Question

The true statement about ‘green-house effect’ is that it is

(a) caused by combination of many gases

(b) caused only by CO₂

(c) caused by CO₂, CFC, CH₄ and NO₂ gases

(d) none of these.

Answer 1

Ans. Correct option: d. none of these.

# Option 1. Incorrect. There is no requirement of any “combination” of gases to exert greenhouse gas. For example, pure CO₂ exhibits greenhouse effect without any combination with other gases.

Note that “caused by combination of many gases” is NOT equivalent to “caused by many cases”. If the statement were to “caused by many cases”, the option would be correct because many cause greenhouse effect.

# Option 2. Incorrect. Many gases like CO₂, CO, fluorocarbons (CFCs, HFCs, PFCs), water vapor, ozone, N₂O, CH₄, cause greenhouse effect.

# Option 2. Incorrect. NO₂ is NOT a greenhouse gas.

#Greenhouse effect: Various molecules like CO₂, CO, fluorocarbons (CFCs, HFCs, PFCs), water vapor, ozone, N₂O, CH₄, efficiently absorb the IR radiation emitted by hot Earth's surface (which is exposed to sunlight during daytime).

These molecules trap (absorb) a large fraction of the IR radiation emitted by hot Earth's surface and increase their vibrational energy. The emitted IR radiation by these gases increases the temperature of the atmosphere and keep is warm for relatively longer time while they return back to ground state by losing energy as heat (IR radiation).

This process of warming the earth's atmosphere is called Greenhouse effect. The gases responsible for greenhouse effect are called greenhouse gases.

# A large fraction of the radiation emitted from Earth's surface and greenhouse gases escape Earth's atmosphere. N₂, NO₂ and O₂ do not absorb IR in the range that is being emitted by Earth's surface. Thus, they don't act as greenhouse gases.

# Note that N₂O is a greenhouse gas but NO₂ is not a greenhouse gas.