Probing the Impact of Nascent Structural Motifs on the Hydration of Newly Formed Atmospheric Clusters

Abstract:

New particle formation (NPF) is a process in which trace vapors in the atmosphere grow into larger, climatically relevant clusters. Clusters containing amines and sulfuric acid have been shown to produce NPF rates similar to those in the ambient atmosphere. However, while water is ubiquitous in the atmosphere, its role in NPF and particle growth is poorly understood and difficult to assess. Water exponentially complicates computational explorations of the potential energy surfaces of small, NPF-relevant clusters, and validating computed structures is made more difficult by a lack of experimental data containing specific structural information, as most laboratory experiments capable of accessing these clusters are not sensitive to water. Small cationic clusters of amines and sulfuric acid that are experimentally accessible have been a useful tool for exploring the structural motifs likely present on the surface of larger cationic and neutral particles. Their increasingly well-assigned vibrational spectra contain all the features of much larger particles, and these vibrational features have been completely mapped to structural features, allowing us to identify climatically relevant surface groups and better predict the behavior of clusters. These clusters also provide starting points to interrogate the role water plays in the growth of these structures. Using a multi-trap gas-phase ion preparation technique and cryogenic ion vibrational predissociation spectroscopy we are able to probe the hydration of these well-studied clusters and investigate how a variety of structural features influence the likely binding sites for water and the relative favorability of those binding sites.