The necessity of microscopy to determine the optical properties of size-selected mineral dust aerosol by cavity ring-down spectroscopy

It is currently unknown whether mineral dust causes a net warming or cooling effect on the Earth system. This uncertainty stems in part from its diverse chemical, physical, and optical properties. To better understand mineral dust optical properties, we are developing the highly sensitive technique of cavity ring-down spectroscopy for the study of size-selected, irregularly shaped particles. In this study, we have compared the fit of measured to calculated extinction cross sections for nonspherical calcium carbonate particles and nearly-spherical ammonium sulfate particles. The size selection process is dependent on the shape of the particles, where nonspherical particles can exhibit higher polydispersities than spherical particles. To measure the polydispersity of the selected sizes, we use transmission electron microscopy to image the particles. We measure the extinction cross sections of the particles using cavity ring-down spectroscopy. Mie scattering theory is used to obtain theoretical extinction cross sections for monodisperse spheres and polydisperse spheres with the size distribution observed in the microscopy images. We find that the extinction cross sections of size-selected ammonium sulfate particles are well modeled by monodisperse and polydisperse spheres. In contrast, the extinction cross sections of size-selected calcium carbonate particles are well modeled only by polydisperse spheres. Our study demonstrates the necessity of having an independent measure of the polydispersity in order to be able to model the observed extinction cross sections. Ultimately, such studies will aid in understanding the direct radiative effects of mineral dust aerosol.