The Origin and Nature of Persistent Combustion-Generated Radicals

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PROJECT SUMMARYThe goal of this work is to determine from what fuels and under what conditions persistent radicals are formed in combustion systems and ultimately incorporated into airborne fine particulate matter (PM2.5). Specific objectives include:1. To determine experimentally how known semiquinone radical precursors react under post-flame combustion conditions and under which conditions they form radicals. This is done using a high-temperature flow reactor/gas chromatograph/mass spectrometer system to study the thermal reactions of hydroquinone and catechol under oxidative and pyrolytic conditions. Effluent is trapped using cryogenic techniques for analyses by electron paramagnetic resonance (EPR). 2. To determine experimentally the combustion conditions under which various common fuels generate stable free radicals. Laboratory reactors are used to study the thermal degradation of various fuels and to trap free radicals for characterization. Candidate fuels include: propane, gasoline, diesel fuel, biodiesel fuel, No. 6 fuel oil, coal, wood, and a chlorinated hydrocarbon mixture.3. To identify which precursors and particle surfaces form and stabilize persistent free radicals. The chemisorbed radical hypothesis is investigated by dosing surrogate particle samples with semiquinone-type radical precursors using a packed-bed reactor system. The particles investigated include activated carbon and silica or alumina doped with iron, copper, magnesium, calcium or zinc. Radical precursors studied include hydroquinone, catechol, phenols, benzenes, and chlorinated analogues..4. To determine if carbonaceous materials form persistent free radicals. The intrinsic radical hypothesis is investigated by generating and modifying soots or chars formed by thermal degradation of fuels. The particles will be characterized for radicals before and after partial oxidation with various oxidants (O2, NO2, OH, O, and Cl). The radical concentrations and lifetimes are determined by EPR.5. To elucidate the structure, stability, and reactivity of semiquinone-type radicals using ab-initio calculation techniques. Density function theory (DFT) procedures are used to calculate the structures, energies, and spin densities of radicals derived from hydroquinone and catechol as well as related radicals identified in the experimental tasks. 6. To demonstrate whether similar persistent radicals are associated with full-scale combustion-derived and ambient PM2.5 samples. A limited number of full-scale combustion and airborne PM2.5 samples are characterized to quantify associated radicals and the metals (iron, copper, magnesium, calcium, zinc) that may have a role in stabilizing them. Broader Impacts The concept of persistent, combustion-generated free radicals is an area that needs exploratory research to establish the nature, origin, and reactivity of these radicals under a variety of conditions. This project is designed to provide this information so that the research community can further explore the impact of persistent radicals under specific conditions and for specific applications. It also supports an ongoing collaboration with Drs. William A. Pryor and Guiseppe Squadrito of the Biodynamics Institute at LSU and other health-effects researchers at LSU andTulane Medical School. The results of this study form a basis for detailed studies of the biological chemistry and health impacts of persistent free radicals.