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.
Highlights
Atmospheric aerosols directly affect the Earth’s radiative balance, across the electromagnetic spectrum from the infrared (IR) to the ultraviolet (UV), by scattering and absorbing solar radiation as well as absorbing and emitting infrared radiation [1,2]
Once the experiment was complete the cell was completely purged of aerosol by closing valves V1 and V4, opening valve V5 and increasing the flows provided by MFC1–MFC3 so that remaining aerosol exited via valve V5 to a fume cupboard
The precise level of dilution was determined from the size distribution returned by the instruments and a mass loading filter sample measurement made within filter holder 1 (FH1)
Summary
Atmospheric aerosols directly affect the Earth’s radiative balance, across the electromagnetic spectrum from the infrared (IR) to the ultraviolet (UV), by scattering and absorbing solar radiation as well as absorbing and emitting infrared radiation [1,2]. Previous high-quality laboratory measurements of the spectral extinction and size distribution of solid aerosol particles have been performed by [49,50,51] These earlier measurements were limited to infrared wavelengths and were performed at the lower resolution of 8 cm−1. The complex refractive indices of amorphous and crystalline silica are well documented in the literature, allowing the experimental extinction data to be evaluated against various scattering models (using the measured size distribution as an input), and allowing an assessment of the accuracy of the experimental method.
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More From: Journal of Quantitative Spectroscopy and Radiative Transfer
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