Greenhouse Gasses Summary

Greenhouse Gases

Overview

• Introduces molecular vibrations.

• Explains the absorption and re-emission of IR radiation.

• Details how greenhouse gases affect Earth’s energy balance.

• Covers the impact of example gases on climate change.

Earth's Energy Balance

• Earth's radiative flux includes UV radiation, visible light, and thermal infrared radiation.

Molecules in the Atmosphere

• Key molecules: Nitrogen $N<em>2$, Oxygen $O</em>2$, Water $H<em>2O$, Carbon Dioxide $CO</em>2$, and Methane $CH_4$.

Absorption of IR Radiation

• IR radiation corresponds to stretching and bending of molecular bonds.

• Molecules have $3N-5$ (linear) or $3N-6$ (nonlinear) normal modes of vibration, where N is the number of atoms.

• Vibration energy depends on bonded atoms and bond type.

Normal Modes of Vibration

• Normal modes are simple motions that combine into complex vibrations.

IR Activity

• If vibration changes the dipole moment, the molecule is IR-active and absorbs IR radiation. If there is no change, it is IR-inactive.

IR-Inactive Molecules

• Nitrogen ($N_2$):

  • No permanent dipole.

  • $\Delta EN = 0$

  • Stretching doesn't induce dipole change.

  • Oxygen ($O_2$) is also IR-inactive for similar reasons.

IR-Active Molecules

• Water ($H_2O$):

  • Bent structure, polar bonds.

  • $\Delta EN \neq 0$

  • Permanent dipole; all three normal modes are IR-active.

• Carbon Dioxide ($CO_2$):

  • Linear, polar bonds, but nonpolar molecule.

  • $\Delta EN \neq 0$

  • Some vibrations induce momentary dipoles, making some modes IR-active.

• Methane ($CH_4$):

  • Some modes are IR-active.

Radiative Flux and Greenhouse Gases

• Greenhouse gases modify radiative flux.

Global Warming Potential (GWP)

• Measures how much heat a gas traps relative to $CO_2$ (GWP = 1).

• Higher GWP means more heat trapped.

• Examples:

  • $CO_2$: GWP = 1

  • $CH_4$: GWP = 21

Key Greenhouse Gases

• Carbon Dioxide ($CO_2$):

  • GWP of 1, largest current impact due to fossil fuel burning.

• Water ($H_2O$):

  • Most abundant, potent due to many IR absorption bands.

  • No GWP calculated, but amplifies warming.

• Methane ($CH_4$):

  • GWP of 21, from fossil fuel production and organic decay.