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 CH 2 O 2 (mass 46) for alkyl vinyl ethers (AVE) and C 2 H 4 O 2 (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 Correspondence to: G. K. Moortgat (moo@mpch-mainz.mpg.de) 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 C 3 H 6 O 2 (mass 74) for ethyl butenyl ether (EBE), trans-3-hexene, and 2,3-dimethyl-2-butene, and C 4 H 8 O 2 (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, 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 oz...

Role of the reaction of stabilized Criegee intermediates with peroxy radicals in particle formation and growth in air

“…Accurate mass determinations (Table 2) on selected OLI-A oligomers show very good agreement with their expected elemental compositions. The ESI-MS results presented here corroborate the oligomerization mechanism proposed by Sadezky and co-workers, 43 however our data show an increased contribution from longer oligomers, i.e., n 4 5, compared to the data reported by Sadezky et al 43 In the presence of an OH scavenger, the reaction between the scavenger and OH radicals contributes to the formation of a different RO 2 radical, which is also expected to initiate oligomerization, thus resulting in the formation of a different oligomer series. As shown in Fig.…”

“…These studies show that particle formation in the trans-3-hexene ozonolysis is consistent with the sequential addition of SCI to RO 2 radicals as previously proposed by Sadezky et al 43 Size-dependent oligomeric and elemental composition of SOA particles show that longer oligomers play a key role in initial particle formation and that subsequent growth occurs by condensation of shorter oligomers and other less oxygenated, more volatile products. In addition, impaction/ATR-FTIR measurements show that trans-3-hexene SOA is less viscous than a-pinene SOA.…”

“…40 Recently, theoretical 45,46 and experimental [41][42][43][44] studies have suggested the gas phase formation of low-volatility oligomers containing SCI as the repeat unit during the ozonolysis of small enol ethers and alkenes (including isoprene) in dry air. Two different mechanisms have been proposed: sequential addition of SCI to peroxy radicals (RO 2 ) (reaction (R1)), 43 or to hydroperoxides (ROOH) (reaction (R2)). 44 Sadezky et al 43 investigated the formation of oligomers from the ozonolysis of a number of simple alkenes including trans-3-hexene in a static chamber, and proposed the structures and mechanisms shown in reaction (R1).…”

“…Two different mechanisms have been proposed: sequential addition of SCI to peroxy radicals (RO 2 ) (reaction (R1)), 43 or to hydroperoxides (ROOH) (reaction (R2)). 44 Sadezky et al 43 investigated the formation of oligomers from the ozonolysis of a number of simple alkenes including trans-3-hexene in a static chamber, and proposed the structures and mechanisms shown in reaction (R1). We report here an expanded investigation of the ozonolysis of trans-3-hexene in which particle composition is probed as a function of size.…”

Reactions between Criegee Intermediates and the Inorganic Acids HCl and HNO3: Kinetics and Atmospheric Implications

“…[1,2] They are thought to have a significant influence on the oxidative capacity of the atmosphere and, through reaction with trace species, to contribute to new particle formation. [3][4][5][6][7][8][9] Unimolecular dissociation of vibrationally hot nascent CIs is a major source of night-time OH in the troposphere. [3,10] Stabilized CIs react quickly (~10 -12 -10 -10 cm 3 s -1 ) with many trace atmospheric gases.…”