Aurora Nephelometer

Light scattering is the measurement of particulates (dust/aerosol) in the air by shining light through sample air and measuring the deflected light (scattering). This light scattering allows us to measure the visibility of air (how far we can see) using the Beer-Lambert law.

Extinction Coefficients σ_ext

Attenuation of light (that is, reduction in its intensity) is usually expressed using the Beer-Lambert law:

I = I₀e^{-σext x}

where:

The relationship between extinction coefficient and visual range is expressed in Koschmieder’s Formula.

L_v = 3.912 / σ_ext

where:

 The larger σ_ext , the more rapidly the light is attenuated (ie reducing visibility).

Assumptions

Light may be attenuated either by scattering off objects or by absorption by objects. Thus the extinction coefficient σ_ext may be broken down into a scattering coefficient σ_scat and an absorption coefficient σ_abs

σ_ext = σ_scat + σ_abs.

Light attenuation equation

For light attenuation in the atmosphere, the objects responsible can be either gas molecules or airborne particles. The scattering and absorption coefficients may therefore be further broken down into

σ_scat = σ_sg + σ_sp

Scattering Coefficient

 σ_abs = σ_ag + σ_ap,

Absorption Coefficient

where the subscripts denote:

s_sp, for example, is the extinction coefficient due to scattering from particulate matter. Scattering due to gas molecules (coefficient σ_sg) is also called ‘Rayleigh scattering’.

NO₂ is the most significant gaseous absorber and soot the most significant particulate absorber. However, except in extremely high concentrations, the effects of absorption are negligible compared to the effects of scattering. Therefore, to a very good approximation,

σ_ext ≈ σ_scat = σ_sg + σ_sp.

Relationship of Extinction coefficient with scattering Coefficient

It is σ_scat that the Ecotech Aurora 3000 measures directly. When the instrument performs a zero adjust in particle-free air (that is, where only Rayleigh scattering is present), the σ_sg component of σ_scat is subtracted leaving  σ_sp as the reported parameter.

Higher particulate concentrations mean more scattering, so σ_sp is a good measure of particulate pollution.

In urban situations σ_sp will be much greater than Rayleigh scattering (σ_sg). σ_sp is therefore also a good measure of atmospheric visibility.