Chemistry PhD Defense

Role of Multiphase Chemistry on the Formation of Aerosol from the Reactions of Monoterpenes with NO3 Radicals and O3 by Megan Claflin, ANYL Student, CU Boulder, Ziemann lab
"Secondary organic aerosols (SOA) have been shown to influence regional and global air quality, climate, and human health. To predict and mitigate the impacts of SOA formation, it is essential to better understand the detailed mechanisms of formation, atmospheric processing, and physical properties of SOA. In this thesis, the gas-and particle-phase reaction products and mechanisms involved in the formation of SOA from the oxidation of monoterpenes in an environmental chamber were studied in an extensive series of laboratory experiments that employed a variety of online and offline analytical methods. In addition, a limited selection of laboratory-generated samples and ambient aerosol samples collected from the Southeast US were analyzed to compare the utility two different methods of functional group analysis.
The research included four major studies. 1) Identification and quantification of the products that form SOA from the reaction of β-pinene with NO3 radicals. The results highlight the importance of ring-opening, alkoxy radical decomposition, and oligomerization reactions, and were used to develop gas- and particle-phase reaction mechanisms. 2) A detailed study of the thermal desorption characteristics of hemiacetal and acetal oligomers for use in understanding mass spectra obtained from the analysis of SOA containing these components. Oligomers formed by a single hemiacetal linkage, when subjected to thermal desorption analysis, will decompose to their precursor aldehyde and alcohol monomers prior to desorption and detection. Oligomers formed by more than one of these linkages, or an acetal or ester linkage, will desorb and be detected intact. 3) Quantification of the functional group composition of SOA formed from α- pinene ozonolysis over a range of α-pinene concentrations and humidities, including autoxidation conditions. The SOA analyses, when combined with results of modeling, provide insight into the effects of RO2• radical reaction regime, humidity, and particle-phase reactions in determining SOA composition. 4) Comparison of two methods for quantifying the functional group composition of organic aerosol: the Fourier Transform Infrared spectroscopy method developed by the Russell group at the Scripps Institute of Oceanography and a derivatization- spectrophotometric method developed by the Ziemann group at the University of Colorado, Boulder. Results for laboratory-generated SOA and ambient aerosol samples show that the two methods agree quite well when results for certain functional groups are combined, and that either is adequate for measuring SOA functional group composition. This study also demonstrates that the functional group composition of SOA can help in elucidating the sources and environmental conditions under which the SOA was formed."