New particle formation, or nucleation, is known to be a globally important source of atmospheric aerosol particles, based on observations and model simulations. However, much of the observational evidence of its importance is based on clear nucleation "events" driven by strong photochemical processes that peak during the daytime. Here, the authors show that over Central Amazonia nearly half of the Aitken-mode sized particles with diameters 10-25 nm stem from a slower but hitherto hidden production of particles. This discovery changes our understanding of aerosol budgets in the lower atmosphere, requiring a reappraisal of measurements and model simulations for this and other regions.