Extension of interferometric particle imaging to small ice-crystal sizes using the Discrete Dipole Approximation

TL;DR

This paper extends Interferometric Particle Imaging (IPI) to smaller ice crystals using Discrete Dipole Approximation (DDA), enabling characterization of particles down to a few micrometers with validated measurement principles.

Contribution

It combines shape modelling, phase field modelling, and DDA to adapt IPI for small ice crystals, expanding its applicability in atmospheric particle analysis.

Findings

IPI remains valid for particles as small as 11.5 wavelengths

Wide viewing angles are necessary but complicate shape interpretation

DDA provides large datasets for inversion method development

Abstract

Interferometric Particle Imaging (IPI) is a powerful technique to characterize aerosol particles, which so far has been applied only to particles larger than about 100 wavelengths. We extend its applicability to smaller ice crystals, combining rigorous modelling of particle shapes with the Phase Field Modelling and light-scattering simulations with the Discrete Dipole Approximation (DDA). Even for particles with the largest dimension of 11.5 wavelengths (and the smallest one comparable to the wavelength), the 2D Fourier transform of the interferometric image remains linked to the 2D autocorrelation of the particle shape at various viewing angles, validating the general measurement principle. However, the sensor must necessarily have wide viewing angle, which complicates interpretation of apparent particle shape, when such particles are observed from the edge. IPI is, thus, shown to be a powerful optical technology for characterizing ice particles down to a few micrometers in the atmosphere. Meanwhile, DDA is a versatile method for such synthetic experiments and can further supply large datasets for development of various inversion methods.