A multiwavelength, multistatic optical scattering instrument is being developed to characterize spherical
aerosols. This instrument uses 405 nm (blue), 532 nm (green) and 655 nm (red) diode lasers and two CCD imagers
to measure the angular distribution of light scattered from aerosols. The incident light is polarized parallel or
perpendicular to the scattering plane; the scattered intensity is measured at backscatter angles ranging from
120° to 170° by CCD imagers. The phase function for each polarization is used to form the polarization ratio,
which is used to characterize the aerosols. This method has proven to be a reliable way to characterize spherical
aerosols by matching the measured polarization ratio with the polarization ratio calculated by the Mie scattering
equations. This method is used to determine the number density, size distribution, and index of refraction of the
aerosols. The sensitivity of the polarization ratio to particle concentration is explored using a narrow distribution
of one micron polystyrene beads in a chamber. The aerosol concentration is found via an inversion technique
that is based on Mie calculations. This study provides the basis for transitioning this instrument to measure
multiple particle size ranges and concentrations for common aerosols in an outdoor environment.
Several laser remote sensing techniques are used to characterize the properties of aerosols. The various techniques
include: backscatter, optical extinction using Raman scatter, and bistatic/multistatic scattering using the polarization
ratio of the scattering phase function. The number density, size, and size distribution are obtained under the
assumption of spherical scatterers. Other measurements can be used to describe additional properties, such as
aerosol type based upon approximate refractive index and detected departure from spherical, when simultaneous
measurements at several wavelengths and several angles are analyzed. Examples are shown to demonstrate our
present capability to characterize aerosol particles using recently developed techniques.
A sensor for measuring scattering at multiple wavelengths and multiple angles has been designed and is being tested for
the characterization of atmospheric aerosols. Charge coupled device (CCD) imagers are used to record scattering
measurements at two polarizations and as a function of angle relative to the co-aligned laser beams. A diffraction grating
is used to spatially separate the wavelengths across the
field-of-view of the CCD array, allowing simultaneous
measurements at multiple wavelengths. Experiments are conducted to measure the scattering intensities for two
polarizations at discrete wavelengths that span the visible spectrum. The data from the CCD images are inverted using a
genetic algorithm and Mie scatter equations to determine aerosol properties of artificially generated fog. The results are
compared with in-situ measurements of the aerosol size distribution and concentration using an aerodynamic particle
sizer spectrometer and a condensation particle counter.
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