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| Titel |
Photochemical Aging of Organic Aerosols: A Laboratory Study |
| VerfasserIn |
Dimitrios K. Papanastasiou, Evangelia Kostenidou, Georgios I. Gkatzelis, Magdalini Psichoudaki, Evangelos Louvaris, Spyros N. Pandis |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250097311
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| Publikation (Nr.) |
 EGU/EGU2014-12876.pdf |
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| Zusammenfassung |
| Organic aerosols (OA) are either emitted directly (primary OA) or formed (secondary OA) in
the atmosphere and consist of an extremely complex mixture of thousands of organic
compounds. Although the scientific community has put significant effort, in the past few
decades, to understand organic aerosol (OA) formation, evolution and fate in the atmosphere,
traditional models often fail to reproduce the ambient OA levels. Secondary organic aerosol
(SOA) formed, in traditional laboratory chamber experiments, from the gas phase oxidation
of known precursors, such as α-pinene, is semi-volatile and with an O:C ratio of around 0.4.
In contrast, OA found in the atmosphere is significantly less volatile, while the O:C ratio
often ranges from 0.5 to 1. In conclusion, there is a significant gap of knowledge
in our understanding of OA formation and photochemical transformation in the
atmosphere. There is increased evidence that homogeneous gas phase aging by
OH radicals might be able to explain, at least in part, the significantly higher OA
mass loadings observed and also the oxidation state and volatility of OA in the
atmosphere.
In this study, laboratory chamber experiments were performed to study the role of the
continued oxidation of first generation volatile and semi-volatile species by OH radicals in
the evolution of the SOA characteristics (mass concentration, volatility, and oxidation state).
Ambient air mixtures or freshly formed SOA from α-pinene ozonolysis were used
as the source of organic aerosols and semi-volatile species. The initial mixture
of organic aerosols and gas phase species (volatile and semi-volatile) was then
exposed to atmospheric concentrations of OH radicals to study the aging of aerosols.
Experiments were performed with various OH radical sources (H2O2 or HONO) and under
various NOx conditions. A suite of instruments was employed to characterize both
the gas and the aerosol phase. A Scanning Mobility Particle Sizer (SMPS) and
a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS)
were used to measure the organic aerosol mass production and oxidation degree
(O:C ratio) following OH aging. A thermodenuder system was used to measure the
volatility distribution change as organic aerosol aged upon continuous oxidation.
Organic gas phase species were characterized with a Proton Transfer Reaction - Mass
Spectrometer (PTR-MS) while NOx and O3 were measured with the use of corresponding
analyzers.
Results from this study show that organic mass production occurs upon exposure
to OH radicals indicating that continuous OH aging of semi-volatile is probably
responsible for at least some of the gap between observed and modeled OA levels in
the atmosphere. Additionally, this chemical aging process leads to a decrease in
volatility and an increase in O:C ratio while the level of change in both properties
depends on OH exposure. The atmospheric implications of this study are discussed. |
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