Colloquium and PhD Defense: Thursday, November 2, 2006 at 3:00 p.m. EN 6085A.

Determining Particle Index of Refraction with Measurements of Scattering at Two Angles

Note: This is a PhD defense.

Abstract:

A new Twin Angle optical particle counter (TAOPC), that measures forward scattering at 40˚ and 74˚ was developed to determine index of refraction of polar stratospheric cloud (PSC). A method to analyze the data to determine index of refraction of single particles was developed. Analysis of the counter response functions at the two angles along with realistic measurement uncertainties were used to estimate the effective size range of the instrument. The lower size limitation for index of refraction determination of single particles with the TAOPC was found to be ~1.5 µm. The instrument was tested in the laboratory with monodisperse spherical particles of known composition and size. The majority of the estimated indices of refraction were within ±1% of the expected indices and size determination was within acceptable error. Concentration of solid non-spherical particles in a PSC can be large enough for the index of refraction determination; however, the assumption of spherical solid particles instead of non-spherical particles introduces large uncertainties in index of refraction and size determination, and may preclude a useful measurement.

The complex index of refraction of mineral dust particles was measured in Laramie, Wyoming, February 2006, with the TAOPC. The index of refraction was determined with the particle number ratio approach (Hu et al., 2006} and was estimated to be in the range of 1.64-1.67 for the real part and 0.009-0.0104 for the imaginary part. Simultaneously with TAOPC measurements, particles were collected on polycarbonated filters for computer-controlled scanning electron microscopy (CCSEM) analysis. Index of refraction calculated from this analysis was in the range of 1.607-1.656 for the real part and 0.006-0.012 for the imaginary part. Particles were also collected on two different filter pack systems in July 2005 and February 2006. Estimates of index of refraction from these measurements compared well with the CCSEM analysis. The TAOPC scattering measurements were also modeled using an independent measurement of size with an Aerodynamic Particle Sizer, and assuming particle refractive index of 1.67-0.009i. The modeled scattering compares with the TAOPC measurements only if the uncertainties on size are increased. The larger uncertainties in scattering probably result from sampling of non-spherical particles and/or a polydisperse index of refraction distribution.