Size-dependent aerosol iron solubility in an urban atmosphere

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Size distribution of aerosol iron (Fe) concentration and solubility determines their environmental and health impacts but they are rarely reported. In this study, size-resolved aerosol particles were collected in Hangzhou, a Chinese megacity upwind of the Asian outflow to the Pacific Ocean. Total and dissolved Fe collected under non-haze, haze, and dust conditions were determined. Results show that total Fe concentrations displayed a unimodal distribution peaking at 3.2–5.6 μm under non-haze and dust conditions, but a bimodal distribution under the haze condition peaking at 3.2–5.6 μm and 0.32–0.56 μm. Dissolved Fe under the non-haze, haze, and dust conditions all displayed bimodal size distributions with one peak in the fine-mode (0.056–1.0 μm) fraction and one peak in the coarse-mode (>1.0 μm) fraction. Fe enrichment factor in fine-mode particles under the haze condition is up to 17.8, suggesting a potential contribution from anthropogenic Fe. Fe solubility in total suspended particles under the haze condition (3.8%) is significantly higher than those under non-haze (2.6%) and particularly dust (0.5%) conditions. Furthermore, Fe solubilities in fine-mode particles under non-haze (10.8%) and haze (10.9%) conditions were much higher than those (1.5% and 2.4%) in coarse-mode particles. We found that Fe solubility was correlated positively with aerosol acidity and negatively with particle size from 0.32 to 5.6 μm. Individual particle analysis further reveals that nanosized iron oxides were internally mixed with acidic sulfates/nitrates under the haze condition. Our results indicate that the strong size dependence of aerosol Fe solubility is associated with the differences in sources of aerosol Fe and its acidic processing.