Correlated IR spectroscopy and visible light scattering measurements of mineral dust aerosol

Abstract

A combined infrared spectroscopy and visible light scattering study of the optical properties of quartz aerosol, a major component of atmospheric dust, is reported. Scattering phase function and polarization measurements for quartz dust at three visible wavelengths (470, 550, 660 nm) are compared with results from T‐matrix theory simulations using a uniform spheroid model for particle shape. Aerosol size distributions were measured simultaneously with light scattering. Particle shape distributions were determined in two ways: (1) analysis of electron microscope images of the dust, and (2) spectral fitting of infrared resonance extinction features. Since the aerosol size and shape distributions were measured, experimental scattering data could be directly compared with T‐matrix simulations with no adjustable parameters. χ2 analysis suggests that T‐matrix simulations based on a uniform spheroid approximation can be used to model the optical properties of irregularly shaped dust particles in the accumulation mode size range, provided the particle shape distribution can be reliably determined. Particle shape distributions derived from electron microscope image analysis give poor fits, indicating that two‐dimensional images may not give an accurate representation of the shape distribution for three‐dimensional particles. However, simulations based on particle shape models inferred from IR spectral analysis give excellent fits to the experimental data. Our work suggests that correlated IR spectral and visible light scattering measurements, together with the use of theoretical light scattering models, may offer a more accurate method for characterizing atmospheric dust loading, and aerosol composition, size, and shape distributions, which are of great importance in climate modeling.