Extinction efficiencies of mixed aerosols measured by aerosol cavity ring down spectrometry

Abstract

A cavity ring down spectroscopy instrument was introduced and designed for measuring extinction efficiencies of pure and mixing aerosols in this paper. Through averaging 500 individual waveforms, the minimal detectable aerosol extinction coefficient of 8.4 × 10-7 m-1 was achieved. By the test results using the NaCl particles, we concluded that this system could measure the extinction efficiencies of an aerosol with an uncertainty less than 3% under laboratory controlled experimental conditions. The refractive indices of different aerosols were retrieved through comparing the measured extinction efficiencies of each aerosol type with which predicted by Mie theory. Aerosols composed of ammonium sulphate and succinic acid with different weight ratios were used to create a model of mixed aerosols using these two materials, whose extinction efficiencies and complex refractive indices were derived. The refractive indices of the mixed aerosols were also calculated by various optical mixing rules. We found that all the molar refraction/absorption mixing rule, the volume ratio linear rule, and Maxwell-Garnett rule did provide comparable results, of which the volume ratio linear rule gave a slightly worse fit than the others. © 2012 The Author(s).