***** * Corresponding author. Journal of Quantitative Spectroscopy & Radiative Transfer 64 (2000) 75}86 Note Optimized response characteristics of an optical particle spectrometer for size measurement of aerosols Wladyslaw W. Szymanski *, Tomasz Ciach, Albert Podgorski, Leon Gradon  Institute of Experimental Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria  Department of Chemical and Process Engineering, Warynskiego 1, PL 00645 Warsaw, Poland Received 4 May 1998 Abstract The accuracy of aerosol particle size measurement based on light scattering from single particles depends strongly on the scattering angular range and particle refractive index. The particle size determination su!ers frequently from multi-valuedness. Based on the Mie theory of light scattering, light collection angles allowing single-valued aerosol measurement were identi"ed and utilized to design a new optical system with dual light scattering angular ranges. The light collecting optics is located co-axially in the forward (10}303) and backward (150}1703) direction with reference to the illumination beam and allows a simultaneous measure- ment of both scattered light #uxes. We found that a weighted superposition of these light #uxes results in monotonically increasing response characteristics versus particle size. In this contribution we analyze the performance of the dual optical system in the particle size range from 0.1 up to 10 m in diameter and show its capability for an accurate particle sizing and an assessment of the complex refractive index of aerosols.  1999 Elsevier Science Ltd. All rights reserved. 1. Introduction Particle spectrometers based on the light scattering from single aerosol particles are widely used for size distribution measurement of aerosols. This quantity combined with the knowledge of the particle complex refractive index is of substantial interest in e.g. atmospheric research or radiative transfer studies, or in modeling of the Earth's radiation budget. The accuracy of these measurements depends upon the response characteristics of an instrument, which is mainly determined by the geometry of the scattered light collection system and by the 0022-4073/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 2 - 4 0 7 3 ( 9 8 ) 0 0 1 3 9 - 3