|Hall Iv, Wiley|
|Pennington, M - University Of Delaware|
|Johnston, Murray - University Of Delaware|
Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/7/2012
Publication Date: 3/5/2013
Citation: Hall Iv, W.A., Pennington, M.R., Johnston, M.V. 2013. Molecular transformations accompanying the aging of laboratory secondary organic aerosol. Environmental Science and Technology. 47(5):2230-2237.
Interpretive Summary: Secondary organic aerosol (SOA) is produced when volatile organic compounds (VOC) in the atmosphere are oxidized to make particulate matter. SOA is important atmospherically because it can cause adverse health effects, and affect global climate change directly, through scattering and absorbing radiation, and indirectly through cloud formation. The reactions that form SOA, as well as the low volatility compounds that comprise it, are largely unknown; particularly the highly oxidized portion of SOA that is thought to be the result of atmospheric aging. In this work, laboratory-generated SOA is reacted with hydroxyl radical to simulate atmospheric aging. Aerosol mass spectrometry and high performance mass spectrometry is used to study what types of reactions may be taking place, and elucidate the structural changes in SOA molecules that occur during aging.
Technical Abstract: The aging of fresh secondary organic aerosol, generated by alpha-pinene ozonolysis in a flow tube reactor, was studied by passing it through a second reaction chamber where hydroxyl radicals were generated. Two types of experiments were performed: plug injection experiments where the particle mass and average O/C mole ratio were measured as a function of time, and continuous injection experiments where aerosol exiting the second chamber was collected on a filter for off line analysis by high performance mass spectrometry. Plug injection indicated the presence of both fragmentation and functionalization reactions: the particle mass decreased over time while the O/C ratio increased. Continuous injection allowed the molecular composition of fresh and aged SOA to be compared. Aged SOA showed an oligomer distribution shifted to lower molecular weight (fragmentation) and the molecular formulas generally exhibited higher O/C ratios and lower H/C ratios (functionalization). Carbon oxidation states for fresh SOA were mostly in the -1 to 0 range, while those for aged oligomers were mostly in the 0 to +2 range. Tandem mass spectrometry experiments showed that fresh oligomers tended to give small neutral losses associated with less oxidized functional groups such as aldehydes and ketones, while aged oligomers tended to give neutral losses associated more highly oxidized groups such as acids and peroxyacids. Product ion spectra of fresh SOA showed monomer building blocks with molecular formulas corresponding to primary ozonolysis products such as terpenylic, norpinonic, pinic and pinonic acids, while aged SOA showed monomer building blocks with molecular formulas corresponding to extremely oxidized products such as dimethyltricarballyic acid.