Source apportionment of water-soluble and water-insoluble metal content of coarse and fine PM in central Los Angeles

In this study, principal component analysis (PCA) was utilized to identify the sources of water-soluble and water-insoluble metal content of coarse and fine PM in the air. The sampling campaign was conducted in winter (December 2021-February 2022) at the University of Southern California Particle Instrumentation Unit (PIU) in central Los Angeles. Ambient PM_2.5 and coarse PM samples were collected on Teflon filters and chemically analyzed to quantify their water-soluble and total metal content. The Multiple Linear Regression (MLR) model was employed to investigate the contribution of each found factor by PCA to the total metal. The most abundant elements in the fine size range were Na, S, Fe, Al, and Ca, while in the coarse range, they were Na, Fe, Ca, Al, and Mg. Ca, Na, and Zn exhibited the highest solubility levels in both size ranges, while Ti, Fe, and Al showed the lowest solubility. Most metals had higher solubility in the fine size fraction. The PCA results indicated that the tire and brake wear source factor contributed significantly to the water-soluble fraction of metals in both size ranges, whereas the soil and re-suspended road dust source factor had the most contribution to the water-insoluble fraction of metals. The primary source of fine water-soluble metal was metal-dominated abrasion due to its higher water-soluble content, typically corresponding to a greater impact of anthropogenic than crustal sources. MLR results confirmed that tire and brake wear are major sources of metal in the water-soluble fractions of both size ranges, while soil and re-suspended road dust are major sources in the water-insoluble portion.