Correct Option
The correct option is Dust and salt particles.
Explanation
Condensation is the process by which water vapour transforms into liquid water. In the free atmosphere, this phase change requires microscopic solid particles to act as surfaces upon which water vapour can condense. These particles are known as hygroscopic nuclei (or condensation nuclei) because of their ability to absorb water.
Detailed Analysis
- Ozone depletion is incorrect: Ozone depletion refers to the thinning of the ozone layer in the stratosphere, primarily caused by chemicals like chlorofluorocarbons (CFCs), not by hygroscopic nuclei.
- Dust and salt particles is correct: Hygroscopic nuclei are primarily composed of microscopic particles such as dust, salt from sea spray, and smoke. These particles have a high affinity for water, allowing condensation to begin even before the air reaches 100% relative humidity. Without these nuclei, condensation would be difficult to achieve in the atmosphere.
- Ionization in the thermosphere is incorrect: Ionization in the thermosphere is caused by high-energy solar radiation stripping electrons from atoms; it is unrelated to the condensation of water vapour.
- Greenhouse effect is incorrect: The greenhouse effect is the warming of Earth's surface due to the trapping of heat by greenhouse gases (like carbon dioxide and methane), not by the physical presence of condensation nuclei.
Key Takeaway
Hygroscopic nuclei, such as salt and dust particles, provide the essential surfaces required for condensation and the subsequent formation of clouds and precipitation.