Molecular phenomena in organic aerosol: water interaction

There is still lack of knowledge about the aerosols physics and chemistry and consequently their impact on climate and health. The complexity of involved chemical interactions makes the characterization of these particles with experimental studies challenging. We discuss the dynamic and thermodynamic properties of organic aerosol by using molecular dynamics (MD) simulations. MD has been successfully used in atmospheric research applications due to its ability to predict macroscopic thermodynamic and dynamic properties for a wide variety of systems using the rules of classical statistical mechanics. In this research, organic-water interaction in organic aerosol is analyzed by investigating the parameters such as mass accommodation coefficient of water on organic coated surfaces, surface tension of organic aerosol, partitioning of organic between surface-bulk and activities of components in organic-water solution. The condensational growth of aerosol particles is sensitive to their ability to accommodate vapor molecules, which is described by the mass accommodation coefficient. However, the underlying processes are not yet fully understood. Growth processes of organic aerosols remain highly uncertain due to the numerous individual species involved. Understanding of the effect of organic on the aerosol hygroscopicity is the first goal of this research. The mass accommodation coefficient of water is estimated on organic-coated water surfaces using MD simulations. We study the effect of straight-chain and branched alcohols on water uptake process. Different methods are tested and discrepancy between applied methods are explained according to their underlying assumptions. It is found that an increase in the carbon density captures the effect of tail-group chain length and extent of branching on lowering the mass accommodation coefficient. Second, we explore the possible synergistic interactions among surfactants on the aerosol surface by MD simulations. The synergism in surface tension reduction efficiency describes the ability of the mixture to lower surface tension at a concentration level that is lower than the concentration levels required to create the same effect with the individual components. Since synergy depends on intermolecular interaction between surfactants, the possibility of having synergistic mixture on anionic-cationic organic aerosols is quite high. The main motivation of the study is to study reasons for inconsistencies in surface tension values of atmospheric particles in laboratory experiments and field studies. The study is extended to understand the relation of synergism with molecular structure by simulating the surfactants that have the same head but different tail groups. The synergistic effect of the considered surfactants is validated and it is found that not only head-group but also tail-group interactions are important to consider for determining the occurrence of synergism. Organic aggregate formation in aerosols can change both the physical and chemical properties of atmospheric particles. Therefore, lastly, we study how the formation of aggregates can change thermodynamic properties relevant for aerosol-water interactions. We use MD to simulate the aggregated and fully-solved organic+water systems at the same molality level. Possible effects of aggregation on the water, solute activities and supersaturation of aerosols are explored.