Abstract
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 eight 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 the 22 days of the campaign on which nucleation events were observed, eight major events were selected for detailed analysis on the basis of indications that the observed air masses were relatively homogeneous. In most of these cases, reasonable agreement is found between IMN 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. The possible reasons leading to substantial differences between observation and theory in some cases are also explored. Statistically, roughly 80% of the nucleation events recorded during the Hyytiälä campaign exhibited mean size-dependent particle overcharging ratios within the range of, or exceeding, those predicted by the IMN model, suggesting that ion nucleation processes were significant during these events. 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. In particular, it is concluded that the ion nucleation model originally developed by Lovejoy et al. (2004) significantly under-predicts ion nucleation rates, and cannot explain the new observations from Hyytiälä regarding the electrical properties of nanoparticles. We also show that, for the well documented conditions of the Hyytiälä project, the binary and ternary homogeneous nucleation rates calculated using the most current theories would fall well below ~10−7 cm−3 s−1, and thus would be negligible.
Highlights
New particle formation – regularly observed worldwide – appears to have clear spatial patterns (Kulmala et al, 2004a; Yu et al, 2008)
By constraining the key parameters in these simulations using observations, we aim to: (1) test the ability of the current ionmediated nucleation (IMN) model to account for the observed concentrations of freshly nucleated particles in the 3–6 nm size range; (2) assess the consistency of observed charged fractions of freshly nucleated particles against IMN model predictions under sampled conditions; and (3) compare nucleation rates predicted by the IMN model with those based on other existing nucleation models/parameterizations to highlight differences between various approaches, and to determine which of these representations are viable in light of the new data from Hyytiala
The unique long-term and multiple-instrument characterizations of nucleation events obtained at a boreal forest site (Hyytiala, Finland) by Kulmala and colleagues provide the most comprehensive set of data available to date to test nucleation theories of atmospheric particle formation
Summary
New particle formation – regularly observed worldwide – appears to have clear spatial patterns (Kulmala et al, 2004a; Yu et al, 2008). 2007a), taken in Hyytiala, Finland, suggest that ions are involved in more than 90% of the particle formation events that can be clearly identified, the relative contributions of ionmediated nucleation versus neutral processes remains controversial (Laakso et al, 2007a; Kulmala et al, 2007; Yu and Turco, 2007; Yu et al, 2008). To resolve the conflicting conclusions with regard to the importance of IMN, we focus here on recent data from the boreal forest experiments noted above, which obtained the most extensive set of relevant parameters to date If these events can be explained in the context of a self-consistent theory, the underlying nucleation mechanisms can be clarified. By constraining the key parameters in these simulations using observations, we aim to: (1) test the ability of the current IMN model to account for the observed concentrations of freshly nucleated particles in the 3–6 nm size range; (2) assess the consistency of observed charged fractions of freshly nucleated particles against IMN model predictions under sampled conditions; and (3) compare nucleation rates predicted by the IMN model with those based on other existing nucleation models/parameterizations to highlight differences between various approaches, and to determine which of these representations are viable in light of the new data from Hyytiala
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