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| Titel |
Oligomer formation during gas-phase ozonolysis of small alkenes and enol ethers: new evidence for the central role of the Criegee Intermediate as oligomer chain unit |
| VerfasserIn |
A. Sadezky, R. Winterhalter, B. Kanawati, A. Römpp, B. Spengler, A. Mellouki, G. Bras, P. Chaimbault, G. K. Moortgat |
| 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 ; 8, no. 10 ; Nr. 8, no. 10 (2008-05-21), S.2667-2699 |
| Datensatznummer |
250006144
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| Publikation (Nr.) |
 copernicus.org/acp-8-2667-2008.pdf |
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| Zusammenfassung |
An important fraction of secondary organic aerosol (SOA) formed by
atmospheric oxidation of diverse volatile organic compounds (VOC) has
recently been shown to consist of high-molecular weight oligomeric species.
In our previous study (Sadezky et al., 2006), we reported the identification
and characterization of oligomers as main constituents of SOA from gas-phase
ozonolysis of small enol ethers. These oligomers contained repeated chain
units of the same chemical composition as the main Criegee Intermediates
(CI) formed during the ozonolysis reaction, which were CH2O2 (mass
46) for alkyl vinyl ethers (AVE) and C2H4O2 (mass 60) for
ethyl propenyl ether (EPE). In the present work, we extend our previous
study to another enol ether (ethyl butenyl ether EBE)
and a variety of structurally related small alkenes (trans-3-hexene,
trans-4-octene and 2,3-dimethyl-2-butene).
Experiments have been carried out in a 570 l spherical glass reactor at
atmospheric conditions in the absence of seed aerosol. SOA formation was
measured by a scanning mobility particle sizer (SMPS). SOA filter samples
were collected and chemically characterized off-line by ESI(+)/TOF MS and
ESI(+)/TOF MS/MS, and elemental compositions were determined by
ESI(+)/FTICR MS and ESI(+)/FTICR MS/MS. The results for all investigated unsaturated compounds
are in excellent agreement with the observations of our previous study. Analysis of the collected SOA filter samples reveal
the presence of oligomeric compounds in the mass range 200 to 800 u as major
constituents. The repeated chain units of these oligomers are shown to
systematically have the same chemical composition as the respective main
Criegee Intermediate (CI) formed during ozonolysis of the unsaturated
compounds, which is C3H6O2 (mass 74) for ethyl butenyl ether
(EBE), trans-3-hexene, and 2,3-dimethyl-2-butene, and C4H8O2 (mass
88) for \textit{trans}-4-octene. Analogous fragmentation pathways among the oligomers
formed by gas-phase ozonolysis of the different alkenes and enol ethers in
our present and previous study, characterized by
successive losses of the respective CI-like chain unit as a neutral
fragment, indicate a similar principal structure. In this work, we confirm the basic structure of a linear
oligoperoxide – [CH(R)-O-O]n – for all detected oligomers, with the
repeated chain unit CH(R)OO corresponding to the respective major CI. The elemental compositions of parent ions, fragment ions and fragmented neutrals determined by accurate mass measurements with the FTICR technique allow us to assign a complete structure to the oligomer molecules. We suggest that the formation of the oligoperoxidic chain units occurs through a new gas-phase reaction mechanism observed for the first time in our present work, which involves the addition of stabilized CI to organic peroxy radicals.
Furthermore, copolymerization of CI simultaneously formed in the gas phase
from two different unsaturated compounds is shown to occur during the
ozonolysis of a mixture of \textit{trans}-3-hexene and ethyl vinyl ether (EVE), leading to
formation of oligomers with mixed chain units C3H6O2 (mass
74) and CH2O2 (mass 46). We therefore suggest oligoperoxide
formation by repeated peroxy radical-stabilized CI addition to be a general reaction pathway of small
stabilized CI in the gas phase, which represents an alternative way to
high-molecular products and thus contributes to SOA formation. |
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