Atmos. Chem. Phys. Discuss., 8, 5683-5723, 2008
© Author(s) 2008. This work is distributed
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Case studies of particle formation events observed in boreal forests: implications for nucleation mechanisms
F. Yu1 and R. Turco2
1Atmospheric Sciences Research Center, State University of New York at Albany, 251 Fuller Road, Albany, NY 12203, USA
2Department of Atmospheric and Oceanic Sciences, University of California at Los Angeles, 405 Hilgard Ave, Los Angeles, CA 90095, USA

Abstract. Aerosol nucleation events observed worldwide may have significant climatic and health implications. However, the specific nucleation mechanisms remain ambiguous. Here, we report case studies of six nucleation events observed during an intensive field campaign at a boreal forest site (Hyytiälä, Finland) in spring 2005. The present analysis is based on comprehensive kinetic simulations using an ion-mediated nucleation (IMN) model in which the key physical and chemical parameters are constrained by a variety of recent measurements. Out of roughly 30 nucleation event days sampled during the campaign, four were initially selected on the basis of indications that the observed air masses were relatively homogeneous. It happens that all four of these days exhibited medium to high electrical overcharging of the nucleated nanoparticles. In each of these well-defined cases, reasonable agreement is found between the predictions and field data for a range of variables, including critical nucleation sizes, size-dependent overcharging ratios, and the concentrations of 1.8–3 nm stable clusters and 3–6 nm particles, and their diurnal variations. However, to extend the scope of the study, one case of weak electrical overcharging, and one of clear undercharging, of the nucleated particles were also selected. These electrical states represented less than about 20% of the total event-days recorded, and among this smaller sample there were no days on which the sampled air masses appeared reasonable uniform over the entire nucleation event. Thus, it is perhaps not surprising that the consistency between model simulations and measurements during these more anomalous periods was less satisfying. We tentatively conclude that the outcomes in these cases were influenced by, among other things, the significant variability in the sampled air masses and the possible role of species other than sulfuric acid in the nucleation process. Statistically, roughly 80% of the nucleation events recorded during the Hyytiälä campaign exhibited mean size-dependent overcharging ratios within the range, or exceeding, those predicted by the IMN model, suggesting that ion nucleation processes are significant. The nucleation rates calculated using the IMN modeling approach are contrasted with those predicted by other theories/models, and key differences between the results are discussed.

Citation: Yu, F. and Turco, R.: Case studies of particle formation events observed in boreal forests: implications for nucleation mechanisms, Atmos. Chem. Phys. Discuss., 8, 5683-5723, doi:10.5194/acpd-8-5683-2008, 2008.
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