Quantity, size distribution, and sources of leaf-level particulate matter from a major steel plant in SW Ohio: Implications for the spatial footprint of an emitter

Despite continued actions to abate harmful air pollutant emissions, air pollution is still a worldwide concern, yet apportioning individual shares of responsibility for pollution is challenging. Here, we present a spatial approach combined with microscopy, elemental composition, and Pb isotopes to trace particulate matter (PM) emissions related to a steel manufacturing plant in Middletown, Ohio. Evergreen leaves were collected in nine sites situated 18 and 32 km upwind and 0–35 km downwind from the steel plant. The relative abundance and size range of spherical Fe-rich particles, as indicators of the steel factory's emissions, were quantified using SEM/EDS. Elemental compositions and Pb isotopes were used for PM source apportionment. The SEM/EDS quantification method was effective for steel particles, while it was less suitable for quantifying fly ash abundances owing to its limitations in detecting ultrafine PM, where fly ash particles are prevalent. Pb isotopes indicated that the average leaf-level PM mass originating from glacial till, steel plant, gasoline, and fly ash, were 44 ± 23, 34 ± 30, 33 ± 17, and 18 ± 11 mg m⁻², respectively, highlighting the steel plant and gasoline as the primary anthropogenic PM sources. Strong correlations between steel spherule mass estimated by MixSIAR and its relative proportion quantified through microscopic investigations (r = 0.94) and pollution load index (r = 0.89) provide support for source apportionment using isotopic methods. The steel spherules quantity decreased exponentially with distance with the steel plant's effective PM footprint extending approximately 32 and 40 km upwind and downwind, respectively, emphasizing its ongoing environmental impact despite pollution control measures.