Modelling optical properties of morphologically complex soot aerosols

Licentiate thesis, 2019

Soot containing aerosol has both adverse impacts on the Earth's climate and on human health. Monitoring soot sources, transport pathways and sinks on global scale requires satellite-borne remote sensing techniques.

A detailed understanding of the soot particle's optical properties is important to improve the interpretation of remote sensing data as well as the use of lidar remote sensing data in chemical transport modelling. The calculations of the optical properties were carried out using the discrete dipole approximation (DDA). Aim of this thesis is to identify key morphological features, which affect the depolarisation ratio.

As soot particles age in the atmosphere, condensation of other compounds from the gas phase onto the particles results in soot aggregates coated by liquid-phase material. Initially, the soot particles are coated by a thin film (i.e., the coating follows the shape of the aggregate). As more liquid phase material is added, the coating becomes increasingly spherical. It is found that this transition from film coating to radial growth of spherical shells is an important process affecting the linear depolarisation ratio. If this transition occurs first at relatively high amounts of coating, then the depolarisation ratio tends to be high. Conversely, if the coating becomes already spherical at low amounts of coating material, then the depolarisation ratio of the coated soot particles is much lower.


The linear depolarisation ratio of thickly coated aggregates was found to be sensitive to changes in the complex refractive index of the coating material, which represents changes in the chemical composition.
These differences in the optical properties, even after averaging over a particle size distribution, are large enough to clearly distinguish the coating materials.