Wildfires and biomass burning release aerosol particles containing brown carbon chromophores into the atmosphere. These chromophores absorb solar radiation and contribute to atmospheric warming. They may also act as cloud condensation nuclei due to the presence of water-soluble material. Knowledge of their physical characteristics that depend on relative humidity is limited and must be further explored to fully understand their role in the atmosphere.   <br/><br/>In this work, a linear quadrupole electrodynamic balance with a broadband light source is used to illuminate levitated aqueous brown carbon particles consisting of 4-nitrocatechol (4NC) for Mie resonance spectroscopy. The back scattered light from the particle is then used to determine the size, refractive index, and dispersion properties simultaneously. We measure the hygroscopic growth and optical properties of pure 4NC as well as mixtures of 4NC containing a non-absorbing inorganic component (ammonium sulfate) over a range of mixing ratios to derive the growth of the mixtures with ammonium sulfate. We also identify the phase transitions in the mixed particles noting the onset of liquid-liquid phase separation and efflorescence. These methods can be applied to other brown carbon species to further classify their hygroscopic properties and phase transitions to better understand their complex interactions in the environment.

Optical properties and phase separation of brown carbon aerosol in levitated droplets

ACS Spring 2022

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