The Development of a Linear-Quadrupole Electrodynamic Balance to Probe the Physical and Optical Properties of Aerosol Particles
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The Development of a Linear-Quadrupole Electrodynamic Balance to Probe the Physical and Optical Properties of Aerosol Particles


Aerosol particles present in the atmosphere influence climate through their interactions with light and their effect on cloud formation, and negatively impact air quality with significant health implications. In the atmosphere, aerosol particles can undergo transformations through an array of processes, including evaporation and condensation of semi-volatile compounds, oxidation initiated by heterogeneous reactions, and photochemistry initiated by solar illumination. The evolution of particle composition influences its physical and optical properties, which need to be understood to fully assess their role in the environment. To study the physical and optical properties of aerosol particles, a linear quadrupole electrodynamic balance (LQ-EDB) was developed and used to levitate single aerosol particles using electric fields. This method allows for contactless probing of particle properties in controlled conditions that mimic the natural environment. To probe the size and optical properties of the particle, a broadband light source was integrated with the LQ-EDB to illuminate the particle, giving rise to morphology dependent resonances (MDRs) that were sampled in a back-scattered geometry with a spectrometer. The wavelength of the MDR’s was used to determine the droplet size, refractive index, and wavelength-dispersion parameters via Mie theory. These methods were initially benchmarked in measurements of the evaporation rates of semi-volatile organic particles and the refractive index of aqueous salt particles. These measurements identified the facility of the methods in exploring the optical properties of aerosol that regulate their interactions with incoming solar radiation. In general, aerosol particles in the atmosphere have a cooling effect due to the scattering of incoming solar radiation. However, particles containing light absorbing species contribute to a warming effect. Using the LQ-EDB, we explored the physical and optical properties of aerosol particles containing light absorbing brown carbon (BrC) chromophores, such as those generated by wildfires and biomass burning. We report the optical properties, hygroscopic growth, and phase behavior as a function of relative humidity for a surrogate BrC particle containing 4-nitrocatechol and ammonium sulfate, revealing complex phase behavior that has implications for the optical properties and chemical reactivity of BrC aerosol. Subsequently, a new LQ-EDB was developed that incorporated heating elements to study the vapor pressures and influence of phase state on the rate of evaporation of a series of linear dicarboxylic acids from 293 to 350 K. These measurements provide clear evidence that phase state, either crystalline or amorphous, is a controlling factor in the rate of evaporation and contribute to our broader understanding of liquid-vapor partitioning in organic aerosol, organic solvents, fuel mixtures, and semi-volatile household products.

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