Do All Atmospheric Gases Absorb and Emit Electromagnetic Radiation?

Yes, all atmospheric gases absorb and emit electromagnetic radiation.  It is often said that only the so-called greenhouse gases emit and absorb radiation.  That is usually in the context of a discussion in which the relevant radiation is infrared radiation.  Sometimes, that discussion ought to acknowledge the oxygen molecules, composing nearly 21% of the atmosphere, absorb and emit radiation in the very near infrared and visible portions of the electromagnetic spectrum.  In other contexts, it is important that both oxygen and nitrogen molecules absorb ultraviolet light.  These absorptions affect the radiation incident from the sun, but not that emitted by the Earth.

I was asked if every gas molecule or atom emits energy when it is at a non-zero temperature.  The answer is yes.  No gas molecule or atom has an emissivity of zero.  Let me simply explain why.

A single charged particle undergoing acceleration or deceleration will emit electromagnetic energy.  Any oscillating dipole will emit electromagnetic energy.  A dipole consists of a positive and a negative charge with offset centers of their charge distribution.  It is rather like a dumbbell with one side positive and the other side negative.  The oscillation is a change of distance between the two opposite charges.  A force is required to cause the two charges to leave their equilibrium position.

Let us consider any atom.  The nucleus has a positive charge and the electron cloud about it is negative.  As long as the electron cloud has a symmetric distribution, there is no dipole.  Now you can accelerate the entire atom and the radiation emitted is very low because the symmetric nuclear positive charge and the surrounding positive electronic charge electromagnetic emissions cancel out at a distance on the order of the diameter of the atom.  If the atom is ionized, so that an electron is missing, then the mere acceleration of the atom will result in electromagnetic emission at a distance.

More commonly, an atom will emit radiation because the electronic cloud centroid is oscillating about the nucleus.  When the atom is not at absolute zero temperature, there will be such an oscillation.  However, it takes a lot of force to pull the electronic cloud away from or push it toward the nucleus because the coulombic and nuclear repulsion forces are very strong.  The oscillation at room temperature is therefor very small and the energy radiated is very low.  This energy is in the microwave range for a single atom, such as an argon atom in our atmosphere.  Nonetheless, the energy is great enough that atoms far away in the universe emit microwave radiation from areas of space at a temperature of only 3K and that has been detected.

It is easier at a modest temperature to stretch a bond between two atoms such as in N2 or O2.  The creation of the bond between these two atoms creates a distortion of the symmetry of the electron cloud of each atom around its nucleus.  The equilibrium length of the bond between them is the condition in which the center of the positive charge in the two nuclei corresponds to the center of the electron cloud distribution, but at non-zero temperature this bond length oscillates and the stretching and compression of the bond causes the electron cloud to have to accelerate and decelerate to try to keep its centroid in the same position as that of the positive charge in the two nuclei. This makes for a weakly oscillating dipole moment and radiation is emitted.  The radiation is very long wavelength microwave radiation.  Such very long wavelength microwave radiation can also be absorbed by these molecules as well.  This is why the atmosphere is fairly opaque to low energy or large wavelength portions of the microwave spectrum.  See the spectrum below:



The 6 cm to 20 cm microwave or radio wave radiation is not absorbed by the atmosphere, but at longer microwave wavelengths the absorption is very strong.  This is also the part of the electromagnetic spectrum in which N2 and O2 are radiating for the most part.  But very long wavelength is also very low energy radiation.  When N2 and O2 molecules collide with other molecules, there will be minor oscillations set off between the centers of the positive and negative charges and microwave radiation will result.  They collide because they have kinetic energy, which means they have a temperature.  N2 and O2 also absorb ultraviolet radiation from the solar insolation spectrum.

More complex molecules with 3 or more atoms have additional degrees of freedom of motion.  Not only can their molecule bonds be stretched, but the angles between them can be changed.  This allows them to absorb and radiate energy at a higher energy than in the microwave range.  H2O, CO2, and CH4 are examples of molecules that have a number of absorption and emission lines in the infrared radiation portion of the electromagnetic spectrum.  Much of the solar insolation radiation is in the infrared range, especially the high energy end of the infrared range.  This is often called the near infrared range.  The radiation from the Earth is in the mid and lower energy portion of the infrared range of electromagnetic radiation.  These parts of the infrared radiation range are called the mid and far infrared, respectively.  This is why these larger molecule gases are properly called infrared-active gases.

They are also commonly called greenhouse gases, though that term has no scientific basis.  Water vapor does change the distribution of heat at the Earth's surface and in the troposphere, but their is no well-founded analogy with the mechanisms by which it does so with a greenhouse.  The same is true of CO2 as well.