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Yes, gas is a noun. Greenhouse here is an adjective describing 'gas'. Together they make a noun phrase, 'Greenhouse gas'.
Nitrous Oxide is the third greenhouse gas.........
No, Ammonia is not considered a greenhouse gas.
Natural gas is a greenhouse gas, methane, and when it is burnt it releases the other greenhouse gas, carbon dioxide.
Yes, ozone is a greenhouse gas. Any gas with three or more atoms in their molecule are greenhouse gases.