Quiet New Particle Formation is a significant aerosol source in the Amazon boundary layer

Abstract. Aerosol particles formed by new particle formation (NPF) are essential for cloud condensation nuclei and can strongly influence cloud properties and climate. However, the mechanisms behind NPF in the Amazon boundary layer have remained elusive. Classical “banana” NPF events, common in other continental regions, are rarely observed in the Amazon, while most detected sub-50 nm particles have been linked to precipitation- and downdraft-related episodes, often called Amazonian banana events. Here, we analyse a decade of particle number size distributions (10–420 nm) from the Amazon Tall Tower Observatory (ATTO) during the wet season and demonstrate the presence of a distinct phenomenon called Quiet NPF. This process represents a subtle but persistent background particle formation, occurring on days without clear banana-type growth signatures. Using a statistical approach, we show that Quiet NPF links freshly formed 10 nm particles to growth into the Aitken mode. This mechanism is characterized by a growth rate of 2.3 ± 0.1 nm h⁻¹, about half that of Amazonian banana events, but occurs much more frequently. Quiet NPF accounts for ~45 % of 10–25 nm particle production during the wet season, revealing an overlooked but important source of nanoparticles that contributes to sustaining Amazonian aerosol populations.