The correct option is 1 only
Explanation
The atmosphere is a mixture of gases, water vapour, and dust particles. However, the composition of the atmosphere is not uniform vertically. The density of gases is highest near the Earth's surface and decreases with increasing altitude. Heavier gases are concentrated in the lower layers, while lighter gases dominate the upper layers.
Statement-wise Analysis:
- Statement 1 is Correct: Oxygen is a relatively heavy gas compared to hydrogen or helium. Due to gravitational separation and the structure of the atmosphere, the concentration of oxygen becomes almost negligible at heights above 120 km.
- Statement 2 is Incorrect: Carbon dioxide is a heavy gas and is largely concentrated in the lower atmosphere. It is found in significant quantities only up to a height of 90 km from the surface of the Earth, not 500 km.
- Statement 3 is Incorrect: Water vapour is not distributed uniformly throughout the atmosphere. It is highly variable and decreases rapidly with altitude. Like carbon dioxide, water vapour is found effectively only up to a height of 90 km. Furthermore, it decreases from the equator towards the poles.
Key Takeaway:
Carbon dioxide and water vapour are found only up to 90 km, while oxygen becomes negligible above 120 km. The upper atmosphere is dominated by lighter gases like helium and hydrogen.