Structural Impacts of the Addition of Succinic Acid to Cationic Ammonium Bisulfate Clusters

The process by which atmospheric trace vapors cluster and grow into climatically-relevant sizes is called new particle formation (NPF). While sulfuric acid and ammonia are the most abundant acid and base in the atmosphere, these two vapors alone cannot adequately explain observed NPF rates, and organic acids have been studied as a third vapor that could stabilize new particles or drive NPF on their own. We used infrared spectroscopy and mass spectrometry to characterize the structure of clusters composed of succinic acid (SuA), ammonia and sulfuric acid. We show that the structures of ammonium-bisulfate clusters are largely retained upon binding of up to two SuA molecules. Computed structures show significant COOH-COOH hydrogen bonding. These observations suggest that, even at these small sizes, these clusters feature incipient inorganic-organic phase separation. We also find that, despite the fact that the inorganic portion of these clusters feature more hydrogen bond donors than acceptors, there are few free carboxylic acid OH groups, suggesting that the cluster surface is relatively non-polar and depleted in binding sites for further vapors. Increasing SuA content is correlated to spectral broadening, suggesting that organic-enriched new particles are likely to be more fluxional than inorganic ones.