What makes a gas greenhouse gas? able to absorb infrared light Must have molecular vibration(s) This excludes monoatomic gases as greenhouse gases (That is why argon, the third most abundant atmospheric constituents is transparent to infrared irradiation) The molecular vibrations must be non symmetric, i.e. infrared active Homonuclear diatomic molecules only have symmetric vibrations. That's why n2, 02 are not greenhouse gases.What makes a gas greenhouse gas? Able to absorb infrared light • Must have molecular vibration(s) – This excludes monoatomic gases as greenhouse gases. (That is why argon, the third most abundant atmospheric constituents is transparent to infrared irradiation) • The molecular vibrations must be non￾symmetric, i.e. infrared active – Homonuclear diatomic molecules only have symmetric vibrations. That’s why N2, O2 are not greenhouse gases