Mainz 2026 – scientific programme

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MO: Fachverband Molekülphysik

MO 29: Cluster and Nanoparticles (joint session MO/A)

MO 29.5: Talk

Thursday, March 5, 2026, 16:30–16:45, P 105

Uptake of Ammonia onto Mixed Sodium Sulfate/Bisulfate Cluster Ions Studied in a Cryogenic Ion Trap — •Kevin Li, Yihui Yan, and Jozef Lengyel — School of Natural Sciences, Technical University of Munich, Garching, Germany

Understanding the earliest stages of atmospheric new particle formation (NPF) requires direct measurements on sub-nanometer molecular clusters, yet such studies remain experimentally challenging. To investigate this size range, we have developed a cryogenic ion trap mass spectrometer for kinetic studies with mass-selected cluster ions under multicollisional conditions. The instrument utilizes a ring-electrode ion trap, which enables stable confinement and allows for long reaction times of up to seconds. Therefore, the trap provides a powerful platform for investigating reaction pathways relevant to atmospheric aerosol formation.

In this study, we characterize the performance of the cryogenic ion trap and show its capabilities in a model study measuring uptake of gaseous ammonia onto mixed sodium sulfate and bisulfate ions. We further highlight that investigating mixed multicomponent clusters can reveal synergistic effects in NPF, where different components jointly enhance the uptake efficiency. In mixed sodium sulfate-bisulfate clusters, bisulfate units govern the number of uptaken ammonia molecules, whereas sulfate units control the reaction rate. With these experiments, we demonstrate the instrument's ability to investigate reaction pathways relevant to NPF.

Keywords: molecular clusters; molecule-cluster collision; mass spectrometry; reaction kinetics; ion trap