The primary objective of the research is to understand the effect of photochemical aging on the polycyclic aromatic hydrocarbon (PAH) composition of Asian particulate matter (PM) during trans-Pacific atmospheric transport to the U.S. West Coast. The principal investigators hypothesize that nitro-PAHs (NPAHs) and oxy-PAHs (OPAHs) are formed on Asian PM during trans-Pacific atmospheric transport. The specific goal of this research is to use aerosol reactors to study the photochemical aging of size-fractionated Asian PM in the laboratory. While incomplete combustion results in the formation of unsubstituted PAHs, both nitro-PAHs and oxy-PAHs result from both direct emission during incomplete combustion and reactions of unsubstituted PAHs with photochemical reactants (ozone, hydroxyl radical and nitrogen oxides) during the daytime and reactions with nitrate radicals at night. It is likely that PAHs react with these oxidants during trans-Pacific transport to form NPAH and OPAH. Existing aerosol samples from both Asia and Oregon, containing Asian PM, will be used for the laboratory studies of PAH aging. Chemical oxidation of PAHs on aerosol samples will be carried out and subsequently analyzed in collaboration with colleagues at the University of Bordeaux.
The activity includes training a graduate student and continued inclusion of a faculty member from Central Oregon Community College (COCC) and her undergraduate students in the research. COCC undergraduate students will learn to conduct statistical manipulations of a large research data set, as well as understand the process of identifying and quantifying PAHs using mass spectrometry. Finally, the results of the research could have global environmental policy implications with regard to combustion emission controls in Asia.
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 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 question as to whether PM-bound PAHs undergo atmospheric transformation to 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 activity in relation to the chemical composition of the extract. The sampling of atmospheric particulate matter with diameter < 2.5 (PM2.5) µm before, during and after the 2008 Olympic Games in Beijing provided insights into the concentrations, chemical composition, photochemistry, and mutagenicity of PAH, NPAH, and oxy-PAH (OPAH) near the emission source. 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 semi-volatile PAHs through the heterogeneous reactions of surface-bound PAHs with NO2, NO3/N2O5, O3, and OH radicals. Five semi-volatile 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 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 OH radical-initiated reaction. This study has shown that NO2 and NO3/N2O5 were effective oxidizing agents in transforming PAHs deposited on filters toNPAHs, 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. This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
