10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Light Scattering Analysis of Irregularly Shaped Dust Particles: A Study Using 3-Dimensional Reconstructions from Focused Ion-Beam (FIB) Tomography and Q-Space Analysis
DIANA ORTIZ-MONTALVO, Joseph Conny, National Institute of Standards and Technology
Abstract Number: 1017 Working Group: Aerosol Physics
Abstract We study the scattering properties of irregularly shaped ambient dust particles. The way in which they scatter and absorb light has implications for aerosol optical remote sensing and aerosol radiative forcing applications. However, understanding light scattering and absorption by non-spherical particles can be very challenging. We used focused ion-beam scanning electron microscopy and energy-dispersive x-ray spectroscopy (FIB-SEM-EDS) to reconstruct three-dimensional (3-D) configurations of dust particles collected from urban and Asian sources. The 3-D reconstructions were then used in a discrete dipole approximation method (DDA) to determine their scattering properties for a range of shapes, sizes, and refractive indices. Scattering properties where obtained using actual shapes of the particles, as well as, (theoretical) equivalently-sized geometrical shapes like spheres, cubes, and tetrahedrons. We use Q-space analysis to interpret the angular distribution of the scattered light obtained for each particle. Q-space analysis has been recently used to distinguish scattering by particles of different shapes, and it involves plotting the scattered intensity versus the scattering wave vector (q or qR) on a log-log scale, where q = 2ksin(θ/2), k = 2π/λ, and R = particle effective radius. Results from a limited number of particles show that when Q-space analysis is applied to 3D models of irregularly shaped particles (from FIB tomography), common patterns appear that agree with previous Q-space studies done on ice crystals and other irregularly shaped particles. More specifically, we found similar Q-space regimes including a forward scattering regime of constant intensity when qR < 1, followed by the Guinier regime when qR ≈ 1, which is then followed by a complex power law regime with a -3 slope regime, a transition regime, and then a -4 slope regime. Currently, Q-space comparisons between actual and geometric shapes are underway with the objective of determining which geometric shape best represents the angular distribution and magnitude of the scattered light. Current work also focuses on the effects of the imaginary part of the refractive index on the light scattering of our dust particles. Preliminary data show that agreement between models for the geometric shapes and the actual shapes of particles varies substantially with particle size. Better agreements were observed for smaller particle sizes. Overall, shape and size cause more variability in the data than differences in chemical composition.