Influence of Inorganic Ions on the Phase Behavior of Organic Rich Aerosol Particles
KIRAN REDDY PITTA, Komal Jaswal, Sylvain Foisy, Miriam Freedman, The Pennsylvania State University
Abstract Number: 277
Working Group: Aerosol Chemistry
Abstract
Phase transitions in atmospheric aerosols influence heterogeneous chemistry, hygroscopicity, and cloud formation. Aerosol particles often contain both organic and inorganic compounds, with organic matter making up 20–90% of the total mass, though only ~10% has been characterized. In contrast, inorganic composition is better understood, with major ions including Na+, K+, Ca2+, Mg2+, Cl−, NO3−, NH4+ and SO42−. Research on the phase transitions of organic/inorganic aerosol particles has primarily focused on (NH4)2SO4, which is a dominant component of the total inorganic mass of continental aerosol. This focus creates gaps in understanding the influence of other prevalent inorganic ions on phase transition. In this study, 2-methylglutaric acid and 1,2,6-hexanetriol, proxies for oxidized organic compounds, were mixed with different salts to investigate the phase transition properties of model droplets using an optical microscope. Experiments were conducted with 20 different salts, and separation relative humidity (SRH), efflorescence relative humidity (ERH), and deliquescence relative humidity (DRH) were compared. Anions followed the Hofmeister series, where strongly salting-out species had higher SRH, whereas cations showed an opposite pattern, with weaker salting-out ions exhibiting higher SRH. Relatively strong hydrating ions exhibited higher values of SRH, ERH, and DRH. In addition, systems with organic compounds and mixed inorganic compounds ((NH4)2SO4/NH4NO3, (NH4)2SO4/NH4Cl, and NH4Cl/ NH4NO3) were also studied. SRH values were compared as the relative concentrations of the salts varied, revealing a suppression in SRH with increasing concentrations of salts containing relatively weakly hydrating ions. Parameterizations of phase transition relative humidity were examined as a function of salt concentration in mixtures containing weakly and strongly hydrating ions. These studies are important in understanding the impact of organic-inorganic interactions on the phase transition properties of aerosol particles, which is important for predicting the role of aerosol particles for climate and air quality.