Electrospray Ionization-Based Synthesis and Validation of Amine-Sulfuric Acid Clusters of Relevance to Atmospheric New Particle Formation.

Atmospheric new particle formation (NPF) is the process by which atmospheric trace gases such as sulfuric acid, ammonia, and amines cluster and grow into climatically relevant particles. The mechanism by which these particles form and grow has remained unclear, in large part due to difficulties in obtaining molecular-level information about the clusters as they grow. Mass spectrometry-based methods using electrospray ionization (ESI) as a cluster source have shed light on this process, but the produced cluster distributions have not been rigorously validated against experiments performed in atmospheric conditions. Ionic clusters are produced by ESI of solutions containing the amine and bisulfate or by spraying a sulfuric acid solution and introducing trace amounts of amine gas into the ESI environment. The amine content of clusters can be altered by increasing the amount of amine introduced into the ESI environment, and certain cluster compositions can only be made by the vapor exchange method. Both approaches are found to yield clusters with the same structures. Aminium bisulfate cluster distributions produced in a controlled and isolated ESI environment can be optimized to closely resemble those observed by chemical ionization in the CLOUD chamber at CERN. These studies indicate that clusters generated by ESI are also observed in traditional atmospheric measurements, which puts ESI mass spectrometry-based studies on firmer footing and broadens the scope of traditional mass spectrometry experiments that may be applied to NPF.