The optical scattering from laser beams propagating through atmospheric aerosols has been shown to be very useful in
describing air pollution aerosol properties. This research explores and extends that capability to particulate matter. The
optical properties of Arizona Road Dust (ARD) samples are measured in a chamber that simulates the particle dispersal
of dust aerosols in the atmospheric environment. Visible, near infrared, and long wave infrared lasers are used. Optical
scattering measurements show the expected dependence of laser wavelength and particle size on the extinction of laser
beams. The extinction at long wavelengths demonstrates reduced scattering, but chemical absorption of dust species
must be considered. The extinction and depolarization of laser wavelengths interacting with several size cuts of ARD are
examined. The measurements include studies of different size distributions, and their evolution over time is recorded by
an Aerodynamic Particle Sizer. We analyze the size-dependent extinction and depolarization of ARD. We present a
method of predicting extinction for an arbitrary ARD size distribution. These studies provide new insights for
understanding the optical propagation of laser beams through airborne particulate matter.
Lidar is a powerful tool for measuring the vertical profiles of aerosols. Dusts are irregularly-shaped particles with varied
composition and strong index of refraction variations in the LWIR. We measure dust indices using ellipsometry and
transmission through KBr pellets. Milling makes the ellipsometry data less dependent on incidence angle, and the results
of measurements on milled materials agree with those from transmission measurements. Measurements show that the
spectrum of a milled Arizona Road Dust (ARD) approaches that of pure quartz, indicating a decrease of absorption
efficiency for particles larger than the absorption length. These indices of refraction will be used in the future to simulate
extinction for the beam of a LWIR lidar.
Lidar is a powerful tool for measuring the vertical profiles of aerosols in the atmosphere using Rayleigh and Raman lidar
techniques. Bistatic lidar can be used to obtain the angular structure of the scattered light. When the aerosols are
uniformly distributed, this information can be analyzed to provide particle size distribution information. However, dusts
tend to be irregularly shaped particles with varied composition. We investigate the impact of the irregular shape using
optical scattering at several wavelengths, scanning electron microscopy, and T-matrix calculations. In particular, we
study the rapid loss of Mie scattering resonances as the particle shape departs from spherical. Different size distributions
produced by different size-cuts of Arizona Road Dust (ARD) are studied.
Aerosol optical scattering experiments are often large, expensive, and provide poor control of dust uniformity and size
distribution. The size distribution of such suspended atmospheric aerosols varies rapidly in time, since larger particles
settle quickly. Even in large chambers, 10 micron particles settle in tens of seconds. We describe lab-scale experiments
with stable particle distributions. A viscous colloidal solution can stabilize the particles for sufficient time to measure
optical scattering properties. Colloids with different concentrations or size distributions enable nearly time independent
studies of prepared distributions. We perform laser aureole scattering from a colloid containing a few percent by volume
of Arizona Road Dust (ARD) in mineral oil and glycerin, and 1-micron polystyrene spheres in water. We discuss aureole
analysis, the differences expected in scattering properties due to the index of refraction of the mineral oil medium versus
air, and the impact of non-spherical shape on the scattering. This research demonstrates that particles suspended in a
viscous medium can be used to simulate aerosol optical scattering in air, while enabling signal averaging, offering
reproducibility, and easing problems resulting from parameter variations in studies of dust properties.
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