Electron spin resonance of particulate soot samples from automobiles to help environmental studies

Photocatalytic Role of Atmospheric Soot Particles under Visible-Light Irradiation: Reactive Oxygen Species Generation, Self-Oxidation Process, and Induced Higher Oxidative Potential and Cytotoxicity

It is known that there are semiconductor oxides involved in mineral dust, which have photocatalytic properties. However, soot particles contained in carbonaceous aerosol and their photoactivity under sunlight are rarely realized. In this study, reactive oxygen species (ROS) such as superoxide anions and hydroxyl radicals were generated upon visible-light irradiation of soot particles, and the production activity was consistent with the carbonaceous core content, indicating that the atmospheric soot particles can serve as a potential photocatalyst. The increase of oxygen-containing functional groups, environmentally persistent free radicals, oxygenated polycyclic aromatic hydrocarbons, and the oxidative potential (OP) of soot after irradiation confirmed the occurrence of visible-light-triggered photocatalytic oxidation of the soot itself. The mechanism analyses suggested that the carbonaceous core caused the production of ROS, which subsequently oxidize the extractable organic species on the soot surface. It is oxidized organic extracts that are responsible for the enhancements of the OP, cell mortality, and intracellular ROS generation. These new findings shed light on both the photocatalytic role of the soot and the importance of ROS during the photochemical self-oxidation of soot triggered by visible light and will promote a more comprehensive understanding of both the atmospheric chemical behavior and health effects of soot particles.

Paramagnetic centers in particulate formed from the oxidative pyrolysis of 1-methylnaphthalene in the presence of Fe(III)2O3 nanoparticles

The identity of radical species associated with particulate formed from the oxidative pyrolysis of 1-methylnaphthalene (1-MN) was investigated using low temperature matrix isolation electron paramagnetic resonance spectroscopy (LTMI-EPR), a specialized technique that provided a method of sampling and analysis of the gas-phase paramagnetic components. A superimposed EPR signal was identified to be a mixture of organic radicals (carbon and oxygen-centered) and soot. The carbon-centered radicals were identified as a mixture of the resonance-stabilized indenyl, cyclopentadienyl, and naphthalene 1-methylene radicals through the theoretical simulation of the radical's hyperfine structure. Formation of these radical species was promoted by the addition of Fe(III)2O3 nanoparticles. Enhanced formation of resonance stabilized radicals from the addition of Fe(III)2O3 nanoparticles can account for the observed increased sooting tendency associated with Fe(III)2O3 nanoparticle addition.

Micro-FTIR study of soot chemical composition-evidence of aliphatic hydrocarbons on nascent soot surfaces

Previous studies suggest that soot formed in premixed flat flames can contain a substantial amount of aliphatic compounds. Presence of these compounds may affect the kinetics of soot mass growth and oxidation in a way that is currently not understood. Using an infrared spectrometer coupled to a microscope (micro-FTIR), we examined the composition of soot sampled from a set of ethylene-argon-oxygen flames recently characterized (A. D. Abid, et al. Combust. Flame, 2008, 154, 775-788), all with an equivalence ratio Φ=2.07 but varying in maximum flame temperatures. Soot was sampled at three distances above the burner surface using a probe sampling technique and deposited on silicon nitride thin film substrates using a cascade impactor. Spectra were taken and analyses performed for samples collected on the lowest five impactor stages with the cut-off sizes of D(50)=10, 18, 32, 56 and 100 nm. The micro-FTIR spectra revealed the presence of aliphatic C–H, aromatic C–H and various oxygenated functional groups, including carbonyl (C=O), C–O–C and C–OH groups. Spectral analyses were made to examine variations of these functional groups with flame temperature, sampling position and particle size. Results indicate that increases in flame temperature leads to higher contents of non-aromatic functionalities. Functional group concentrations were found to be ordered as follows: [C=O]<[C–O]<[aliphatic C–H]. Aliphatic C–H was found to exist in significant quantities, with very little oxygenated groups present. The ratio of these chemical functionalities to aromatic C–H remains constant for particle sizes spanning 10-100 nm. The results confirm a previous experimental finding: a significant amount of aliphatic compounds is present in nascent soot formed in the flames studied, especially towards larger distances above the burner surface.

Electron spin resonance. Part two: a diagnostic method in the environmental sciences

A review is presented of some of the ways in which electron spin resonance (ESR) spectroscopy may be useful to investigate systems of relevance to the environmental sciences. Specifically considered are: quantititave ESR, photocatalysis for pollution control; sorption and mobility of molecules in zeolites; free radicals produced by mechanical action and by shock waves from explosives; measurement of peroxyl radicals and nitrate radicals in air; determination of particulate matter polyaromatic hydrocarbons (PAH), soot and black carbon in air; estimation of nitrate and nitrite in vegetables and fruit; lipid-peroxidation by solid particles (silica, asbestos, coal dust); ESR of soils and other biogenic substances: formation of soil organic matter carbon capture and sequestration (CCS) and no-till farming; detection of reactive oxygen species in the photosynthetic apparatus of higher plants under light stress; molecular mobility and intracellular glasses in seeds and pollen; molecular mobility in dry cotton; characterisation of the surface of carbon black used for chromatography; ESR dating for archaeology and determining seawater levels; measurement of the quality of tea-leaves by ESR; green-catalysts and catalytic media; studies of petroleum (crude oil); fuels; methane hydrate; fuel cells; photovoltaics; source rocks; kerogen; carbonaceous chondrites to find an ESR-based marker for extraterrestrial origin; samples from the Moon taken on the Apollo 11 and Apollo 12 missions to understand space-weathering; ESR studies of organic matter in regard to oil and gas formation in the North Sea; solvation by ionic liquids as green solvents, ESR in food and nutraceutical research.