Atmospheric aerosol particles contribute to over four million premature deaths annually and play a critical role in modulating Earth’s climate. Most atmospheric particles and more than 50% of the cloud condensation nuclei are formed through a secondary process named new particle formation involving unique precursor vapors. This article summarizes current knowledge of how new atmospheric particles form, based on experiments at the CERN CLOUD chamber. While the role of sulfuric acid has long been known, other vapors like highly oxygenated organic molecules and iodine oxoacids are also important, along with stabilizers like ammonia, amines, and ions from cosmic rays. We explain how findings from CLOUD experiments help us understand particle formation in various atmospheric conditions and improve air quality and climate models.