Parameterization of the Separation Relative Humidity in Submicron Aerosol Particles

MIRIAM FREEDMAN, Qishen Huang, Kiran Reddy Pitta, The Pennsylvania State University

     Abstract Number: 436
     Working Group: Aerosol Chemistry

Abstract
Liquid-liquid phase separation (LLPS) of atmospheric particles composed of organic and inorganic components has the potential to significantly impact climate, air quality, and human health. However, the complex composition, small size, and history-dependent properties of these aerosol particles results in great uncertainty in aerosol phase state for modeling atmospheric processes. In this study, we worked with model submicron aerosol particles composed of organic compounds mixed with ammonium sulfate and established a parameterization for the separation relative humidity (SRH) that accounts for the chemical composition, particle size, and equilibration time. We evaluated different variables that describe chemical composition: O/C ratio, partition coefficient, solubility, and polarizability. We found that the O/C ratio and polarizability can be used to distinguish homogenous vs. LLPS morphologies. Polynomial fittings show that the O/C ratio fits the SRH of micrometer droplets best, whereas the partition coefficient provides the best fit for the SRH of submicron aerosol. By using a scaling factor to translate the parameterization for SRH of micrometer droplets to the SRH for submicron aerosol particles, we incorporate the effects of size and the equilibration time on LLPS. Our parameterization will aid in modeling SRH, potentially leading to more accurate predictions of aerosol mass, optical properties, water uptake, and heterogeneous chemistry.