Electrospray Ionization Mass Spectrometry Can Provide Molecular-Level Insights into Key Clusters Related to Atmospheric New Particle Formation

Studies on the microscopic mechanisms of new particle formation (NPF) have long been hindered by the lack of analytical tools capable of characterizing critical early stage clusters in the key 1-2 nm size range. Here, we repurpose and advance electrospray ionization mass spectrometry (ESI-MS)─a mature and widely accessible technique─into a robust platform for unraveling the molecular mechanisms of atmospheric nucleation. Validated across diverse chemical systems, ESI-MS enables reliable identification of key nucleation clusters and shows strong consistency with measurements from the Cosmics Leaving Outdoor Droplets (CLOUD) chamber. Experiments in binary, ternary, and quaternary systems involving various acids and bases yield consistent cluster compositions, sizes, and growth pathways. The integrated ESI-MS workflow further enables rapid screening of nucleation precursors. Using this strategy, citric acid is identified as an effective organic promoter that participates in cluster growth within classical systems such as sulfuric acid-ammonia. This work demonstrates that ESI-MS can serve as a valuable complementary experimental approach for NPF research, providing useful molecular-level insights into key clusters involved in atmospheric nucleation processes.