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Atmospheric iron solubility varies depending on whether the particles are collected in rural or urban areas, with urban areas showing increased iron solubility. In this study, we investigate if the iron species present in different environments affects its ultimate solubility. Field data are presented from the Platte River Air Pollution and Photochemistry Experiment (PRAPPE), aimed at understanding the interactions between organic carbon and trace elements in atmospheric particulate matter (PM). 24-hr PM2.5 samples were collected during the summer and winter (2016–2017), at three different sites on the Eastern Colorado plains: an urban, agricultural, and a mixed site. Downtown Denver had an average total and water-soluble iron air concentration of 181.2 and 7.7 ng m−3, respectively. Platteville, the mixed site, had an average of total iron of 76.1 ng m−3, with average water-soluble iron concentration of 9.1 ng m−3. Jackson State Park (rural/agricultural) had the lowest total iron average of 31.5 ng m−3 and the lowest water-soluble iron average, 1.3 ng m−3. The iron oxidation state and chemical speciation of 97 samples across all sites and seasons was probed by X-ray absorption near edge structure (XANES) spectroscopy. The most common iron phases observed were almandine (Fe₃Al₂Si₃O₁₂) (Denver 21%, Platteville 16%, Jackson 24%), magnetite (Fe3O4) (Denver 9%, Platteville 4%, Jackson 5%) and Fe (III)dextran (Denver 5%, Platteville 13%, Jackson 5%), a surrogate for Fe-organic complexes. Additionally, native iron [Fe(0)] was found in significant amounts at all sites. No correlation was observed between iron solubility and iron oxidation state or chemical speciation.

Background: Wildfires can significantly impact water quality and supply. However logistical difficulties and high variability in in situ data collection have limited previous analyses. Aims: We simulated wildfire and rainfall effects at varying terrain slopes in a controlled setting to isolate driver-response relationships. Methods: Custom-designed laboratory-scale burn and rainfall simulators were applied to 154 soil samples, measuring subsequent runoff and constituent responses. Simulated conditions included low, moderate, and high burn intensities (~100–600°C); 10-, 200-, and 1000-year storm events (~14–51 mm/h); and 10–29° terrain slopes. Key results: Simulators can control key drivers, with burn intensities highly correlated (R 2 = 0.64) with heat treatment durations. Increasing burn intensity treatments generally saw significant (α = 0.05) increases in responses, with runoff and sedimentation increasing by ~30–70% with each intensity increment. Carbon and nitrogen peaked at moderate intensities (~250°C), however, with concentrations ~200–250% of unburned samples. Conclusions: Distinct responses at each burn intensity indicate nuanced changes in soil physical and chemical composition with increased heating, exacerbating driving mechanisms of runoff and sedimentation while reducing carbon and nitrogen through volatilisation. Implications: This work furthers our understanding of interactions between complex geographic features and the mosaic of burn intensities which exist in wildfire-affected landscapes. Laboratory-scale experiments simulated varying burn intensities, revealing nuanced changes in soil physical properties and chemical composition with increased heating levels. Burn effects on water quality and supply were observed independently and in combination with rainfall intensity and terrain slope. [ABSTRACT FROM AUTHOR]