Optical properties of non-absorbing mineral dust components and mixtures

Mineral dust is the second largest emission by mass into the atmosphere. Aerosol particles affect the radiative forcing budget by directly scattering and absorbing light, acting as cloud condensation and ice nuclei, and by providing surfaces for heterogeneous chemistry. Factors that affect how the particles scatter and absorb light include their composition, shape, size, and concentration. In this study, we characterize the most common components of mineral dust, quartz, and aluminosilicate clay minerals. In addition, we apply our results from calcite, feldspars, quartz, and aluminosilicate clay minerals to model the optical properties of Arizona test dust (ATD). We use cavity ring-down spectroscopy to measure the extinction cross sections of size-selected particles, electron microscopy to characterize the size selection, and Mie theory as well as the discrete dipole approximation as models. For quartz, the extinction cross sections can be well modeled assuming the particles are spheroids or spheres. For clay minerals, even spheroids fail to model the extinction cross sections, potentially due to orientation effects and lift forces in our flow system. In addition, aluminosilicate clay minerals experience weak size selectivity in the differential mobility analyzer. For ATD, the extinction cross sections are best modeled by treating each component of the mixture separately in terms of shape and size distribution. Through the application to ATD, our study outlines the procedure that can be used to model the optical properties of complex airborne dust mixtures.