Production of "potential" cloud condensation nuclei associated with atmospheric new-particle formation in northern Finland

During a 33‐month measurement period at a high‐latitude background site in northern Finland, a total of 103 new particle formation events were observed. In 19 cases, movement of the air masses allowed the observation of particle formation and growth over a sufficiently long time to investigate the production of “potential” cloud condensation nuclei (CCN) resulting from new‐particle formation. All the CCN formation events took place in relatively clean air that had traversed the Northern Atlantic/Arctic Ocean prior to arrival at our measurement site. The number of formed new, “potential” CCN varied significantly between the different events and correlated weakly with the number of new‐particles formed during the same events. This is consistent with recent theoretical suggestions of some sort of decoupling between atmospheric new‐particle formation and growth. The vapours responsible for the “potential” CCN production could not be identified but were mostly something else than sulfuric acid resulting from the oxidation of SO2 in the gas phase. Although atmospheric new‐particle formation is likely to give only a minor contribution to the overall CCN budget at our measurement site, the situation may be different over areas where new‐particle formation events are more common. At the very least, our results demonstrate that because of atmospheric new‐particle formation, it may not be possible to find any universal relation between the aerosol mass and cloud droplet number concentration that is applicable to clean conditions.