in the Earth system. For example, a gas present in a small amount may still have an important radiative or chemical role.

A is correct. When moist air cools to saturation, water vapour can condense on tiny particles suspended in the air. Dust and other suitable aerosols can provide these surfaces, called condensation nuclei, helping cloud droplets form. Dust does not produce oxygen or remove nitrogen as part of this process.

A is correct. Stratospheric ozone absorbs much of the Sun's harmful ultraviolet radiation, including nearly all UV-C and a substantial part of UV-B. This reduces the amount of biologically harmful radiation reaching Earth's surface. Ozone is not the oxygen directly used for respiration, and it does not fix nitrogen.

A is correct because nitrogen is an essential part of proteins and other biological molecules. Most organisms cannot use atmospheric nitrogen gas directly; nitrogen fixation converts it into usable compounds that enter the nitrogen cycle. The other options describe effects that nitrogen does not have.

A is correct: oxygen is classed as a permanent gas because its proportion is comparatively stable, while water vapour varies greatly with place, temperature, and weather. The other pairs do not give a permanent gas first and a variable component second.

A is correct. Near sunrise and sunset, sunlight travels through a longer path in the atmosphere. Dust and other aerosols can scatter and absorb different wavelengths by different amounts, changing the colours that reach the observer. The exact effect depends on particle size and atmospheric conditions.

A is correct because stratospheric ozone absorbs much of the Sun’s harmful ultraviolet radiation, especially UV-B and nearly all UV-C, reducing the amount that reaches Earth's surface. This protection is not complete, and ozone near the ground can be a pollutant.

A is correct. Respiration, decomposition, fuel burning, and other processes add carbon dioxide to the atmosphere, while plants and oceans absorb some of it. These sources and sinks vary, so its atmospheric concentration is not perfectly constant.

A is correct. Breaking waves and bursting bubbles send tiny seawater droplets into the air. As their water evaporates, the dissolved salts remain as airborne particles. This sea spray is the main source of sea-salt particles.

A is correct because photosynthesis releases oxygen into the atmosphere, while respiration uses oxygen to release energy from food. Together, these processes link living organisms with atmospheric oxygen, although they are not the only processes affecting the oxygen cycle.

A is correct. Dust particles can act as condensation nuclei: water vapour can condense on their surfaces to form droplets, which may grow and contribute to precipitation. Dust alone does not guarantee precipitation; suitable moisture and atmospheric conditions are also needed. Nitrogen, oxygen, and helium are gases, not solid particles.

A is correct. Nitrogen makes up much of air and is relatively unreactive under ordinary conditions. Mixed with oxygen, it dilutes the oxygen concentration, so many fuels burn less vigorously than they would in nearly pure oxygen. This does not mean nitrogen never takes part in chemical reactions.

A is correct. Dry air contains about 78% nitrogen by volume, and nitrogen is relatively unreactive under ordinary atmospheric conditions. Thus, most of the air does not readily support combustion. Water vapour is variable, and air is a mixture of gases rather than pure oxygen.

The governing concept is that the atmosphere contains permanent gases as well as variable gases and suspended particles. Water vapour influences humidity, clouds, rainfall, and heat retention, while dust and aerosols can provide condensation surfaces and affect radiation. Therefore, option A correctly gives the complete reason. Option B denies the obvious presence of gases, whereas C and D wrongly describe the atmosphere as being made only by mountains or solid rocks.

Correct answer: B. Nitrogen makes up most of the atmosphere, and molecular nitrogen is relatively unreactive under ordinary conditions. Its abundance dilutes the more reactive oxygen and helps moderate the air’s overall chemical reactivity. It does not destroy oxygen, and water vapour remains variable in amount.

Correct answer: A. Aerosols are tiny solid or liquid particles suspended in air. They can scatter light and some can absorb it, reducing the clear light reaching an observer from distant objects. This is why haze, smoke, or dust can reduce visibility.

B is correct. Evaporation from oceans, lakes, and land, along with transpiration from plants, supplies most atmospheric water vapour near Earth's surface. As moist air rises, it cools; some water vapour condenses and returns to the surface as precipitation. These processes help make water vapour less abundant at greater heights.

The homosphere is the lower atmospheric region in which turbulence and vertical mixing keep the proportions of major gases relatively uniform. This does not mean that every gas has exactly the same concentration everywhere; water vapour and pollutants can vary considerably. Above it, the heterosphere shows stronger separation by molecular mass. The exosphere is the outermost atmospheric region, while the magnetosphere and lithosphere are not the required concept.

Explosive volcanic eruptions can inject ash, fine dust, sulfur-bearing gases, and other aerosols into the atmosphere. These materials may temporarily raise the concentration of suspended particles and can influence visibility, air quality, and radiation. They do not create ocean waves, make atmospheric oxygen pure, or permanently alter the normal ratio of noble gases. Therefore option B is the only scientifically consistent group.