vs log wavelength in nm allows solution of the equation below for two parameters.
Optical depth is a dimensionless parameter which quantifies the attenuation of the Sun's direct beam. A total optical depth of 1 would attenuate the direct beam by a factor of
The total optical depth is comprised of molecular optical depth (due to scattering), gaseous optical depth (due to absorption), and cloud and aerosol optical depths (due to scattering and absorption). Molecular optical depth depends only upon surface pressure and wavelength. Gaseous optical depth depends on the total column amount of the gas multiplied by its absorption cross section at that wavelength. Gases such as ozone, NO2, and SO2 absorb sunlight at certain wavelength regions of the spectrum. For a clear sky the optical depths due to gaseous absorption can be calculated for each wavelength allowing the aerosol optical depth to be separated.
Once the optical depth of the aerosol is obtained at a number of wavelengths,
what can be learned about the physical properties of the aerosol? A plot
of log
vs log wavelength in nm allows solution of the equation below for two parameters.
The parameter 
is the total aerosol optical depth at 500 nm (often called the turbidity)
and 
is a measure of the average radius of the aerosol size distribution.
Aerosol distributions with many large particles (radius > 0.1.x 10-6
m) have an 
of close to zero and distributions with mostly small particles (radius
< 0.01 x 10-6 m) have an
of around 2. Large articles often serve as cloud condensation nuclei
for cloud droplets while the very small and often newly formed particles
usually collide with other particles and form larger particles.