Date of Award
Master of Science (MS)
Investigation of aerosol interactions with electromagnetic waves provides insights into the scattering particles. Aerosol phase function, an angular distribution of scattered light, is a value required to calculate parameters used in direct radiative forcing (DRF) models in the atmosphere. Currently no direct measurement of phase function is available hence it is estimated from ground observations such as backscatter fraction b and subsequently relating the parameters via Henyey-Greenstein (HG) approximation. This method has shown to introduce errors in radiation transfer models. HG phase function, in particular, does not account for particle microphysical properties such as shape and refractive index. Given the need for more accurate characterization of aerosol phase function, we developed and tested a prototype instrument for measuring directionality of aerosol light scattering. The study focused on increasing detection sensitivity and mechanical stability by understanding the relationships among instrument components in detail. The prototype was approximately 18" x 12" x 10" in dimensions, designed to be portable for use outside the laboratory and had a final detectable scattering angle range of approximately 12° < θ < 167°. The design incorporated a low-angle elastic scattering detection setup by Ferri  and a custom-made elliptical mirror to capture scattered light across the x-plane, a plane perpendicular to the optical axis of the beam. Each of the 512 channels on linear photodiode detectors was matched to a range of polar scattering angles. The angle resolutions were approximately 0.03° and 0.5° for forward and side scattering respectively. The prototype was deployed in the NASA-NOAA FireX-AQ campaign in McCall, Idaho to measure the scattering phase function of aerosols emitted from biomass fuel samples from the Nethker wildfire site. Assuming a constant intensity at θ < 12°, we found the asymmetry parameter g of aerosol field samples to be 0.73 ± 0.02. Subsequent analysis on scanning mobility particle sizer (SMPS) and transmission electron microscopy (TEM) showed that the samples had 500 nm mean diameter. The results demonstrated the feasibility of building a portable optical device for directly measuring aerosol phase function. Main challenges encountered during the field operation were off-axis aberration, focal position and volume control, and inflexible detection system. Following the field campaign, the aforementioned challenges were analyzed and potential near-term solutions were explored using computational modeling in Synopsys® LightTools.
Dr. Rajan Chakrabarty
Dr. Jian Wang Dr. Brent Williams Dr. Timothy Onasch