APPLICATION OF THE T-MATRIX METHOD TO THE MEASUREMENT OF ASPHERICAL (ELLIPSOIDAL) PARTICLES WITH FORWARD SCATTERING OPTICAL PARTICLE COUNTERS

The response of the FSSP-300 and MASP optical particle counters with forward scattering geometries has been numerically simulated adopting the T-matrix method in order to assess the instrumental properties when exposed to clouds of aspherical particles (here: rotationally symmetric ellipsoids). The resulting scattering cross sections as function of particle size are compared to Mie theory calculations for spherical particles of the same refractive index, scattering geometry, and light wavelength. Based on these T-matrix calculations the FSSP-300 instrument is found suitable for use in populations of aspherical particles with aspect ratios larger than 0.5, if various caveats are taken into consideration. From the T-matrix method new tables result, which relate the instrumentʹs size bin limits to actual particle sizes for a given refractive index. Two data sets of measurements in a cirrus cloud and an aircraft contrail are reduced by means of these new assignment tables in order to demonstrate the applicability of the method to atmospheric clouds. This paper also discusses in detail the limitations inherent in the use of the T-matix method as replacement for the Mie theory calculations commonly adopted for the FSSP-300 type of instruments.