Predicting and parameterizing the glass transition temperature of atmospheric organic aerosol components via molecular dynamics simulations

Abstract

Atmospheric aerosols contain thousands of organic compounds that exhibit an array of functionalities, structures and characteristics. Quantifying the role of these organic aerosols in climate and air quality requires an understanding of their physical properties. A key property determining their behavior is the glass transition temperature (T_g). T_g defines the phase state of aerosols, which in turn influences crucial aerosol processes. Molecular Dynamics (MD) simulations were implemented to predict T_g of a range of atmospheric organic compounds. The predictions were used to develop a T_g parameterization. The predictions and the parameterization link T_g with molecular characteristics such as the type and number of functional groups present in the molecule, its architecture, as well its carbon and oxygen content. The MD simulations suggest that T_g is sensitive to the functional groups in the organic molecule with the following order: –COOH > –OH > –CO. This trend is maintained even when more than one of these functional groups is present in a molecule. Molecular structure was also found to play a significant role. Cyclic structures exhibited consistently higher predicted T_g values compared to linear counterparts. T_g, as expected, increased as the number of carbon atoms increased. The parameterization was evaluated using a leave-one-out approach, providing insights into the contributions of various molecular features.