1Department of Physics, University of Helsinki, P.O. Box 64, 00014, Helsinki, Finland
2Helsinki Institute of Physics, University of Helsinki, P.O. Box 64, 00014, Helsinki, Finland
3Institute of Physics, University of Tartu, Ülikooli 18, 50090 Tartu, Estonia
4School of Physics & Centre for Climate and Air Pollution Studies, Environmental Change Institute, National University of Ireland Galway, Galway, Ireland
*currently at: Université Blaise Pascal, INSU, UMR6016, Laboratoire de Météorologie Physique, 63177 Aubiere, France
Abstract. The detailed mechanism of secondary new particle formation in the atmosphere is still under debate. It is proposed that particle formation happens via activation of 1–2 nm atmospheric neutral molecular clusters and/or large molecules. Since traditional instrumentation does not reach these sizes, the hypothesis has not yet been verified. By directly measuring particle size distributions down to mobility diameters of about 1.3 nm with a pulse-height CPC, we provide evidence of the nucleation mechanism in a coastal environment (Mace Head, Ireland) and in a boreal forest (Hyytiälä, Finland). In both places neutral sub-3 nm condensation nuclei (nano-CN) were continuously present, even when no new particle formation was detected. In Mace Head, however, the concentration of the nano-CN was far too low to account for the particle formation rates during particle bursts. Thus the results imply that at coastal sites new particle formation initiates, as previously proposed, via homogenous nucleation from biogenic iodine vapors. In contrast, activation of pre-existing nano-CN remains a possible explanation in the boreal forest, but the observed concentrations are not the limiting factor for the particle formation events.