Application of Quasi-Elastic Light Scattering (QELS) Spectroscopy to Probe Single Aerosol Particle Microphysics

Atmospheric aerosol particles both directly and indirectly affect the climate, and the extent of their effects is directly related to the physicochemical properties of the aerosol particle(s).¹ Aerosol particles in the atmosphere can interact directly with incoming radiation and uptake water to activate into cloud droplets. Instrumentation has been developed to measure surface tension, viscosity, hygroscopicity, phase state, and water diffusivity of single, suspended particles and thus can be used to infer how these physicochemical properties of particles can affect their impact on climate. It is important to do model studies on single particles with known composition in order to isolate a single component of the complex atmospheric system and to measure particles suspended in air, as they are in the atmosphere.  Aerosols exhibit a high surface-area-to-volume ratio which results in different physicochemical properties when compared to that of a similar bulk solution, thus demonstrating a need to quantify the unique properties of a single, suspended aerosol particle.² Particle levitation using a Linear Quadrupole Electrodynamic Balance (LQ-EDB) traps single charged super-micron aerosol particles in a gradient electric field which allows for particle characterization using Raman spectroscopy.³ Recent studies using LQ-EDB paired with cavity-enhanced Raman spectroscopic techniques have quantified several aerosol physicochemical properties, highlighting that viscosity, water diffusivity, phase state, and surface tension all have dependencies on relative humidity (RH), particle chemical composition, and particle size. ^4-7 Understanding the physicochemical characteristics of particles is critical to accurately predict how particles scatter and/or absorb incoming solar radiation, as well as predict particle hygroscopicity, surface tension, and phase state which influence their ability to activate into cloud droplets. Continued advancement with instrumentation that employs single particle measurements using a LQ-EDB and various spectroscopic techniques can provide measurements of these key aerosol particle properties. 

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