Atmospheric aerosol particles with sizes ranging from 3 to 10 nm are known to form all around the globe at low and high altitudes. Although the processes responsible for their formation take place at below 3 nm, instruments so far have not been able to detect such small particles. Moreover, such measurements have only detected ionic particles to date.

Now, Markku Kulmala of the University of Helsinki and colleagues have used three recently developed instruments to measure neutral particle clusters between 1 and 2 nm across in the troposphere – the area just above the Earth's surface. These particles were detected in Hyytiälä in southern Finland and Birmingham in the UK.

The researchers say that a pool of these neutral clusters exist at almost all times and that atmospheric nucleation to form aerosols begins at 1.5 to 2 nm. They also say that neutral nucleation dominates over the ion-induced mechanism previously thought to be responsible for aerosol formation.

The atmospheric nanoparticles play an important role in climate by absorbing and reflecting heat. They can also start to grow and form clouds in a process known as cloud condensation nucleation. Indeed, Kulmala says that the formation of new atmospheric particles makes up about 30–50% of the global aerosol load, which means that it has an important effect on the world's radiative budget.

The nanoparticles are thought to form after the neutral clusters are "activated" – typically with sulphuric acid and/or organic condensable vapours produced by car exhausts and industrial emissions. In this case, they may have a detrimental effect on human health, producing respiratory and other problems. In coastal areas, water-insoluble and iodine compounds may also be responsible for the formation of the clusters.

The researchers now hope to elucidate the exact chemical composition of the neutral clusters and identify where they come from. They believe that the phenomenon should be included in large-scale atmospheric models, like those that describe regional air quality and global climate.

The team made its measurements using the Neutral Cluster-Air Ion Spectrometer, the UF-02proto Condensation Particle Counter and a Grimm nanoDMA and Faraday Cup Electrometer. It obtained its data over several weeks in Spring 2006.

The work was reported in Sciencexpress.