Hygroscopicity and Phase Transformations of Internally Mixed Organic–Inorganic Proxies of Biomass Burning Aerosol

Biomass burning (BB) is a major source of atmospheric aerosols through both direct emissions and the secondary formation of particles. In addition to releasing large amounts of organic compounds in both the gas and particle phases, biomass burning plumes also emit inorganic species. Potassium salts are directly emitted, while ammonium salts can form due to reactions with ammonia. Aerosol particles formed from biomass burning are therefore a complex internal mixture of organic molecules and inorganic components. The molecular interactions between these species and water dictate the hygroscopic growth and phase behavior of the aerosol. In this work, we explore the hygroscopic growth and phase behavior for a series of mixed particles containing common water-soluble biomass burning compounds, phthalic acid, 4-nitrocatechol, and levoglucosan, and salts, potassium chloride, ammonium sulfate, sodium chloride, and potassium sulfate. These measurements were carried out using a linear quadrupole electrodynamic balance (LQ-EDB) coupled with Mie resonance spectroscopy to probe single particles as a function of the relative humidity. The morphology of these samples was observed to span from well-mixed aqueous solutions to fully effloresced particles with a variety of phase-separated states identified in between. From light scattering and hygroscopic growth measurements, we infer the phase of the particles under atmospherically relevant conditions and report the onset of phase transitions. We break down the contributions of individual components to the hygroscopicity using the Zdanovskii–Stokes–Robinson relation and compare these to predictions from a semiempirical thermodynamic model (AIOMFAC). For fully deliquesced particles, the predictions generally agree with observations, while particles that have undergone phase transitions show the largest deviations. Overall, this work highlights the limitations of assuming that BB particles are well-mixed and provides important physicochemical data to predict and interpret the humidity response of the BB aerosol.