Optical characterization of complex aerosol and cloud particles: Remote sensing and climatological implications

Optical characterization of aerosol and cloud particles has been a challenge to researchers involved in a wide range of disciplines including remote sensing and climate studies. This thesis addresses several important atmospheric radiation problems involving cloud and aerosol particles with complex structure. We solve these problems by (i) extensively using state-of-the-art theoretical techniques to compute radiative properties of nonspherical and composite atmospheric particulates; (ii) combining theoretical and high-quality laboratory data for single scattering by irregular dust-like aerosols; (iii) applying advanced retrieval algorithms to analyze satellite observations of tropospheric aerosols; and (iv) validating satellite retrievals with high-quality ground-based data.; In Chapter 1, the superposition T-matrix method is used to compute electromagnetic scattering by semi-external aerosol mixtures in the form of polydisperse, randomly oriented two-particle clusters with touching components. The results are compared with those for composition-equivalent external aerosol mixtures. It is concluded that aggregation had a relatively weak effect on radiative properties of composite aerosols.; In Chapter 2, scattering and absorption characteristics of water cloud droplets containing black carbon (BC) inclusions are calculated in the visible spectral range by a combination of ray-tracing and Monte Carlo techniques. In addition, Lorenz-Mie calculations are performed assuming that the same amount of BC particles are mixed with water droplets externally. It is concluded that under normal conditions the effect of BC inclusions on the radiative properties of cloud droplets is weak.; In Chapter 3, we compare and combine the results of laboratory measurements of the Stokes scattering matrix for nonspherical quartz aerosols at a visible wavelength in the scattering angle range 5°–173° and the results of Lorenz-Mie computations for projected-area-equivalent spheres with the refractive index of quartz and have constructed a synthetic normalized phase function.; In Chapter 4, we use the synthetic phase function constructed in Chapter 3 to analyze the potential effect of particle nonsphericity on the results of retrievals of mineral tropospheric aerosols based on radiance observations from Advanced Very High Resolution Radiometer (AVHRR).; Chapter 5 presents the validation results of the aerosol optical thickness retrieved from AVHRR channel 1 and 2 radiances. The satellite retrieved optical thickness is compared with the accumulated historical ship-borne sun-photometer measurements. Comparisons of single-scattering albedo and Ångström exponent values retrieved from the AVHRR data and those measured in situ at Sable Island indicate that the currently adopted value 0.003 can be a reasonable choice for the imaginary part of the aerosol refractive index in global satellite retrievals.; In chapter 6, we analyze existing lidar observations of polar stratospheric clouds (PSCs) and derive several constraints on PSC particle microphysical properties based on extensive T-matrix computations of light scattering by polydispersions of randomly oriented, rotationally symmetric nonspherical particles.