seen in an ESI-MS should not be extrapolated to infer concentrations of similar species during atmospheric particle formation events. Figure shows cationic ammonium bisulfate clusters produced by ESI. The composition of the ions, (NH4 + ) x (HSO4 -) x-1 , is identical to the ammonium bisulfate clusters detected by the atmospheric simulation chamber, Cosmics Leaving Outdoor Droplets (CLOUD), using CI-APiTOF. This demonstrates that ESI is effective in producing ions that are atmospherically relevant in a laboratory setting. The success of ESI in generating atmospherically relevant particles has led to the development of customized sources, such as the electrostatic precipitation ESI (EP-ESI) source.(75) EP-ESI involves a two-step process: aerosols are first ionized by corona discharge in an ionization chamber, then deposited onto a metal filament by electrophoretic motion. The filament acts as the emitter, and the deposited aerosols dissolve into the Taylor cone of the electrospray, eventually forming ions. In a proof-of-concept study, EP-ESI detected nanogram quantities of collected materials, making it valuable for chemical analysis of PM 2.5 and smaller particles in lab studies. The synthesis of particles/molecular clusters via ESI is sensitive to the environment in the ionization chamber. A custom-designed nano-ESI emitter, housed in a air-tight capsule with controlled temperature and pressure, has been used to produce ammonium and aminium bisulfate clusters. The clusters, produced by electrospraying the respective salt solutions or through exchange reactions in the gas phase between nascent ions and amine vapors, were compositionally identical to those detected by CLOUD. This setup separates the ESI emitter from changes in the laboratory environment, and has been used to synthesize many atmospherically relevant clusters.(76; 33; 77; 78; 79)