Mie theory has been used in this work to obtain a theoretical calculation of the size distribution of aerosol particles by using tabulated mean of the Angstrom wavelength exponent ct,. Comparison was done between an industrial polluted area (Helwan, which is a neighbor to Cairo city), and an agricultural relatively unpolluted area (Mansoura, about 140 km from Cairo). The results show that the size distribution obeys the Junge power law. The size of particles in the polluted area is larger than that in the unpolluted area.