Atmospheric Chemistry and Physics Discussions
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10.5194/acpd-8-95-2008
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http://www.atmos-chem-phys-discuss.net/8/95/2008/acpd-8-95-2008.pdf
95
135
2008-01-04
CCN activity and droplet growth kinetics of fresh and aged monoterpene secondary organic aerosol
G. J. Engelhart
A. Asa-Awuku
A. Nenes
S. N. Pandis
Dept. of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, USA
School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA
School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA
Department of Chemical Engineering, University of Patras, Patra, Greece
The ability of secondary organic aerosol (SOA) produced from the ozonolysis
of α-pinene and monoterpene mixtures (α-pinene, β-pinene, limonene and 3-carene) to become cloud droplets was investigated.
Monoterpene SOA is quite active and would likely be a good source of cloud
condensation nuclei (CCN) in the atmosphere. A static CCN counter and a
Scanning Mobility CCN Analyser (a Scanning Mobility Particle Sizer coupled
with a Continuous Flow counter) were used for the CCN measurements. A
decrease in CCN activation diameter for α-pinene SOA of
approximately 3 nm h<sup>−1</sup> was observed as the aerosol continued to react
with oxidants. Hydroxyl radicals further oxidize the SOA particles thereby
enhancing the particle CCN activity with time. The initial concentrations of
ozone and monoterpene precursor (for concentrations lower than 40 ppb) do
not appear to affect the activity of the resulting SOA. Köhler Theory
Analysis (KTA) is used to infer the molar mass of the SOA sampled online and
offline from atomized filter samples. KTA suggests that the aged aerosol
(both from α-pinene and the mixed monoterpene oxidation) is
primarily water-soluble (around 70–80%), with an estimated average molar
mass of 180±55 g mol<sup>−1</sup> (consistent with existing SOA speciation
studies). CCN activity measurements of the SOA mixed with
(NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> suggest that the organic can depress surface
tension by as much as 10 nM m<sup>−1</sup> (with respect to pure water). The
droplet growth kinetics of SOA samples are similar to
(NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>, except at low supersaturation, where SOA tends to
grow more slowly. The CCN activity of α-pinene and mixed monoterpene
SOA can be modelled by a very simple implementation of Köhler theory,
assuming complete dissolution of the particles, no dissociation into ions,
molecular weight of 180 g mol<sup>−1</sup>, density of 1.5 g cm<sup>−3</sup>, and
surface tension to within 10–15% of water.
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