Association of organic species with seasonal and spatial variability in redox activity of ultrafine particles in the Los Angeles Basin

Abstract

Recent toxicological studies have linked oxidative potential of ultrafine particles (dp< 0.1-0.2 µm) to adverse health effects, ranging from respiratory problems to cardiovascular diseases and lately to neurological diseases. In order to investigate the redox activity of this important size range and in particular its correlation with inorganic and organic ultrafine PM species, a year-long sampling campaign of quasi-ultrafine particles (dp< 0.25 µm) was conducted at 9 distinctive sites representing source, near-freeway and rural receptor locations across the Los Angeles air basin. Redox activity of the ultrafine PM samples was measured using the Dithiothreitol (DTT) assay and detailed chemical analysis was performed to measure the concentrations of inorganic and organic species. Redox activity per unit volume (m3) of air (an indicator of oxidative potential associated with exposure to PM) showed distinct seasonal and spatial variations, ranging between 0.2 and 1.4 (nmol DTT/min/m3 air). Peaks generally occurred during the fall and comparable values were observed during winter and summer. Noticeable peaks were also observed at near-freeway locations representing the source sites for vehicular and industrial emissions. Regression analysis indicated strong correlation of DTT with organic carbon (OC) across all seasons and strong winter-time correlations with tracers of primary vehicular emissions including polycyclic aromatic hydrocarbons (PAHs), alkanes, hopanes and steranes. During the summer, correlations with semi-volatile compounds, such as PAHs were significantly weaker. Water soluble organic carbon (WSOC) and also tracers of biogenic secondary organic aerosol (SOA), however, displayed higher correlations with DTT during the summer time compared to winter suggesting that the contribution of organic species from both primary and secondary sources is important to the overall oxidative potential of ultrafine PM.