Investigation of scattering of soot with a three-wavelength integrating nephelometer

Sammanfattning: Soot particles are one of the major contributors to climate change and global warming, mostly due to their absorbing effect. It is of great interest to gain better knowledge about soot particles optical properties, since there are great uncertainty how they affect the atmosphere. When soot particles are present in the atmosphere they disturb the sunlight albedo when absorbing light and by so heat up the atmosphere. To be able to study the soot particles optical properties in this thesis an instrument that usually is used for monitoring air pollution has been used, i.e. a three-wavelength integrating nephelometer. The instrument measures the scattering of light by particles or gas. It performs the measurement with a LED light source that operates at three different wavelengths i.e. 635 nm, 525 nm and 450 nm, measuring the total scatter (9-170 degrees) for one second for each wavelength, then a shutter is turned on and the same measurement procedure is performed for the backscatter (90-170 degrees). To be able to study the soot particles a soot generator (miniCAST) has been used. The soot generator mixes fuel, nitrogen and air in different combinations that produces soot with different characters. Five different soot types has been studied in this thesis and they are called OP-7, OP-6, OP-5, OP-3 and OP-1. Measurements have been made on both small primary particles (OP-7), and large aggregated soot particles (OP-1), hence different sized soot have different optical properties. The results presented in this thesis are the first measurements on soot made with the nephelometer at the department of Combustion Physics in Lund, therefore some improvements to the set-up has to be made in future experiments. However, the focus of this thesis work has been to commissioning the nephelometer and to fully understand what functions the instrument has and how they can be applied to this type of experiments. Calibration of the instrument has been made several times. Typical measurements on outdoor air have been made, this to be able to see what the nephelometer is usually used for. The results from the soot experiments show that the scattering properties vary a lot for the different particles. Large particles, i.e. OP-3 and OP-1, results in a smaller SAE typically around 2, SAE stands for Scattering Ångström Exponent a parameter used in climate research models and explain the scattering parameter wavelength dependence. For small particles such as in OP-7 and OP-6 SAE=4. These two results corresponds well with what theory predicts about particles SAE's for different sizes, i.e. smaller particles results in larger SAE's whereas larger particles have smaller SAE's. A larger SAE's for backward scatter is obtained for all measurements made in this work, OP-experiments and outdoor measurements, why this is the case is still unclear and will be studied in the future and explained with mathematical modeling and analysis. Also the relative scattering, i.e. ratio between forward and backward scatter shows an increasing ratio and also an increasing separation between the three wavelengths when approaching larger particle sizes.