AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Exploring Gelation in Model Marine Aerosol Particles: Micro-Rheological Observations of Ternary Water-Monosaccharide-Calcium Ion Microdroplets
RYAN DAVIS, David Richards, Kristin Trobaugh, Trinity University
Abstract Number: 73 Working Group: Aerosol Chemistry
Abstract Laboratory and field studies have demonstrated that atmospheric aerosols can exist as highly viscous liquids, semi-solids, and glasses. Such (semi-)solid particles are characterized by long timescales of equilibration with gas-phase species and slow diffusion of reactants throughout the particle bulk. Understanding the properties of semi-solid atmospheric aerosols is thus important to accurately represent aerosol water uptake and multi-phase chemistry in atmospheric models. In the past decade, a number of studies have characterized the rheological properties, such as viscosity, of model organic aerosols. These studies have shown many binary aqueous organics, such as aqueous sucrose, exhibit behavior consistent with viscous Newtonian fluids, with a predictable increase in viscosity upon decreasing relative humidity (RH). To date, studies have focused on those systems that exhibit such Newtonian behavior. However, it is known that marine-derived organic and inorganic material can potentially form Non-Newtonian fluids, such as gels; The behavior and properties of non-Newtonian gels remain understudied and it is unclear what aerosol compounds can form gels and under what conditions.
Here, using a dual-stage quadrupole trap as a micro-rheological tool, we present observations consistent with the formation of non-Newtonian gels in ternary microdroplets composed of aqueous monosaccharides and Ca2+ salts. For all of the monosaccharides studied, no gelation is observed in the binary monosaccharide nor ternary NaCl-monosaccharide systems, consistent with past studies. By contrast, in the presence of Ca2+ (i.e., in ternary CaCl2-monosacchride systems), gel formation is observed as high as ~60% RH. Gelation is found to depend on the molecular functionality and stereochemistry of the monosaccharides, as well as the identity of the counter-anion of the salt. As will be discussed, these observations provide a conceptual basis to predict gel formation in mixed organic-inorganic aerosol.