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 forwar d scattering geometries has been numerically simulated adopting the T-matri x method in order to assess the instrumental properties when exposed to clo uds of aspherical particles (here: rotationally symmetric ellipsoids). The resulting scattering cross sections as function of particle size are compar ed to Mie theory calculations for spherical particles of the same refractiv e index, scattering geometry, and light wavelength. Based on these T-matrix calculations the FSSP-300 instrument is found suitable for use in populati ons of aspherical particles with aspect ratios larger than 0.5, if various caveats are taken into consideration. From the T-matrix method new tables r esult, which relate the instrument's size bin limits to actual particle siz es 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 atmosphe ric clouds. This paper also discusses in detail the limitations inherent in the use of the T-matix method as replacement for the Mie theory calculatio ns commonly adopted for the FSSP-300 type of instruments. (C) 2000 Elsevier Science Ltd. All rights reserved.