Determining the unique refractive index properties of solid polystyrene aerosol using broadband Mie scattering from optically trapped beads

Abstract

A method is described to measure the refractive index dispersion with wavelength of optically trapped solid particles in air. Knowledge of the refraction properties of solid particles is critical for the study of aerosol; both in the laboratory and in the atmosphere for climate studies. Single micron-sized polystyrene beads were optically trapped in air using a vertically aligned counter-propagating configuration of focussed laser beams. Each bead was illuminated using white light from a broadband light emitting diode (LED) and elastic scattering within the bead was collected onto a spectrograph. The resulting Mie spectra were analysed to accurately determine polystyrene bead radii to ±0.4 nm and values of the refractive index to ±0.0005 over a wavelength range of 480-700 nm. We demonstrate that optical trapping combined with elastic scattering can be used to both accurately size polystyrene beads suspended in air and determine their wavelength dependent refractive index. The refractive index dispersions are in close agreement with reported values for polystyrene beads in aqueous dispersion. Our results also demonstrate a variation in the refractive index of polystyrene, from bead to bead, in a commercial sample. The measured variation highlights that care must be taken when using polystyrene beads as a calibration aerosol.

Retrievals of aerosol optical and microphysical properties from Imaging Polar Nephelometer scattering measurements

“…This notion is confirmed in the sensitivity studies previously described, where it was found that there was very little sensitivity to sphericity in the case of small particles (r < 200 nm). Retrievals of the monodisperse PSL spheres produced real refractive index values that were within the range of existing measurements available in the literature at all three wavelengths (Bateman et al, 1959;Ma et al, 2003;Sultanova et al, 2003;Jones et al, 2013). The spectral dependence of the retrieved values, as well as the three most recently reported Cauchy's equation parameterizations of PSL refractive index can be found in Fig.…”

“…Uncertainties in these assumptions can generate significant changes in the resulting geometric PSD, but the presence of APS data can still be used as an optically independent, qualitative confirmation regarding the presence of significant coarse mode. The UH-SAS (Ultra-High Sensitivity Aerosol Spectrometer) data are shown for two different calibration aerosols, PSL spheres and ammonium sulfate, which have real refractive indices of 1.61 (Jones et al, 2013) and 1.53 (Tang, 1996) respectively. The LAS (Laser Aerosol Spectrometer) data shown correspond to calibration with PSL spheres.…”

“…This hypothe- Figure 9. Retrieved real part of the refractive index for PSL spheres, alongside three previous, modern measurements of polystyrene refractive indices (Ma et al, 2003;Jones et al, 2013;Sultanova et al, 2003). The subplot shows the retrieved size distribution (blue) along side the manufacturer's specified central radius (red dashes) and FW67 (red dots).…”

The study of thin films on solid aerosol particles using optical trapping and Mie scattering from a broadband white LED

“…Dry silica beads were delivered to the empty trapping cell using an atomiser connected to a diffusion dryer 20 . The Silica beads were obtained from Bangs Laboratories, SS04N, Lot number 7920 and specified as non-porous 1.035 µm radius beads.…”

“…A number of studies have been conducted on coated systems using both contact techniques [12][13][14] and non-contact techniques [15][16][17][18][19][20] .…”