The interaction between light and aerosols is a defining characteristic of particulate matter, leading to significant real-world consequences, the most noticeable of which is the reduction of atmospheric visibility. These same optical properties—how particles scatter and absorb light—also form the basis for some of the most powerful and widely used aerosol measurement techniques.

The nature of the interaction between a particle and light is primarily determined by the dimensionless size parameter, α, which relates the particle’s circumference to the wavelength (λ) of the incident light:

α = π * Dp / λ (Eq. 24)

5.1 Light Scattering and Extinction

Depending on the value of the size parameter α, light-particle interactions are categorized into three primary regimes:

1. Rayleigh Scattering (α < ~2): Occurs when particles are much smaller than the wavelength of light. In this regime, the scattered light intensity is proportional to the sixth power of the particle diameter (∝ Dp^6), making it extremely sensitive to size.
2. Mie Scattering (α ≈ 2 to 10): An intermediate regime where the particle size is comparable to the wavelength of light. The scattering patterns are complex and highly dependent on size, shape, and angle.
3. Geometrical Optics (α > ~10): Occurs when particles are much larger than the wavelength of light. Here, scattering is proportional to the particle’s cross-sectional area, scaling with the square of its diameter (∝ Dp^2).

A particle’s material properties also play a critical role, defined by its complex index of refraction, m:

m = n1 – i*n2 (Eq. 27)

The real part (n1) governs how light bends, or refracts, while the imaginary part (n2) is responsible for the absorption of light.

Light extinction is the total attenuation of a light beam as it passes through an aerosol, resulting from the combined effects of scattering and absorption. The reduction in light intensity (I) over a path length (l) follows the Beer-Lambert law:

I = I₀ * exp(-γ * l) (Eq. 28)

The term γ is the extinction coefficient, which represents the total effective cross-sectional area of particles per unit volume of air:

γ = ∫ Cext * n(Dp) dDp (Eq. 29)

where Cext is the extinction cross-section of a single particle, which is the sum of its scattering and absorption cross-sections. For visible light in the atmosphere, the extinction coefficient reaches its maximum for particles with diameters around 0.5 μm.

5.2 Atmospheric Visibility

The extinction coefficient is directly and quantitatively linked to atmospheric visibility. Visual range (Lv) is defined as the maximum distance at which an object can be distinguished from its background. This relationship is captured by the well-known Koschmieder equation:

Lv = 3.912 / γ (Eq. 32)

This equation provides a powerful, direct link between a fundamental aerosol property (γ) and a tangible human perception (visibility). For example, an aerosol consisting of 0.5 μm particles at a number concentration of 10⁴ particles/cm³ would produce an extinction coefficient of 6.5 x 10⁻⁵ cm⁻¹, resulting in a calculated daylight visual range of approximately 0.6 km.

The dynamic, optical, and electrical properties of aerosols are not just theoretical concepts; they are the very principles exploited by the practical instruments used to measure and quantify them.