|Abstract or Summary
- The trans-Pacific atmospheric transport of particulate matter (PM)-bound polycyclic aromatic hydrocarbons (PAHs) to remote sites in western North America has been well documented and has triggered research questions regarding to atmospheric transformation of PM-bound PAHs and the potential impacts on human health from their inhalation exposure. In this dissertation, field measurements, theoretical studies, laboratory experiments, and mutagenicity studies were used to begin the address the questions as to whether PM-bound PAHs undergo atmospheric transformation into mutagenic nitro-PAHs (NPAHs) during trans-Pacific atmospheric transport. PM extracts were tested in the Salmonella mutagenicity assay, using Salmonella typhimurium strain TA98 (with and without metabolic activation), to determine the mutagenic activities in relation to the chemical composition of the extracts. The sampling of atmospheric PM with diameter < 2.5 μm (PM₂.₅) before, during, and after the Olympic Games 2008 in Beijing provided some insights into the concentrations, chemical composition, photochemistry, and mutagenicity at the source of emission. The PAH, NPAH and OPAH composition of the PM₂.₅ was similar throughout the sampling periods, which included the period when a wide range of combustion sources were controlled. In addition, it showed that PAHs were associated with both local and regional emissions, while the NPAH and OPAH concentrations were only correlated with the NO concentrations, indicating that the NPAH and OPAH were primarily associated with local emissions. The characteristic NPAH ratios suggested a predominance of photochemical formation of NPAHs through OH radical-initiated reactions in the atmosphere. Subsequently, the heterogeneous reactions of PAHs bound to Beijing ambient PM with various oxidants, including NO₃/N₂O₅, OH radical and O₃, were studied using an environmental reaction chamber under simulated trans-Pacific transport conditions. In addition, PM collected from Riverside, CA was simultaneously exposed along with the Beijing PM in order to allow us to compare the reactivity between two different sites. In general, O₃ was most effective in degrading PM-bound PAHs with more than five rings, except for benzo[a]pyrene which was degraded by O₃ and NO₃/N₂O₅ equally well. However, the NPAHs were most effectively formed during the NO₃/N₂O₅ exposure. The reactivity of the PM could be explained by the degree to which the PM had been photochemically aged because the accumulation of degradation products on the surface of PM appeared to inhibit further atmospheric degradation of parent PAHs. For the NO₃/N₂O₅ exposure, the increase in direct-acting mutagenicity was associated with the formation of mutagenic NPAHs. Additional laboratory experiments were carried out in order to identify NPAH products of 5- to 6-ring PAHs through the heterogeneous reactions of surface-bound PAHs with NO₂, NO₃/N₂O₅, O₃, and OH radicals. Five PAHs, benzo[a]pyrene-d₁₂, benzo[k]fluoranthene-d₁₂, benzo[g,h,i]perylene-d₁₂, dibenzo(a,i)pyrene-d₁₄, and dibenzo[a,l]pyrene, were spiked onto quartz fiber filters and exposed in the chamber. Some of the identified NPAH products have not yet been measured in the environment. In parallel to the laboratory experiments, a theoretical study was conducted to assist in predicting the formation of NPAH isomers based on the gas-phase OH radical-initiated reaction. This study has shown that NO₂ and NO₃/N₂O₅ were effective oxidizing agents in transforming PAHs deposited on filters to NPAHs, under these experimental conditions. The lighter of the PAHs studied, including benzo[a]pyrene-d₁₂, benzo[k]fluoranthene-d₁₂ and benzo[ghi]perylene-d₁₂, yielded more than one mono-nitro isomer product, whereas dibenzo[a,l]pyrene and dibenzo[a,i]pyrene-d₁₄ resulted in the formation of only one mono-nitro isomer product. The direct-acting mutagenicity increased the most after NO₃/N₂O₅ exposure, particularly for benzo[k]fluoranthene-d₁₂ in which dinitro PAHs were observed.