EPR Measurements and Redox Chemistry of Fine Particulate Matter

Redox chemistry influences many processes including atmospheric aging as well as adverse health effects of ambient particulate matter (PM). By atmospheric processing and emissions, PM composition undergoes constant change, where condensed-phase and gas-phase molecules interact in multiphase processes including redox- and radical reactions. Upon deposition of PM in the human airways, redox-active constituents may induce oxidative stress in the lung. The term oxidative stress describes a condition in which the formation of reactive species (RS) outcompetes the regulation capacity of natural antioxidants, potentially resulting in oxidation of biomolecules, inflammation, and even cell death or tissue damage. Herein RS include radicals like •OH and O2-• as well as non-radical species like H2O2 and NO2-. Due to the complexity of online measurements in airborne particles and the short lifetimes of many radicals, there is a lack of understanding about the atmospheric relevance and reaction rates of many of these redox reactions involving PM. Therefore, in this PhD project electron paramagnetic resonance (EPR) spectroscopy was used to explore some of these processes in PM. This technique is particularly suitable to study radical reactions as it enables the retrieval of qualitative as well as quantitative information about radicals. The concrete research studies in this thesis focused on the following aspects: (1) Characterization of the impact synthetic cerium oxide nanoparticles (CeNPs) have on OH radical concentrations in a surrogate lung fluid (SLF). Therein, the potential of these particles to produce •OH – a particularly harmful RS - in suspensions was tested against their antioxidant activity using spin-trapping and EPR spectroscopy. According to the investigations, CeNPs exhibit predominantly antioxidant properties in SLF, which mimics physiologically relevant conditions. Furthermore, the particle size has a pronounced impact on this activity, with smaller particles showing a stronger effect. This size-dependent antioxidant activity is attributed to the higher concentration of active surface CeO2 species, which influence •OH concentrations in solution by heterogeneous redox reactions. (2) Elucidation of the role of highly oxygenated molecules (HOMs) play in the RS formation of PM upon suspension in aqueous solution. The concentration of HOMs in ambient PM and laboratory-generated secondary organic aerosol (SOA) were quantified using mass spectrometry and radical formation in aqueous solution was studied by EPR spin-trapping. For ambient PM a positive correlation of HOMs abundance with the radical formation in pure water was observed. Among laboratory-generated particles biogenic precursors formed SOA with a higher radical yield than SOA from anthropogenic precursors. The results indicate that HOMs could play a significant role in the production of RS by PM. (3) Identification of key chemical molecules that determine the production of reactive species by ambient particulate matter from different environments. While there is some understanding that aerosols from clean and polluted environments differ in their potential to generate RS in solution, convincing links to specific PM constituents are missing. Radical and H2O2 yields from dissolution of PM confirm differences in the RS production potential by filter samples from a remote forest and two urban sites. Further results indicate that the specific proportions of different RS concentrations measured for the three sites can be reasonably mimicked using only transition metals, organic hydroperoxide, H2O2, humic-, and fulvic acids as PM surrogate mixtures. This suggests that these very abundant constituents could be of central importance for the RS yields of PM. (4) Investigation of the occurrence of environmentally persistent free radicals (EPFR) in remote and urban environments and their distribution within PM of different size between 10 nm and 10 µm. EPFR are molecular structures that were identified as constituents of ambient PM and also of other environmental samples like soil and microplastic. The understanding of this class of pollutants is still limited due to a shortage of data about their concentration and spatial distribution. The results demonstrate, that EPFR occur not only in populated areas but also in remote regions with limited anthropogenic influence. Among the filter samples from different environments the abundance of EPFR is highly correlated with PM mass concentration. Moreover, in urban air samples, the size-resolved EPFR distribution reveals a pronounced maximum in the sub-micrometer size range, while remote regions have a more variable EPFR size distribution. Based on these insights, the deposition of EPFR in human airways was modeled, whereby the results show that most of the radicals potentially deposit deep in the lung, because they are concentrated in particulate matter smaller than 1 µm. (5) Determination how prevalent EPFR are indoor air, house dust, and on indoor surfaces, as well as how EPFR interact with gas-phase oxidants. In a coordinated study in Mainz, indoor and outdoor samples were collected simultaneously. Concentrations of EPFR in indoor air were found to be linked with concentrations in outdoor air. The results further demonstrate that the number of EPFR in surface films and house dust exceeds the total amount of EPFR in PM substantially for typical indoor settings. Exposure experiments of those samples indicate that there may be interactions between EPFR and typical gas-phase oxidants (e.g. O3 and NO2). Therewith EPFR could be involved in redox reactions and represent a reactivity reservoir in PM as well as other condensed phases indoors