Wavelength extention in AAS
From this source broadenings occur due to


  
  Doppler effect
  This effect arises because atoms will have different components of velocity along the line of observation.
  
  Lorentz effect
  This effect occurs as a result of the concentration of foreign atoms present in the environment of the emitting or absorbing atoms. The magnitude of the broadening varies with the pressure of the foreign gases and their physical properties.
  Quenching effect
  In a low-pressure spectral source, quenching collision can occur in flames as the result of the presence of foreign gas molecules with vibration levels very close to the excited state of the resonance line.
  Self absorption or self-reversal effect
  The atoms of the same kind as that emitting radiation will absorb maximum radiation at the center of the line than at the wings, resulting in the change of shape of the line as well as its intensity. This effect becomes serious if the vapor, which is absorbing radiation is considerably cooler than that which is emitting radiation.
  


As Greg pointed out in the comment, You can find more detailed explanation on the topic here

Pressure broadening should be dependent on the concentration of atoms. The other type of broadening shouldn't be dependent on the concentration