"Assessment of oxidative potential of ambient water-soluble and insoluble of PM2.5

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Gao, Dong
Weber, Rodney J.
Mulholland, James A.
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Oxidative stress has been proposed as a major mechanism responsible for adverse health effects associated with particulate matter (PM) pollution. Various methods have been developed to measure PM oxidative potential (OP), the potential for particles to generate reactive oxygen species and elicit oxidative stress. But no consensus has been reached as to the best OP assay. Both water-soluble and insoluble PM components contribute to PM OP, but the water-insoluble OP fraction has been less studied. This dissertation aims to characterize water-soluble PM OP measured by different OP assays and water-insoluble OP in terms of temporal variability and chemical determinants. This dissertation provides a direct inter-comparison between two health-relevant acellular OP assays, the synthetic respiratory tract lining fluid (RTLF) assay and the dithiothreitol (DTT) assay. These assays were used to measure the water-soluble OP of ambient fine PM collected in urban Atlanta over a year-long period. The results showed that these assays were driven by different groups of aerosol species, ranging from organic species to transition metal ions. The OP responses in the RTLF assay were affected by the composition of synthetic lung fluid, which emphasizes the importance of developing a “standard” technique for OP assays. Multivariate regression models for these OP metrics capture interactions among species, expanding our understanding of the relationships among species in the OP assessment. To develop a method for quantifying total PM OP, we compared three commonly used extraction methods for total OP assessment, involving methanol extraction (1) with or (2) without filtering the extracts, followed by solvent removal and reconstitution with water, and (3) water extraction without removing the particle-laden filter. The results indicated that performing the OP assay directly on the water extracts that still contained the particle-laden filter was a more effective way to capture water-insoluble OP compared to organic solvent extraction. An automated system was developed based on the DTT assay to facilitate the total OP analysis. The water-soluble and total OP of ambient particles collected at two urban sites and one roadside site were analyzed, with water-insoluble OP determined by difference. The results clearly demonstrated a measurable OP contribution from water-insoluble PM, which accounted for 20–35 % of total OP. The spatial and temporal variations in OP measures suggested that the insoluble OP contributors were largely secondary and related to biomass burning emissions. Multivariate regression analyses indicated that water-insoluble OP was related to incomplete combustion products and surface properties of soot and water-insoluble metals. Overall, assessing water-insoluble or total PM OP may provide important information in elucidating the health risks related to PM exposure and ultimately in promulgating effective control strategies to protect public health.
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