The size-dependent charge fraction of sub-3-nm particles as a key diagnostic of competitive nucleation mechanisms under atmospheric conditions 1Atmospheric Sciences Research Center, State University of New York at Albany, 251 Fuller Road, Albany, New York 12203, USA
16 Sep 2011
2Department of Atmospheric and Oceanic Sciences, University of California at Los Angeles, 405 Hilgard Ave, Los Angeles, California 90095, USA
Received: 15 March 2011 – Published in Atmos. Chem. Phys. Discuss.: 12 April 2011 Abstract. A clear physical understanding of atmospheric particle
nucleation mechanisms is critical in assessing the influences of aerosols on
climate and climate variability. Currently, several mechanisms have been
proposed and are being employed to interpret field observations of
nucleation events. Roughly speaking, the two most likely candidates are
neutral cluster nucleation (NCN) and ion-mediated nucleation (IMN). Detailed
nucleation event data has been obtained in boreal forests. In one set of
analyses of these measurements, NCN was suggested as the dominant formation
mode, while in another, it was IMN. Information on the electrical charge
distribution carried by the nucleating clusters is one key for identifying
the relative contributions of neutral and ion-mediated processes under
various conditions. Fortunately, ground-breaking measurements of the charged
states or fractions of ambient nanometer-sized particles soon after
undergoing nucleation are now available to help resolve the main pathways.
In the present study, the size-dependent "apparent" formation rates and
fractions of charged and neutral particles in a boreal forest setting are
simulated with a detailed kinetic model. We show that the predicted
"apparent" formation rates of charged and neutral particles at 2 nm for
eight representative case study days agree well with the corresponding
values based on observations. In the simulations, the "apparent"
contribution of ion-based nucleation increases by up to ~one order of
magnitude as the size of "sampled" particles is decreased from 2 nm to
~1.5 nm. These results suggest that most of the neutral particles
sampled in the field at sizes around 2 nm are in reality initially formed on
ionic cores that are neutralized before the particles grow to this size.
Thus, although the apparent rate of formation of neutral 2-nm particles
might seem to be dominated by a neutral clustering process, in fact those
particles may be largely the result of an ion-induced nucleation mechanism.
This point is clarified when the formation rates of smaller particles (e.g.,
~1.5 nm) are explicitly analyzed (noting that measurements at these
smaller sizes are not yet available), indicating that IMN dominates NCN
processes under typical circumstances in the boreal forest cases
Revised: 17 July 2011 – Accepted: 05 September 2011 – Published: 16 September 2011
Citation: Yu, F. and Turco, R. P.: The size-dependent charge fraction of sub-3-nm particles as a key diagnostic of competitive nucleation mechanisms under atmospheric conditions, Atmos. Chem. Phys., 11, 9451-9463, doi:10.5194/acp-11-9451-2011, 2011.