AS&T Article Highlight

AS&T Article Highlight

By Benjamin Murphy

“Why would apparent κ linearly change with O/C?
Assessing the role of volatility, solubility, and surface
activity of organic aerosols”

Shunsuke Nakao


Computational models continue to be fraught by the challenge of representing the vast number and complex behavior of organic compounds
present in the atmosphere. This complexity creates diffculties when
predicting impacts of organic particles and vapors on the formation and
properties of clouds. Nearly all large-scale models (and many laboratory-scale models) thus represent these compounds with a reduced set
of surrogate species chosen to reproduce the distribution of important
properties that govern atmospheric processing of organics (e.g. volatility, carbon number, the ratio of oxygen atoms to carbon atoms (O/C),
polarity, solubility, etc). However, it is often not clear how to connect
these properties to important microphysical phenomena. For example,
as Nakao points out in this study, studies have disagreed about the
degree of correlation between O/C and κ, a parameter that quantifes a
chemical component’s hygroscopicity and connects mixture information
to bulk cloud condensation nuclei (CCN) activity.

Nakao has laid out a straight-forward framework that links existing
organic aerosol theories (i.e. 2D-Volatility Basis Set) with parameterizations of organic solubility and fnally with κ-Köhler theory in order to
calculate bulk CCN activity. Importantly, this comprehensive approach
accounts for the effects of both solubility and molecular size simultaneously on bulk mixture behavior. The author uses an example population
of compounds, in this case they are laboratory-observed oxidation products of α-pinene ozonolysis, in order to demonstrate why models should
utilize distributions of properties like O/C and volatility when parameterizing κ, rather than relying on averaged estimates.


Figure 2. Simulation results of α-pinene SOA without inorganic seed. The 2D-VBS distribution is taken
from Donahue et al. (2012). Ddry = 100 nm. (a) Distribution of organic aerosol volume in terms of O/C
and logC*. The solid lines correspond to intrinsic κ. The dotted lines correspond to volume-based
solubility C.


He is also able to report a practical upper-limit on κ values associated
with organics equal to 0.3, given the relevant volatility and O/C values
observed in the ambient.


The study further explores the impact of surface activity, accounting for
both depletion of bulk-phase solute concentration and surface tension
depression. Because relationships between properties describing surface activity and molecular properties like volatility and O/C are lacking,
1000 simulations are performed using plausible ranges for parameters
describing surface activity. The study fnds that the solute and surface
tension effects generally counter-balance each other if one assumes the
compounds are all highly soluble. The author points out though, that
more studies are needed to constrain the behavior of mixtures with surface-active, slightly soluble compounds as the undissolved portion may
enter the bulk to replace the molecules that partition to the surface, and
thus replenish the depleted solute.


Finally, Nakao predicts κ as a function of O/C using several atmospherically relevant mixtures of organic compounds from previous literature.
The analysis reproduces the generally linear relationship between κ and
O/C found in several studies, but is also able to explain observations
where does not seem to vary with O/C or decreases when ambient particles are heated in a thermodenuder. The framework Nakao has put forth
resolves a number of mysteries, misunderstandings, and missing pieces
in the literature involving organic aerosol and cloud interactions. It has
the potential to be a crucial asset to coupled chemistry-meteorological
models in the future, and enable evaluation of lumped distributions (e.g.
2D-VBS, etc) using κ measurements.

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