(442c) Polymorphism of Glutaric Acid Aerosols

Because of the abundance of dicarboxylic acids in the atmosphere, they are commonly studied in the aerosol field, specifically investigating their hygroscopic and cloud condensation nuclei activity properties along with their behavior in inorganic mixtures. Glutaric acid, being a water soluble dicarboxylic acid is commonly used to model water activity in dicarboxylic acid aerosols. Studying trends associated with glutaric acid aerosols in the atmosphere, such as CCN activity and water uptake behavior is common. Glutaric acids behavior in the presence of inorganic particles, such as coating inorganic aerosols and forming co-crystals, is also frequently studied. However, in several circumstances there is some variability within the data associated with glutaric acid, specifically DRH and hygroscopic data, which is hypothesized to be caused by its polymorphism.

Glutaric acid exhibits dimorphism, meaning the molecule can form into two different types of crystal structures, an alpha and a beta polymorph, which can manifest themselves as a bimodal aerosol size distribution when analyzed by SMPS or other mobility sizing methods. Glutaric acid naturally forms in the beta polymorph, which is more stable. The alpha polymorph is the metastable phase, meaning it is less stable. In this work we investigated the formation of the alpha glutaric acid polymorph in aerosols generated from an aqueous glutaric acid solution. In no published research has the formation of both polymorphs from an aqueous solution been documented. Identifying the presence of polymorphism in glutaric acid aerosols will ideally explain the variability present in studies associated with glutaric acid aerosols. Understanding and defining under which conditions the alpha polymorph is present will allow others to avoid it when investigating glutaric acid as a dicarboxylic acid.

Additionally, bimodal distributions have been observed with select mixtures of organic and inorganic substances, however the mechanism creating bimodality appears to be different than polymorphism. These results will be presented and explained.