Aerosol Optical Properties and Radiative Effects

How aerosol particles scatter and absorb light, and the resulting direct and indirect effects on Earth's radiation balance.

Definition

Aerosol optical properties describe how particles interact with radiation, and aerosol radiative effects are the resulting perturbations to the flow of solar and terrestrial energy through the atmosphere.

Scope

Covers Mie scattering theory applied to aerosols, the scattering and absorption efficiencies and single-scattering albedo, aerosol optical depth, the direct radiative effect of scattering and absorbing particles, the role of black carbon, and the indirect effects mediated through clouds.

Core questions

• How do particle size and composition determine scattering and absorption?
• What is aerosol optical depth and how is it measured?
• How do aerosols force climate directly and indirectly through clouds?

Key theories

Mie scattering theory

An exact solution of light scattering by spherical particles that relates a particle's size and refractive index to its scattering and absorption efficiencies.

Aerosol radiative forcing

Scattering aerosols raise planetary albedo and cool, absorbing aerosols warm the atmosphere, and aerosol effects on clouds add a further, more uncertain forcing.

Mechanisms

When particle size is comparable to the wavelength of light, scattering and absorption are described by Mie theory, which yields efficiencies depending on the size parameter and complex refractive index. The single-scattering albedo sets the balance between cooling by scattering and warming by absorption. Integrated over the column, extinction gives the aerosol optical depth. Direct effects alter the radiation balance immediately, while indirect effects arise as aerosols change cloud droplet number, brightness and lifetime.

Clinical relevance

Aerosol optical properties govern visibility, the retrieval of aerosol loading from satellites, and the magnitude of aerosol climate forcing, a leading source of uncertainty in climate projections.

History

Gustav Mie's 1908 solution for scattering by spheres provided the optical foundation, and quantification of aerosol radiative forcing became a major effort from the 1990s as its importance for the climate budget was recognized in IPCC assessments.

Key figures

• Gustav Mie
• Craig Bohren

Seminal works

• bohrenHuffman1983
• boucher2013

Frequently asked questions

What is aerosol optical depth?

Aerosol optical depth is a measure of how much aerosol in a vertical column attenuates light by scattering and absorption; higher values mean hazier, more particle-laden air.