Mass extinction spectra and size distribution measurements of quartz and amorphous silica aerosol at 0.33–19 µm compared to modelled extinction using Mie, CDE, and T-matrix theories

Abstract

Simultaneous measurements were made of the spectral extinction (from 0.33–19 µm) and particle size distribution of silica aerosol dispersed in nitrogen gas. Two optical systems were used to measure the extinction spectra over a wide spectral range: a Fourier transform spectrometer in the infrared and two diffraction grating spectrometers covering visible and ultraviolet wavelengths. The particle size distribution was measured using a scanning mobility particle sizer and an optical particle counter. The measurements were applied to one amorphous and two crsystalline silica (quartz) samples. In the infrared peak values of the mass extinction coefficient (MEC) of the crystalline samples were 1.63 ± 0.23 m2g−1 at 9.06 µm and 1.53 ± 0.26 m2g−1 at 9.14 µm with corresponding effective radii of 0.267 and 0.331 µm, respectively. For the amorphous sample the peak MEC value was 1.37 ± 0.18 m2g−1 at 8.98 µm and the effective radius of the particles was 0.374 µm. Using the measured size distribution and literature values of the complex refractive index as inputs, three scattering models were evaluated for modelling the extinction: Mie theory, the Rayleigh continuous distribution of ellipsoids (CDE) model, and T-matrix modelling of a distribution of spheroids. Mie theory provided poor fits to the infrared extinction of quartz (R2 < 0.19), although the discrepancies were significantly lower for Mie theory and the amorphous silica sample (R2=0.86). The CDE model provided improved fits in the infrared compared to Mie theory, with R2 > 0.82 for crsytalline sillica and R2=0.98 for amorphous silica. The T-matrix approach was able to fit the amorphous infrared extinction data with an R2 value of 0.995. Allowing for the possibility of reduced crystallinity in the milled crystal samples, using a mixture of amorphous and crystalline T-matrix cross-sections provided fits with R2 values greater than 0.97 for the infrared extinction of the crystalline samples.