The growth of freshly formed aerosol particles can be the bottleneck in their survival to cloud condensation nuclei. It is therefore crucial to understand how particles grow in the atmosphere. Insufficient experimental data has impeded a profound understanding of nano-particle growth under atmospheric conditions. Here we study nano-particle growth in the CLOUD (Cosmics Leaving OUtdoors Droplets) chamber, starting from the formation of molecular clusters. We present measured growth rates at sub-3 nm sizes with different atmospherically relevant concentrations of sulphuric acid, water, ammonia and dimethylamine. We find that atmospheric ions and small acid-base clusters, which are not generally accounted for in the measurement of sulphuric acid vapour, can participate in the growth process, leading to enhanced growth rates. The availability of compounds capable of stabilizing sulphuric acid clusters governs the magnitude of these effects and thus the exact growth mechanism. We bring these observations into a coherent framework and discuss their significance in the atmosphere.

Atmospheric new-particle formation is a major source of cloud condensation nuclei (CCN)¹ and a large contributor to the current uncertainty associated with aerosol–cloud–climate interactions. Significant effort in both the field and laboratory settings has been put into unravelling aerosol particle formation mechanisms and improving predictions and parameterizations of particle formation rates^{2, 3, 4, 5}. However, new-particle formation and subsequent CCN production is limited by the particle growth rate (GR) in the 1–3 nm diameter size range, rather than by the formation rate of molecular clusters, which are frequently present in the atmosphere^{3, 6, 7, 8}. As particles smaller than 3 nm contain only a handful of molecules, they diffuse almost as rapidly as gases and their survival probability depends critically on their growth rate. Interpretation of field measurement data suffers seriously from problems in isolating the different growth processes, such as vapour condensation, coagulation and various chemical reactions. The roles of different precursor vapours and ions in growth are still unclear, as the relevant concentrations in the atmosphere are extremely low. Laboratory studies conducted so far have not directly addressed the nano-particle growth issue.

Even though sulphuric acid vapour has been established as the main driving component for particle formation in the atmosphere^{2, 6, 9, 10}, numerous field studies have shown that the growth rates often substantially exceed the predictions based on measured sulphuric acid concentrations, and the remaining fraction of growth has usually been attributed to condensation of low-volatility organic vapours^{3, 8, 10, 11, 12, 13}. This is supported by quantitative measurements of the particle composition in the size range above 10 nm (refs 13, 14). It has recently been speculated that amines or other alkaline vapours could also participate in the growth process—possibly via salt formation^{15, 16, 17, 18}—and aminium salts have indeed been observed in 10 nm particles¹⁵. It has also been debated to what extent the electric charge of the initial clusters can assist the growth process, which affects the relative importance of the neutral and charged particle formation pathways^19, 20, 21.

In this study, the observed nano-particle growth can be directly connected to the precursor vapour concentrations and the gas-to-particle formation processes starting from molecular clusters. Although sulphuric acid is responsible for the nano-particle growth in all the studied experimental conditions, the exact growth mechanism is strongly tied to the presence of compounds capable of stabilizing sulphuric acid clusters. At low concentrations of base compounds, nano-particles grow mainly via the uptake of sulphuric acid and base monomers according to the traditional theory for vapour condensation, and the growth of the smallest particles is accelerated by the presence of electric charges. However, when a strongly basic compound is present, the nano-particle growth can be greatly enhanced by acid-base clusters. To the best of our knowledge, this is the first time these phenomena have been observed and quantified in a controlled system, which is required for understanding their role in nature.