Optical extinction efficiency measurements on fine and accumulation mode aerosol using single particle cavity ring-down spectroscopy

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.

Accurate Measurements of Aerosol Hygroscopic Growth over a Wide Range in Relative Humidity

“…In a postacquisition analysis step, the radii data are corrected by taking into account the variation of the refractive index with mass fraction of solute (mfs) by applying the molar refraction mixing rule, 37 which has been demonstrated to be the best mixing rule to describe the refractive index for a number of inorganic systems. 38 The molar refraction (R) of a component i is defined as …”

Assessing the accuracy of complex refractive index retrievals from single aerosol particle cavity ring-down spectroscopy

“…However, the light of the intra-cavity ring down beam in an SP-CRDS experiment is a standing wave. As explained in our previous publications Cotterell et al 2015a;Mason et al 2015Mason et al , 2014), the measured s ext value for a particle located at a node or anti-node of the intra-cavity standing wave differs from the value predicted by Mie theory. Moreover, a particle confined in the core of a BB optical trap can move via Brownian motion along the longitudinal axis of the optical cavity over distances of micrometres, sampling several nodes and anti-nodes of the standing wave and the various phases in-between.…”

“…We have previously demonstrated measurements of s ext with size variation, both for the evaporation of semi-volatile organic aerosol and for the hygroscopic response of aqueous droplets containing atmospherically-relevant inorganic solutes, to changes in relative humidity (RH). From SP-CRDS measurements of single component evaporation, the RI retrieval precision was 0.5-0.8% (Cotterell et al 2015a;Mason et al 2015). From SP-CRDS measurements of aqueous droplets containing atmospherically-relevant inorganic solutes, a forthcoming publication demonstrates the RI retrieval precision is »0.0025 (»0.17%).…”

“…The radius offset parameter is a value that is added to all the radius values for a RDT versus radius data set, and has the same definition as the radius offset parameter that was varied in Refs. (Mason et al 2015) and (Cotterell et al 2015a). In the case of data sets simulating a hygroscopic response measurement, w, n 532,1 and n 532,2 were varied.…”

“…Furthermore, analysis of E-CRDS measurements typically neglects the broadening of measured s ext values due to the effect of the CRDS standing wave Mason et al 2014). We have recently shown that a modification must be made to Mie theory to account for the intra-cavity standing wave of a two-mirror CRDS instrument in order to model correctly the s ext variations with radius Reid et al 2014;Cotterell et al 2015a;Mason et al 2015Mason et al , 2014.…”