Modelling optical properties of morphologically complex aerosols

Doktorsavhandling, 2021

The interpretation of remote sensing data of atmospheric aerosol particles requires a thorough understanding of the links between microphysical and optical properties. Morphologically complex aerosol models describe the particles’ morphology in detail. Based on the calculations with realistic particle models, simplified models can be devised, which incorporate essential microphysical properties for reproducing the optical properties. In this thesis, such models are developed and tested for soot aerosols, for mineral dust, and for dried and partially dissolved sea salt aerosol.
A tunable model for coated soot aggregates is presented, and corresponding uncertainty estimates are performed. One of the main sources of uncertainty for thickly coated soot is the chemical composition of the coating, as represented by its refractive index. These uncertainties are so substantial, they are investigated as a potential source of information. The calculated lidar-measurable (spectral) quantities are distinct for two coating materials.
The non-sphericity of a particle is identified as an essential morphological property affecting the linear depolarisation ratio. For coated soot another important property is the amount of carbon interacting with the incident wave, as it affects the absorption cross section. Combining these two insights resulted in the core grey shell dimer (CGS2) model, which is introduced in this thesis.
For dry sea salt aerosol different random geometries are investigated, to simultaneously calculate linear depolarisation and extinction-to-backscatter ratio of dried sea salt aerosol particles. The results indicate that convex polyhedra are best suited to represent dried sea salt aerosol particles. Thus, the coated convex polyhedra model is proposed as the basis for modelling dissolving sea salt in a further study. For dissolving sea salt three simplified, equally well-performing models are presented, which identify the change in particle sphericity as a key morphological feature.
A spheroidal model with a single refractive index and a single aspect ratio is fitted to laboratory measurements of 131 different dust samples. The scattering of the measurements about the model can mainly be explained by changes in morphology and dielectric properties, and to a lesser degree by the width of the particle size distribution.
These results are expected to significantly advance our capacity to exploit and interpret polarimetric remote sensing observations of morphologically complex and chemically heterogeneous aerosol. This will be important for constraining Earth-system climate and air-quality forecasting models, and for evaluating and improving parameterisations of aerosol processes in these environmental modelling systems