Natural Science Forum / Physics / Optics / April 2005

Assuming you're talking about normal infrared 4 to 20um,
and perhaps longer distances in air.

The absorbtion there is due to chemical componds and other
components in the air.
Scattering is done by particles much smaller than the
wavelength.

This is purely coincidence. If the scattering particles were
bigger, the air would not be transparent anymore, perhaps
such as in a cloud. If you have substances in the air that
absorb at shorter wavelength, you'd call that coloring the air,
such as dye or soot.

Rene

In a gas I think they're the same thing. Otherwise the former is
determined by the energy levels of the outer electrons, the latter by
the scatterer dimensions.

If you know complex numbers, the explanation is that the scattering is
due to the real part of the dielectric constant, and the absorption to
the imaginary part. The are related by Kramers-Kronig transforms.

This is a mathematical representation of the behavior of driven
oscillators: the absorption (dissipation) is due to the response that is
in phase with the driving field, the scattering is due to the component
of the motion that is 90 degrees out of phase with the driving field.

Something about the Lorentz oscillator:
http://webphysics.davidson.edu/Projects/AnAntonelli/node5.html