Measurements of the evaporation and hygroscopic response of single fine-mode aerosol particles using a Bessel beam optical trap

Abstract

A single horizontally-propagating zeroth order Bessel laser beam with a counter-propagating gas flow 9 was used to confine single fine-mode aerosol particles over extended periods of time, during which 10 process measurements were performed. Particle sizes were measured by the analysis of the angular 11 variation of light scattered at 532 nm by a particle in the Bessel beam, using either a probe beam at 12 405 nm or 633 nm. The vapour pressures of glycerol and 1,2,6-hexanetriol particles were determined 13 to be 7.5 2.6 mPa and 0.20 0.02 mPa respectively. The lower volatility of hexanetriol allowed better 14 definition of the trapping environment relative humidity profile over the measurement time period, 15 thus higher precision measurements were obtained compared to those for glycerol. The size evolution 16 of a hexanetriol particle, as well as its refractive index at wavelengths 532 nm and 405 nm, were 17 determined by modelling its position along the Bessel beam propagation length while collecting phase 18 functions with the 405 nm probe beam. Measurements of the hygroscopic growth of sodium chloride 19 and ammonium sulfate have been performed on particles as small as 350 nm in radius, with growth 20 curves well described by widely used equilibrium state models. These are the smallest particles for 21 which single-particle hygroscopicity has been measured and represent the first measurements of 22 hygroscopicity on fine mode and near-accumulation mode aerosols, the size regimes bearing the most 23 atmospheric relevance in terms of loading, light extinction and scattering. Finally, the technique is 24 contrasted with other single particle and ensemble methods, and limitations are assessed. 25 26 2

Deviations from Plane-Wave Mie Scattering and Precise Retrieval of Refractive Index for a Single Spherical Particle in an Optical Cavity

“…We can also compare the retrieved refractive index to a recent and independent measurement for 1,2,6-hexanetriol droplets from our laboratory of n532 = 1.482  0.001 (with 1 SD uncertainty). 16 The rate of evaporation from the hexanetriol particle is sufficiently low that a trapped particle samples multiple positions within the ring-down cavity standing wave while maintaining a given radius. As figure 3 demonstrates, the envelope of extinction efficiency points is therefore well defined.…”

Electrodynamic balance–mass spectrometry of single particles as a new platform for atmospheric chemistry research

“…Using electrodynamic or optical forces, multiple laboratories have analyzed levitated droplets using optical techniques such as Raman spectroscopy and Mie resonance spectroscopy (earlier work reviewed in Krieger et al, 2012). A number of different properties have been studied in this way, including vapor pressures Cotterell et al, 2014;Huisman et al, 2013;Krieger et al, 2017), hygroscopic growth Cotterell et al, 2014;Rovelli et al, 2016), optical properties (Mason et al, 2015), liquid-liquid phase separation , diffusivities and diffusion coefficients (Bastelberger et al, 2017; Published by Copernicus Publications on behalf of the European Geosciences Union. 34 A. W. Birdsall et al: Electrodynamic balance-mass spectrometry of single particles Lienhard et al, 2014), and oxidative aging (Dennis-Smither et al, 2014).…”

Angular scattering of light by a homogeneous spherical particle in a zeroth-order Bessel beam and its relationship to plane wave scattering

“…[18][19][20][21][22] In the field of aerosol science, Bessel beams are very appealing for the study of single aerosol particles and instrumentation 2 incorporating such beams has been recently demonstrated. [23][24][25][26][27][28][29] However, there remains the need for an approach to accurately characterize a particle once it has been trapped in such a beam. For a homogeneous spherical particle the radius and refractive index must be accurately determined.…”