Determining particle index of refraction with measurements of scattering at two angles

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). The counter response function at the two angles, along with realistic measurement uncertainties were used to estimate the effective size range of the instrument. A method to analyze the data to determine index of refraction was developed. The error analysis indicates that measurement uncertainties at 40° are between 4 and 10% for particles >1.5 mum and between 5 and 15% at 74°. For the index of refraction determination the measurement uncertainties lead to index of refraction uncertainties between 1 and 2.5%. The lower size limitation for index of refraction determination of single particles with the TAOPC was found to be ∼1.5 mum. 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. Tests of polydisperse aerosol were less conclusive. The measurement uncertainties were used to evaluate possible index of refraction determination of PSC particles. Concentration of solid non-spherical particles >1.5 mum 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 0.008-0.012 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.; A T-matrix database, developed in collaboration between Danish Meteorological Institute, the ETH Zurich, the CNR-ISAC and the University of Wyoming is used to determine scattering by randomly oriented non-spherical axially symmetric particles with a range of real indices of refraction, size and shape. It is shown that use of the T-matrix database requires caution when calculating scattering by spherical particles due to low size resolution in the T-matrix database and since spherical particles are independent of particle orientation; however, the low size resolution calculations are acceptable for non-spherical particles due to the averaging over particle orientation.