Exploring the Hygroscopicity, Phase State and Morphology of Biomass-Burning and Brown Carbon Aerosol Particles

PRAKRITI SINGH, Malsha Amugoda, James F. Davies, University of California, Riverside

     Abstract Number: 193
     Working Group: Aerosol Chemistry

Abstract
Atmospheric aerosols impact air quality, human health, visibility, cloud physics, and global climate. Biomass burning, an important source of aerosols and gas phase emissions, can produce significant amounts of brown carbon (BrC), a class of organic compounds that absorb light at visible wavelengths (<450 nm). BrC aerosol contain a mixture of water-soluble compounds, such as nitroaromatics, imidazoles, other oxygen-rich molecules with conjugated structures, and inorganic ammonium, nitrate, chloride, sulfate salts. Their physical characteristics, including size, refractive index, and phase morphology, are influenced by the prevailing environmental relative humidity (RH) and chemical composition. In this work we report the hygroscopicity of mixtures of organic components using an electrodynamic balance to perform single-particle levitation. We measure hygroscopicity of levoglucosan, phthalic acid, and 4-nitrocatechol, internally mixed with inorganic salts, such as (NH)2SO4, NaCl, KCl, and K2SO4. Measurements of hygroscopicity are compared to predictions using the ZSR mixing rule and to semi-empirical predictions using the AIOMFAC model. Our measurements show that hygroscopic BrC particles in mixtures with inorganic salts will exist in the form of a homogenous aqueous solution at high RH. However, they may undergo liquid-liquid phase separation (LLPS) and/or partial solidification at low RH, leading to a range of possible phase states with implications for their evolving role in the atmosphere, such as their optical characteristics and the reactivity of chromophores towards photolysis. Finally, we explore the hygroscopic growth and phase of particles containing BrC compounds that form during aqueous phase chemistry in dilute aqueous solutions representative of cloud droplets. Overall, this work seeks to connect the molecular composition of particles with their physiochemical properties to better inform their role in the atmosphere, their effect on air quality, and their interactions with solar radiation.