PROJECT SUMMARY (Project 3: Jon Chorover, Robert Root, Mónica Ramírez-Andreotta) Mine tailings wastes are continuously generated and deposited at the land-surface where they serve as point sources for airborne particulate matter (PM). As mandated by the Clean Air Act, the U.S. Environmental Protection Agency (EPA) has established regulatory standards for PM of <2.5 ìm (PM2.5) and <10 ìm (PM10) that indicate the maximum PM concentration to be present in outdoor air irrespective of PM composition. The health risks associated with PM from mine tailings (mt-PM) are greater than for most other sources because they contain elevated concentrations of toxic metals and metalloids, such as arsenic, lead, cadmium, and zinc. These toxic metal(loid)s occur as surface-adsorbed molecular species or as atoms co-precipitated into a wide range of crystalline and amorphous structures. The local bonding environment of a metal(loid) in a given particle – also known as its molecular speciation – is defined by its oxidation state, coordination number, and nearest neighbor atom composition. These nanoscale characteristics change over the course of mt-PM diagenesis because of weathering and local biogeochemical conditions. Furthermore, as tailings undergo weathering, toxic metal(loid)s are enriched in smaller, neoformed particles that are more susceptible to aerosolization than the originally-deposited mt-PM. The mt-PM also contain redox active species, such as iron, that can generate reactive oxygen species in vivo. We hypothesize that weathering-driven alterations in molecular speciation of metal(loid)s in mt-PM controls their bioaccessibility, bioavailability, and toxic effects during inhalation exposure, which is the primary exposure pathway being studied in the UA DUST Center. We further postulate that mt-PM transport into adjacent residential soils and into ecosystems subjected to wildfire leads to increased human health risk for proximal communities. These communiti