Optical Properties

Electromagnetic radiation incident on an aerosol interacts with the electric fields of the electrons and protons in a process known as scattering, described by Maxwell’s equations. Light scattering by an aerosol particle may be elastic, where the emitted light is the same wavelength as the incident radiation, or inelastic, where the wavelength of the emitted light is increased due to a loss of energy following absorption. The processes by which particles interact with light are shown here schematically. The optical properties of an individual aerosol particle determine the amount of scattering and absorption which take place during its interaction with light and depend upon the size, shape, morphology and refractive index of the particle and the wavelength of the light.

Modes of interaction of light with an aerosol particle.

The size of the particle influences the regime of light scattering which is important. Very large particles act like a bulk sample and simple geometric lens optics may be applied to understand their interactions with light. For very small particles, with radius much smaller than the wavelength of light, Rayleigh scattering theory can be applied and scattered radiation is emitted uniformly in all directions. When particles are in the size regime with radii 20 – 100x the wavelength  of light, more complex scattering treatments must be employed to describe the interactions.

As a consequence of their interaction with light, aerosol particles in the atmosphere perturb the radiative balance of the atmosphere. Incoming solar radiation covers a broad range of the electrodynamic spectrum and, with the vast diversity of aerosol composition and particles sizes, quantifying the influence of aerosol on incoming solar radiation is a challenging yet vital component of predicting the radiative balance of current and future climates.

Next: Physical Properties

Figures © 2014 James Davies