A laboratory investigation of light scattering from representative components of mineral dust aerosol at a wavelength of 550 nm

To test the applicability of Mie theory in climate models and remote sensing data retrievals, we have studied the scattering phase function and linear polarization of representative mineral dust aerosol components at a wavelength of 550 nm. The mineral components investigated include the silicate clays, kaolinite, illite, and montmorillonite, and non-clay minerals, quartz, calcite, gypsum, and hematite, as well as Arizona road dust. In each case the aerosol size distribution was simultaneously monitored with an aerodynamic particle sizer. Particle diameters in this study fall in the accumulation mode size range characteristic of long-range transport aerosols. Our results show significant discrepancies between the experimental and Mie theory phase functions. The model shortcomings are due to particle shape effects for these non-spherical mineral dust particles. We find intriguing differences in the scattering between the silicate clay and non-clay components of mineral dust aerosol in this size range. For the non-clay minerals the most significant errors are found at large scattering angles where Mie theory substantially overestimates the backscattering signal. For the silicate clay minerals, there is more variability in the comparison to Mie theory. These findings have important consequences for the radiative forcing component of global climate models and remote sensing measurements that rely on Mie theory to characterize atmospheric dust. We also present experimentally based synthetic phase functions at 550 nm, for both silicate clay and non-clay mineral dust aerosols in the size parameter range X = 2-5, which may be useful for empirical models of the scattering due to particles in the accumulation mode size range.