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Simultaneous Retrieval of the Size and Refractive Index of Suspended Droplets in a Linear Quadrupole Electrodynamic Balance
journal contribution
posted on 2020-02-25, 21:29 authored by Chelsea
L. Price, Alison Bain, Brandon J. Wallace, Thomas C. Preston, James F. DaviesSingle-particle
trapping is an effective strategy to explore the
physical and optical properties of aerosol with high precision. Laser-based
methods are commonly used to probe the size, optical properties, and
composition of nonlight-absorbing droplets in optical and electrodynamic
traps. However, these methods cannot be applied to droplets containing
photoactive chromophores, and thus, single-particle methods have been
restricted to only a subset of atmospherically relevant particle compositions.
In this work, we explore the application of a broadband light scattering
approach, Mie resonance spectroscopy, to simultaneously probe the
size and the refractive index (RI) of droplets in a linear quadrupole
electrodynamic balance. We examine the evaporation of poly(ethylene
glycol)s and compare the calculated vapor pressures with literature
values to benchmark the size accuracy without prior constraint on
the RI. We then explore the hygroscopic growth and deliquescence of
sodium chloride droplets, measuring RI at the deliquescence relative
humidity and demonstrating agreement to literature values. These data
allow the wavelength dependence of the RI of aqueous NaCl to be determined
using a first-order Cauchy equation, and we effectively reproduce
literature data from multiple techniques. We finally discuss measurements
from a light-absorbing aqueous droplet containing humic acid and interpret
the spectra via the imaginary component of the RI. The approach described
here allows the radius of nonabsorbing droplets to be determined within
0.1%, the refractive index within 0.2%, and the first-order term in
the Cauchy dispersion equation within ∼5%.