Characterization of oligomers in secondary organic aerosol using advanced mass spectrometry techniques

Biogenic secondary organic aerosol (SOA) forms from the reaction of gas phase organic molecules from biological sources with an atmospheric oxidant. Although biogenic SOA can comprise up to 80% of the particulate mass suspended in the atmosphere, the reactions that form SOA and the chemical identities of the compounds it contains are poorly understood, especially the oligomeric species that form the non-volatile core of SOA. In this dissertation, mass spectrometric techniques are used to characterize the oligomers found in SOA throughout their lifetime. Fresh aerosol was generated in a Tedlar bag and flow tube reactor (FTR) to determine: (1) the relevance of laboratory-generated oligomers to the atmosphere, and (2) the formation routes, and identity of the oligomers. Fresh SOA generated in the FTR was then reacted in a chamber designed to simulate photooxidation to (3) study the aging of SOA oligomers and determine if they are a source of highly oxidized atmospheric SOA.;A scanning mobility particle sizer (SMPS) was found to accurately measure the concentration and size distribution of SOA. The SOA was then collected onto Teflon coated, glass fiber filters. Filter phase reactions were found to be minimal or non-existent. Various extraction solvents were tested, and acetonitrile was found to have high extraction efficiency without causing side reactions with the sample. Through the method of standard additions, the concentration of oligomeric species in the non-volatile core of the SOA collected and extracted was determined to be ∼50% for laboratory SOA. SOA generated in the FTR was shown to have similar behavior as a class of organic aerosol found in the atmosphere. High resolution mass spectra revealed that oligomers undergo thermal degradation to volatile compounds when heated to high temperatures, so thermodenuders cannot be used to determine SOA volatility.;High resolution tandem mass spectrometry (MSMS) was used to determine which compounds react to form oligomers and what their routes of formation are. By examining the product ions formed by dissociating oligomeric precursor ions, the monomers that are most likely to react were determined. Additionally, by searching precursor ions for the expected products of reported oligomerization reactions and examining their fragmentation spectra, several reported reactions were confirmed. These include the reactions of hydroperoxides, carbonyls and stabilized Criegee intermediates.;Finally, an aerosol reaction chamber was constructed to test the theory that the oligomers found in SOA are sources of the highly oxidized class of organic aerosol found in the atmosphere after undergoing photo-oxidative aging. Freshly formed SOA was exposed to high levels of hydroxyl radical and then analyzed both online by the nanoaerosol mass (NAMS) spectrometer and off line by high resolution mass spectrometry. The average O:C and H:C ratios of the aged compounds were in the range reported for highly oxidized atmospheric SOA. Additionally, the extent of evaporation caused by the fragmentation of oligomers into smaller volatile species was not found to be significant enough to be a sink of atmospheric SOA, although the time scale of the experiment may not have been sufficient for evaporation to occur.