The study combines the cycles of sulphur, nitrogen and carbon in the ecosystem, as it shows that the molecular clusters need sulphuric acid, amines and oxygenated organics for growth.

When the clusters reach a size of 1.5-2 nm, their growth increases considerably. The measurements were conducted at the University of Helsinki SMEAR II (Station for Measuring Forest Ecosystem-Atmosphere Relations) measurement station in Hyytiala, southern Finland, which is among the most comprehensive stations in the world for atmosphere and biosphere research.

During the last five years, the researchers at the University of Helsinki Physics Department have developed a Particle Size Magnifier (PSM), which is the first particle counter able to detect clusters and particles as small as 1 nm in diameter. The instrument is commercially available through the spin-off company Airmodus.

The scientists have also put effort into developing mass spectrometric methods for measuring the composition of the recently born clusters. The results in this study would not have been achieved without this technical development.

Professor Kulmala predicted the existence of neutral molecular clusters already in the year 2000 and their growth mechanisms in 2004.

He says: "Years of systematical research are now bearing fruit. My theoretical predictions have been proven to reflect the reality."

He stresses that knowledge of the formation and growth mechanisms of nanoparticles is needed for understanding the interactions within the climate system. Assessing the global impact requires an extensive data bank and a world-wide observation network.

Direct Observations of Atmospheric Aerosol Nucleation, Science, 22.2.2013, Manuscript Number: science.1227385