Abstract
A new experiment is presented for the measurement of single aerosol particle extinction efficiencies, Qext, combining cavity ring-down spectroscopy (CRDS, λ = 405 nm) with a Bessel beam trap (λ = 532 nm) in tandem with phase function (PF) measurements. This approach allows direct measurements of the changing optical cross sections of individual aerosol particles over indefinite time-frames facilitating some of the most comprehensive measurements of the optical properties of aerosol particles so far made. Using volatile 1,2,6-hexanetriol droplets, Qext is measured over a continuous radius range with the measured Qext envelope well described by fitted cavity standing wave (CSW) Mie simulations. These fits allow the refractive index at 405 nm to be determined. Measurements are also presented of Qext variation with RH for two hygroscopic aqueous inorganic systems ((NH4)2SO4 and NaNO3). For the PF and the CSW Mie simulations, the refractive index, nλ, is parameterised in terms of the particle radius. The radius and refractive index at 532 nm are determined from PFs, while the refractive index at 405 nm is determined by comparison of the measured Qext to CSW Mie simulations. The refractive indices determined at the shorter wavelength are larger than at the longer wavelength consistent with the expected dispersion behaviour. The measured values at 405 nm are compared to estimates from volume mixing and molar refraction mixing rules, with the latter giving superior agreement. In addition, the first single-particle Qext measurements for accumulation mode aerosol are presented for droplets with radii as small as ∼300 nm.
Highlights
Ensemble cavity ring-down spectroscopy (E-CRDS) involves measuring Qext by studying the extinction of light by a collection of hundreds to thousands of particles introduced into an optical cavity formed by two highly reflective mirrors at a separation of B1 m
We have previously shown that a zeroth-order Bessel laser beam (BB)[15,16,17] optical trap can be used in the measurement of single particle processes, such as evaporation and hygroscopic response, for particle radii ranging from B5 mm to as small as B350 nm.[18,19,20,21]
We have recently demonstrated that optical extinction measurements on single particles can be made using a BB (l = 532 nm) to control the position of a single aerosol particle within a CRD beam.[20,25,26]
Summary
Ensemble cavity ring-down spectroscopy (E-CRDS) is a wellestablished technique used routinely in the measurement of Qext for aerosol particles with sub-micron radii.[3,4,5] E-CRDS involves measuring Qext by studying the extinction of light by a collection of hundreds to thousands of particles introduced into an optical cavity formed by two highly reflective mirrors at a separation of B1 m. In tandem with size determination from angular light scattering measurements, the variation of Qext was measured for the simple case of slowly evaporating 1,2,6-hexanetriol droplets[25] and for the more complicated case of hygroscopic growth of sodium chloride droplets.[20] does optical confinement of a single particle remove the assumptions inherent in E-CRDS measurements and improve the size determination accuracy, but it allows the same single particle to be studied over a long period of time. Paper reduced BB core, the first single particle measurements of Qext for droplets with radii as small as B250 nm are reported
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