The correct option is 2 only
Explanation
The atmosphere is a mixture of gases, water vapour, and dust particles. The composition of the atmosphere is not static; it changes with time, place, and altitude. Heavy gases are concentrated in the lower layers, while lighter gases are found in the upper layers.
Statement-wise Analysis:
- Statement 1 is Incorrect: Carbon dioxide is meteorologically a very important gas. It is transparent to incoming solar radiation (shortwave) but opaque to outgoing terrestrial radiation (longwave). It absorbs a part of terrestrial radiation and reflects some part of it back towards the earth’s surface, largely responsible for the greenhouse effect.
- Statement 2 is Correct: The proportion of gases changes in the higher layers of the atmosphere. Oxygen will be almost in negligible quantity at the height of 120 km. It is not found in substantial quantities at this altitude.
- Statement 3 is Incorrect: Carbon dioxide and water vapour are relatively heavy components of the atmosphere. They are found only up to 90 km from the surface of the earth. Consequently, they do not exist in uniform concentration up to the exosphere (which is the outermost layer extending beyond 400 km). Furthermore, the concentration of water vapour is highly variable and decreases rapidly with altitude.
Key Takeaway:
Carbon dioxide and water vapour are confined to the lower atmosphere (up to 90 km), while oxygen becomes negligible at 120 km. Carbon dioxide acts as a greenhouse gas by trapping outgoing terrestrial radiation.