Modelling optical properties of morphologically complex soot aerosols

Soot containing aerosol has both adverse impacts on the Earth\u27s 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\u27s 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

Modelling optical properties of morphologically complex aerosols

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 system

Calculation of optical properties of light-absorbing carbon with weakly absorbing coating: A model with tunable transition from film-coating to spherical-shell coating

Optical properties of particles consisting of light-absorbing carbon (or soot) and a weakly absorbing coating material are computed at a wavelength of 355 nm and 532 nm. A morphological particle model is used, in which small amounts of coating are applied as a thin film to the surface of the aggregate, while heavily coated aggregates are enclosed in a spherical shell. As the amount of coating material is increased, a gradual transition from film-coating to spherical-shell coating is accounted for. The speed of this transition can be varied by specifying a single parameter. Two different choices of this parameter, corresponding to a slow and a rapid transition from film-coating to spherical-shell coating, respectively, are investigated. For low soot volume fractions the impact of this transition on the linear depolarisation ratio δlis most pronounced. The model that describes a rapid transition to a spherical coating yields results for δlthat are more consistent with existing lidar field measurements than the slow-transition model. At 532 nm the relative uncertainty in modelled δlfor a rapid transition values due to uncertainties in the aggregate\u27s geometry and chemical composition are estimated to range from 109 to 243%, depending on the soot volume fraction. At 355 nm the relative uncertainties were estimated to range from 90.9 to 200%

Modeling Optical Properties of Non-Cubical Sea-Salt Particles

Dry sodium chloride forms cubic crystals, while marine aerosol particles often display more or less irregular deviations from this ideal form. In this study, three non-ideal cuboidal and octahedral model geometries are investigated. Superellipsoids are tested as a model to simulate the linear backscatter depolarization ratio and the extinction-to-backscatter ratio. Gaussian random cubes as well as convex polyhedra are investigated as possible model candidates to quantify the error introduced by simplified model geometries, such as superellipsoids. Uncertainties in the real and imaginary part of the refractive index are studied, and their effect on the optical properties is compared to that caused by morphological variations. Optical calculations were performed at a wavelength of 532\ua0nm using the discrete dipole approximation and the T-matrix method. The considered size range is representative for marine aerosol generated at low to moderate wind speeds. It is found that cuboidal superellipsoids predict depolarization and extinction-to-backscatter ratios that are consistent with observations. On the other hand, octahedral superellipsoids strongly overestimate the depolarization ratio. Gaussian random surface perturbations result in a positive shift of the depolarization ratio compared to cuboidal superellipsoids. By contrast, convex polyhedra yield results that more or less randomly scatter about those of regular cubes. Thus, convex polyhedra are a promising candidate for modeling random errors, while Gaussian random cubes are not. Uncertainties in the refractive index result in perturbations of the depolarization and extinction-to-backscatter ratio that are of comparable magnitude as those caused by perturbations of the geometry

Aerosol optics model for black carbon applicable to remote sensing, chemical data assimilation, and climate modelling

Aerosol optics models are an integral part of of climate models and of retrieval methods for global remote sensing observations. Such large-scale environmental applications place tight constraints on the affordable model complexity, which are difficult to reconcile with the considerable level of detail that is needed to capture the sensitivity of optical properties to morphological aerosol characteristics. Here, we develop a novel core-grey-shell dimer model and demonstrate its potential for reproducing radiometric and polarimetric properties of black carbon aerosols. The depolarisation is mainly sensitive to the relative size of the monomers, while the optical cross sections depend on the core-shell partitioning of black carbon. The optimum choice of these parameters is fairly stable across particle sizes and soot volume fraction, as is demonstrated by comparison with a more realistic coated aggregate model

Optical properties of water-coated sea salt model particles

We investigate the optical properties of marine aerosol in dependence of the water content. To this end we develop a model geometry that realistically mimics the morphological changes as the salt particles take up more water. The results are compared to morphologically simpler models, namely, homogeneous and inhomogeneous superellipsoids, as well as cube-sphere hybrids. The reference model yields depolarization ratios, depending on size and water uptake, in the range from 0 to 0.36 \ub1 0.12. Overall, the simple models can reproduce optical properties of the reference model. The overall nonsphericity, as well as inhomogeneity are identified as key morphological parameter, while rounding of edges only has a minor impact on optical properties

Cautious note on using the discrete dipole approximation for inhomogeneous, spherical scatterers with high optical contrast

The discrete dipole approximation (DDA) is capable of treating scatterers with arbitrary shape and composition. However, large values of the refractive index require additional considerations. DDA calculations are performed for small spheres with a stronger absorbing inclusion and compared to T-matrix results. A natural phenomenon with strong optical contrasts is the melting of ice hydrometeors at microwave frequencies, and hence corresponding refractive indices are chosen. The obtained extinction and absorption efficiencies are found to depend mainly on the dipole size, whereas the phase function closely follows the T-matrix results by choosing a smaller stopping criterion and changing the polarizability formulation. (C) 2022 Optica Publishing Grou

Modelling optical properties of atmospheric black carbon aerosols

The optical properties of atmospheric black carbon (BC) aerosols are needed to model the direct radiative forcing of the climate system, as well as for interpreting and assimilating remote sensing observations from satellites. Modelling efforts during the past decade have predominantly been based on using morphologically highly realistic representations of the particle geometry in conjunction with numerically exact methods for solving the light-scattering problem. We review (i) the present state of knowledge about the morphological, dielectric, and compositional properties of BC aerosols, (ii) the state-of-the-art in numerical light-scattering methods frequently applied to black carbon, and (iii) the recent literature on modelling optical properties of BC aggregates, both bare and internally mixed with liquid-phase material. From this review we formulate some key lessons learned regarding those morphological properties that have a dominant impact on the optical properties. These morphological key features can form the basis for devising simplified model particles that can be used in large-scale applications. We illustrate this approach with one example appropriate for climate modelling, and one example relevant to the interpretation of lidar remote sensing data. \ua9 202

Aerosol-optics model for the backscatter depolarisation ratio of mineral dust particles

The size-dependence of the linear depolarisation ratio of mineral dust aerosols is investigated. Laboratory measurements on 131 different aerosol samples with varying size distributions and mineralogical compositions are fitted with a homogeneous spheroid model. A minimum-bias and minimum-variance fit of the data is obtained for prolate model particles with a refractive index of 1.525+0.001i and an aspect ratio of 0.87. The model error is analysed by varying the input parameters to the light-scattering computations. It is found that the scattering of the measurements about the model can mainly be explained by variation of the morphology and dielectric properties, and to a much lesser extent by variation in the geometric standard deviation of the size distribution. The modelling of the data is extended by using size-shape distributions of spheroids. The results indicate that there is some freedom in choosing the best-fit weights of the shape-distribution of spheroids, which could potentially be useful when extending the model to multiple wavelengths, or to including additional optical parameters other than depolarisation. However, it is also found that the most reasonable fits of the data are obtained by mildly aspherical prolate and oblate spheroids, which limits the freedom of adjusting the best-fit weights. (C) 2020 The Authors. Published by Elsevier Ltd

Coating material-dependent differences in modelled lidar-measurable quantities for heavily coated soot particles

The optical properties of thickly coated soot particles are sensitive to the chemical composition, thus to the refractive index of the coating material. For 58 differently sized coated soot aggregates the extinction-to-backscatter ratio (lidar ratio) and the depolarisation ratio are computed at a wavelength of 355 nm, 532 nm and 1064 nm for two different coating materials: a toluene-based coating and a sulphate coating. Additionally the \uc5ngstr\uf6m exponents between 355 nm and 532 nm as well as between 532 nm and 1064 nm are calculated. The extinction-to-backscatter ratio is found to allow a distinction between the coating materials at all three wavelengths, and the depolarisation ratio allows for a distinction at 355 and 532 nm