Aerosol Phase Function Measurements
Atmospheric particles (aerosols) scatter light in different directions.
The aerosol phase function describes how much light is scattered in
each direction. When the particles are smaller than
the wavelength of light then more light is scattered into the
sideways direction. When they become larger that the
wavelength, more is scattered into the forward direction. If
the particles are large water drops then the rainbow will appear
at ~145 degree scattering angle. The scattering angles are defined as
shown in the following figure.
In order to measure the aerosol phase function we use a polar
nephelometer which measures the amount of light scattered at each
angle. The basic design of this system is shown in the figure below. In
addition to the scanning angle detector, scattered light measurements
are also made at a fixed angle. Dividing the scanning angle
measurements by the fixed angle measurements allows us to correct
possible changes in particle concentration and laser intensity
(during the time the scanning detector moves across the different
scattering angles).
Initial polar nephelometer measurements were carried out at Bellows Beach (Porter et al., 2007). Measurements with a second generation system were carried out during the SEAS experiment at Bellows Beach on Oahu (Clarke, et. al., 2002 and Porter et al., 2002). Using the sea salt phase function measurements, Dr. Barry Lienert carried out inversions using Mie theory (Lienert et al., 2002 and Porter et al., 2003).
We are now working on a small version of the polar nephelometer which will allow it to be used on ballons or aircraft.
