Formation of secondary organic aerosol and oligomers from the ozonolysis of enol ethers

by a. Sadezky, P. Chaimbault, a. Mellouki, a. Römpp, R. Winterhalter, G. Le Bras, G. K. Moortgat
Atmospheric Chemistry and Physics ()
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Formation of secondary org. aerosol has been obsd. in the gas phase ozonolysis of a series of enol ethers, among them several alkyl vinyl ethers (AVE, ROCH = CH2), such as Et, Pr, Bu, iso-Bu, t-Bu vinyl ether, and Et propenyl ether (EPE, C2H5OCH = CHCH3). The ozonolysis has been studied in a 570-L spherical glass reactor at ambient pressure (730 Torr) and room temp. (296 K). Gas phase reaction products were studied by in-situ FTIR spectroscopy, and secondary org. aerosol (SOA) formation was monitored by a scanning mobility particle sizer (SMPS). The chem. compn. of the formed SOA was analyzed by a hybrid mass spectrometer using electrospray ionization (ESI). The main stable gas phase reaction product is the resp. alkyl formate ROC(O)H, formed with yields of 60 to 80%, implying that similar yields of the corresponding excited Criegee Intermediates (CI) CH2O2 for the AVE and CH3CHO2 for EPE are generated. Measured SOA yields are 2-4% for all enol ethers. SOA formation is strongly reduced or suppressed by the presence of an excess of formic acid, which acts as an efficient CI scavenger. Chem. anal. of the formed SOA by ESI(+)/MS-TOF allows to identify oligomeric compds. in the mass range 200-800 u as its major constituents. Repetitive chain units are identified as CH2O2 (mass 46) for the AVE and C2H4O2 (mass 60) for EPE and thus have the same chem. compns. as the resp. major Criegee Intermediates formed during ozonolysis of these ethers. The oligomeric structure and chain unit identity are confirmed by HPLC/ESI(+)/MS-TOF and ESI(+)/MS/MS-TOF expts., whereby successive and systematic loss of a fragment with mass 46 for the AVE (and mass 60 for EPE) is obsd. It is proposed that the oligomer has the following basic structure of an oligoperoxide, -[CH(R)-O-O]n-, where R = H for the AVE and R = CH3 for the EPE. Oligoperoxide formation is thus suggested to be another, so far unknown reaction of stabilized CI in the gas phase ozonolysis of O-contg. alkenes leading to SOA formation. [on SciFinder (R)]

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