Extinction Coefficient and Optical Thickness

If there are dN particles in the size range dj, to dp + d dp) per unit volume of air, this corresponds to a total particle cross-sectional area of Kd /4)dNdz over the light path length, dz, per unit area normal to the beam. The attenuation of light over this length is given by the relation 

The contributions to h(k) from a given particle size range depend on the extinction cross section and on the particle size distribution function. The integral (5.16) can be rearranged as follows  

This function Itas been evaluated fora measured atmospheric. size distribution and is shown in Fig. 5.7 as a function of particle size. The area under the curve is proportional to h. The figure shows that the principal contributions to b come from the size range between 0.1 and 3 m. This occurs frequently for urban aerosols. 

The reduction in the intensity of the light beam passing through the aerosol is obtained by integrating (5.16) between any two points, z = and z = L - 

SCATTERING OVER THE VISIBLE WAVELENGTH RANGE AEROSOL CONTRIBUTIONS BY VOLUME 

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