"Atomic Attenuation Coefficient" is a measurement of how strongly a chemical species or substance absorbs or scatters light at a given wavelength, per the number of atoms in the substance.
$\mu_a = -\frac{\mu}{n}$, where $\mu$ is the linear attenuation coefficient and $n$ is the number density of the atoms in the substance.
Applicable units are those of quantitykind:Area
Atomic Attenuation Coefficient