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

Abstract

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.

Impacts of SO2, Relative Humidity, and Seed Acidity on Secondary Organic Aerosol Formation in the Ozonolysis of Butyl Vinyl Ether

“…These differences were mainly caused by different reaction mechanisms. In the absence of SO 2 , the overall O/C observed in the current study is in agreement with the O/C ratio of oligoperoxides characterized by Sadezky et al 19,20 The slope of zero was attributed to the addition of alcohol or peroxide functional groups to the carbon backbone. This further reinforces previously reported results showing that oligoperoxides formed through the oligomerization pathway of sCI account for the majority of BVE-derived SOA in the absence of SO 2 .…”

“…This further reinforces previously reported results showing that oligoperoxides formed through the oligomerization pathway of sCI account for the majority of BVE-derived SOA in the absence of SO 2 . 19,20 For the case with SO 2 , the evolution of SOA composition was generally consistent with the classical path that O/C increases when H/C decreases. 47 As the data resulted in a Van Krevelen slope steeper than −2 in Figure 2, this suggests the substantial addition of ketone or aldehyde.…”

“…14 In addition, Sadezky et al further proved that oligoperoxides produced by repeated peroxy radicalstabilized Criegee intermediate (sCI) addition are crucial SOA components in the absence of SO 2 . 19,20 More recent studies have also reported that anthroponotic emissions have a significant impact on SOA formation. 21−24 For example, the homogeneous reaction between SO 2 and sCI is widely considered to be an important source of both atmospheric sulfate and SOA.…”

Competition between HO2 and H2O2 Reactions with CH2OO/anti-CH3CHOO in the Oligomer Formation: A Theoretical Perspective

“…61 Compared to the barriers of the 4CH 2 OO + HO 2 and 4CH 2 OO + H 2 O 2 reactions, it can be found that the first two CH 2 OO addition reactions in the former case are favored obviously, while the last two CH 2 OO addition reactions in the latter case are preferable. This conclusion is not fully consistent with previous results obtained by Moortgat and co-workers, 24 in which the oligomer formation follows the sequential addition of CIs to RO 2 radicals.…”

“…Recently, experimental studies have proposed that oligomers containing SCI as the chain units are the dominant components of SOA. 24,25 However, the formation mechanisms of oligomer ROO(CH 2 OO) n H are not consistent. Sadezky et al 24 proposed that the mechanism contains sequential addition of CH 2 OO to RO 2 radical leading to the ROO(CH 2 OO) n radical adduct, and then the oligomer ROO(CH 2 OO) n H is formed by abstraction of (3)…”