DUST VARIATION ALONG AN URBAN GRADIENT IN PHILADELPHIA

Although atmospheric dust is a less well-known input to watersheds, it can deposit in streams and on the land surface to impact aqueous chemistry and soil formation. Dust collection across an urban gradient was carried out to determine the grain size distribution, mass flux, mineralogy, and geochemistry of atmospheric dust. Individual dust collectors were placed in the urban center of Philadelphia, in the outer urban ring, and in the suburbs. Data collection from each site occurred August 2021, December 2021, March 2022, and June 2022. Samples were centrifuged and condensed into a 50 mL tube for each site before being run through a particle size analysis machine. An additional 1 mL of sample was plated onto glycolated and unglycolated slides for X-ray diffraction. The remaining contents were evaporated and weighed to determine the dust mass for each site and then sent to a lab for trace metal analysis. Findings revealed that the median dust size for all three sites ranged from 18 μm to 30 μm. The suburban site had the largest mean particle compared to the downtown and urban ring sites. A sampler placed on the ground in the urban ring had the largest daily dust flux and dust flux variation, so the site was moved to a rooftop to match the other locations. The downtown site had the least variation between collections in daily dust flux. XRD analysis revealed that all samples contained similar mineralogy profiles, therefore indicating uniformity in dust structure and composition. Trace metal analysis revealed that the downtown site contained the highest levels of common urban contaminants (e.g., Pb, Cu, and Sn). Although iron was also higher in the downtown location, for these elements the concentrations observed were higher even when normalized to the iron concentration.