The interaction of light with aerosol particles gives rise to optical phenomena such as scattering and absorption, which significantly impact atmospheric visibility. This interaction is primarily characterized by the dimensionless parameter α = πDp / λ, the ratio of the particle circumference to the wavelength of light.

Light Scattering and Extinction

• Light Scattering: When light strikes a particle, it is redirected in various directions. The intensity and angular distribution of scattered light depend on the particle’s size, shape, and refractive index.
  • Rayleigh Scattering (α < 2): Applies to particles much smaller than the wavelength of light. Scattered intensity is proportional to the sixth power of the particle diameter (Dp⁶).
  • Mie Scattering (2 < α < 10): A complex regime where scattering patterns are oscillatory.
  • Geometrical Optics (α > 10): Applies to particles much larger than the wavelength of light. Scattered intensity approaches being proportional to the square of the particle diameter (Dp²).
• Light Extinction: The attenuation of a light beam passing through an aerosol is called extinction. It is the sum of light lost to both scattering and absorption. The degree of attenuation is quantified by the extinction coefficient (γ). For visible light in the atmosphere, extinction is typically dominated by scattering and is maximal for particles with diameters around 0.5 µm.

Impact on Visibility

Atmospheric visibility is inversely related to the light extinction coefficient. The quantitative relationship is given by the Koschmieder equation:

Lv = 3.912 / γ

where Lv is the visual range. For an aerosol of 0.5 µm particles at a concentration of 10⁴ particles/cm³, the visual range can be reduced to as little as 0.6 km.