# Nephelometer Functionality

The nephelometer has been commercially available for forty years with MRI/Belfort first marketing the 1550, 1560 and 1590 series of nephelometers.

Integrating nephelometers directly measure the light scattered by aerosols and gases in an enclosed sample volume. The sampling chamber and light source are confined to a small volume so that the instrument makes a “point” or localised measurement of scattering, continuously and in real-time. The total measurements are then combined with a backscatter measurement that will only sample between 90° and 170° to give a more in-depth analysis of particle scattering.

This information can be combined with data measured by other aerosol instruments and then inserted into mathematical models to derive the following additional parameters:

**Aerosol asymmetry parameter (g)**
Defined as the cosine-weighted average of the phase function, where the
phase function is the probability of radiation being scattered in a
given direction. It can be derived, under certain assumptions, from the
measured backscatter fraction (Ogren et al, 2006).

**Angstom Exponent ( Å) **Can
be used to describe the dependency between aerosol optical depth and
wavelength. The Ångström exponent is inversely related to the average
size of aerosol particles: the smaller the particles, the larger the
exponent. Its determination using integrating nephelometers and
satellite data provides valuable information into the optical depth of
the atmosphere and the radiative forcing effect of aerosol.

**Single Scattering Albedo**
The ratio of light scattering to light extinction by atmospheric
particles. It is an important parameter when assessing the climatic
effects of aerosols. It is understood that aerosols with a
single-scattering albedo greater than 0.85 generally cool the planet,
while those with less than 0.85 warm the planet (Hansen et al. 1981).
This is also dependent on the surface albedo and the backscatter
fraction.