Atmospheric Chemistry and Physics Discussions
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2008
10.5194/acpd-8-5683-2008
http://www.atmos-chem-phys-discuss.net/8/5683/2008/
http://www.atmos-chem-phys-discuss.net/8/5683/2008/acpd-8-5683-2008.html
http://www.atmos-chem-phys-discuss.net/8/5683/2008/acpd-8-5683-2008.pdf
5683
5723
2008-03-19
Case studies of particle formation events observed in boreal forests: implications for nucleation mechanisms
F. Yu
yfq@asrc.cestm.albany.edu
R. Turco
Atmospheric Sciences Research Center, State University of New York at Albany, 251 Fuller Road, Albany, NY 12203, USA
Department of Atmospheric and Oceanic Sciences, University of California at Los Angeles, 405 Hilgard Ave, Los Angeles, CA 90095, USA
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.
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