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Aerosol nucleation events observed worldwide may havesignificant climatic and health implications. However, the specificnucleation mechanisms remain ambiguous. Here, we report case studies ofeight nucleation events observed during an intensive field campaign at aboreal forest site (Hyytiälä, Finland) in spring 2005. The presentanalysis is based on comprehensive kinetic simulations using an ion-mediatednucleation (IMN) model in which the key physical and chemical parameters areconstrained by a variety of recent measurements. Out of the 22 days of thecampaign on which nucleation events were observed, eight major events wereselected for detailed analysis on the basis of indications that the observedair masses were relatively homogeneous. In most of these cases, reasonableagreement is found between IMN predictions and field data for a range ofvariables, including critical nucleation sizes, size-dependent overchargingratios, and the concentrations of 1.8–3 nm stable clusters and 3–6 nm particles,and their diurnal variations. The possible reasons leading tosubstantial differences between observation and theory in some cases arealso explored. Statistically, roughly 80% of the nucleation eventsrecorded during the Hyytiälä campaign exhibited mean size-dependentparticle overcharging ratios within the range of, or exceeding, thosepredicted by the IMN model, suggesting that ion nucleation processes weresignificant during these events. The nucleation rates calculated using theIMN modeling approach are contrasted with those predicted by othertheories/models, and key differences between the results are discussed. Inparticular, it is concluded that the ion nucleation model originallydeveloped by Lovejoy et al. (2004) significantly under-predicts ionnucleation rates, and cannot explain the new observations fromHyytiälä regarding the electrical properties of nanoparticles. Wealso show that, for the well documented conditions of the Hyytiäläproject, the binary and ternary homogeneous nucleation rates calculatedusing the most current theories would fall well below ~10−7 cm−3 s−1,and thus would be negligible

Case studies of particle formation events observed in boreal forests: implications for nucleation mechanisms