Physical optics beam tracer models for smooth and rough non-spherical particles

Imaging methods are widely used for particle characterisation, however for small particles optical aberrations and constrained depth of field restrict the quality of the information obtainable. Such constraints do not apply to instruments such as the Aerosol Ice Interface Transition Spectrometer (AIITS) [1], which acquire far field scattering patterns. Obtaining quantitative morphological data by inversion of the patterns can be very challenging. Therefore, the creation of databases of two-dimensional (2D) scattering patterns of known particle morphologies is extremely useful for particle characterization. Exact methods such as T-matrix [2] and semi-exact methods like the finite difference time domain (FDTD) method [3] and the discrete dipole approximation (DDA) [4] can be used for computations of light-scattering properties for non-axisymmetric particles. Approximate methods, such as the geometric optics approximation or physical optics [5,6,7] have to be used for scatterers much larger than the wavelength of radiation.Here we present two beam tracer methods, a fast one suitable for facetted objects [8] and a beam tracer method using very fine beamlets, suitable for particles with complex shapes and/or surface roughness. For the two methods we show comparisons with DDA computations by A. Penttilä and T. Nousiainen [9] for hexagonal prisms with smooth and rough surfaces, respectively. Particle models with Gaussian random surfaces were obtained using the method developed by C T. Collier [9]. The beam tracer models have been applied for interpretation of AIITS scattering images during the NERC and NASA Co-ordinated Airborne Studies in the Tropics and Airborne Tropical Tropopause Experiment [10]. References[1] E. Hirst, C Stopford, P. H. Kaye, R. S. Greenaway and J D Dorsey. The Aerosol Ice Interface Transition Spectrometer – A new particle phase. ATTREX Science Meeting. Boulder, United States. 30 September 2013.[2] M. I. Mishchenko, N. T. Zakharova, N. G. Khlebtsov, T. Wriedt, G. Videen. Comprehensive thematic T-matrix reference database: a 2013–2014 update. J Quant Spectrosc Radiat Transf 2014;146:349–54. [3] P. Yang, K. N. Liou. In: M. I. Mishchenko, J. W. Hovenier, L. D. Travis, editors. Light scattering by nonspherical particles. New York: Academic Press; 1999. p. 173–221 .[4] M. A. Yurkin, A. G. Hoekstra. The discrete-dipole-approximation code ADDA: capabilities and known limitations. J Quant Spectrosc Radiat Transf 2011;112:2234–47 [5] P. Yang and K. Liou. Geometric-optics-integral-equation method for light scattering by nonspheric ice crystals. Applied Optics, vol. 35, no. 33, pp. 6568-6584, 1996.[6] A. V. Konoshonkin, N. V. Kustova and A. G. Borovoi, Y. Grynko, J. Förster. Light scattering by ice crystals of cirrus clouds: comparison of the physical optics methods. J Quant Spectrosc Radiat Transf 2016; 54-67.[7] E. Hesse, L. Taylor, C. T. Collier, A. Penttilä, T. Nousiainen, Z. Ulanowski. Discussion of a physical optics method & its application to absorbing smooth and slightly rough hexagonal prisms. J Quant Spectrosc Radiat Transf 2018; 12-23.[8] L. C. Taylor. A Beam Tracing Model for Electromagnetic Scattering by Atmospheric Ice Crystals, PhD thesis, University of Hertfordshire, 2016.[9] C. T. Collier, E. Hesse, L. Taylor, Z. Ulanowski, A. Penttilä, T. Nousiainen. Effects of surface roughness with two scales on light scattering by hexagonal ice crystals large compared to the wavelength: DDA results. J Quant Spectrosc Radiat Transf 2016;182:225–39.[10] E. R. Mathen, E. Hesse, A. J. Baran. Analysis and Modelling of TTL ice crystals based on in-situ measurement of scattering patterns. 19th Electromagnetic and Light Scattering Conference. Online. 12-16 July 2021.