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
Authors: Sadezky A. , Winterhalter R. , Kanawati B. , Römpp A. , Spengler B. , Mellouki A. , Le Bras G. , Chaimbault P. , Moortgat G. K. 
Published: April 2010

Abstract. 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 (Sadezky et al., 2006) 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(+)/MS-TOF and ESI(+)/MS/MS-TOF, and elemental compositions were confirmed by ESI(+)/MS/MS-FTICR. The results for all investigated unsaturated compounds are in excellent agreement with the observations of our previous study (Sadezky et al., 2006). 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 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 (Sadezky et al., 2006), characterized by successive losses of the respective CI-like chain unit as a neutral fragment, indicate a similar principal structure. As in our previous work (Sadezky et al., 2006), we suggest 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. 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 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 to be a general, so far unknown 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.