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| Titel |
Modeling the gas-particle partitioning of secondary organic aerosol: the importance of liquid-liquid phase separation |
| VerfasserIn |
A. Zuend, J. H. Seinfeld |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 12, no. 9 ; Nr. 12, no. 9 (2012-05-03), S.3857-3882 |
| Datensatznummer |
250011108
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| Publikation (Nr.) |
 copernicus.org/acp-12-3857-2012.pdf |
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| Zusammenfassung |
| The partitioning of semivolatile organic compounds
between the gas phase and aerosol particles is an important source of
secondary organic aerosol (SOA). Gas-particle partitioning of organic and
inorganic species is influenced by the physical state and water content of
aerosols, and therefore ambient relative humidity (RH), as well as
temperature and organic loading levels. We introduce a novel combination of
the thermodynamic models AIOMFAC (for liquid mixture non-ideality) and
EVAPORATION (for pure compound vapor pressures) with oxidation product
information from the Master Chemical Mechanism (MCM) for the computation of
gas-particle partitioning of organic compounds and water. The presence and
impact of a liquid-liquid phase separation in the condensed phase is
calculated as a function of variations in relative humidity, organic loading
levels, and associated changes in aerosol composition. We show that a complex
system of water, ammonium sulfate, and SOA from the ozonolysis of
α-pinene exhibits liquid-liquid phase separation over a wide range of
relative humidities (simulated from 30% to 99% RH). Since fully coupled
phase separation and gas-particle partitioning calculations are
computationally expensive, several simplified model approaches are tested
with regard to computational costs and accuracy of predictions compared to
the benchmark calculation. It is shown that forcing a liquid one-phase
aerosol with or without consideration of non-ideal mixing bears the potential
for vastly incorrect partitioning predictions. Assuming an ideal mixture
leads to substantial overestimation of the particulate organic mass, by more
than 100% at RH values of 80% and by more than 200% at RH values of
95%. Moreover, the simplified one-phase cases stress two key points for
accurate gas-particle partitioning calculations: (1) non-ideality in the
condensed phase needs to be considered and (2) liquid-liquid phase separation
is a consequence of considerable deviations from ideal mixing in solutions
containing inorganic ions and organics that cannot be ignored.
Computationally much more efficient calculations relying on the assumption of
a complete organic/electrolyte phase separation below a certain RH
successfully reproduce gas-particle partitioning in systems in which the
average oxygen-to-carbon (O:C) ratio is lower than ~0.6, as in
the case of α-pinene SOA, and bear the potential for implementation in
atmospheric chemical transport models and chemistry-climate models. A full
equilibrium calculation is the method of choice for accurate offline (box
model) computations, where high computational costs are acceptable. Such a
calculation enables the most detailed predictions of phase compositions and
provides necessary information on whether assuming a complete
organic/electrolyte phase separation is a good approximation for a given
aerosol system. Based on the group-contribution concept of AIOMFAC and
O:C ratios as a proxy for polarity and hygroscopicity of organic
mixtures, the results from the α-pinene system are also discussed from
a more general point of view. |
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