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Safe and optimized utilization of waste-to-energy (WtE) fly ash (FA) requires a detailed understanding of the physicochemical properties of its metal constituents. This study provides a comprehensive analysis of the chemical form of Zn in fine (<1 μm) and coarse (>1 μm) FA particles, hypothesized to originate from different formation mechanisms. Size-selective aerosol sampling was performed during standard operation in the flue gas channel at a WtE facility. Additionally, FA samples from the air pollution control filters at the facility and boiler deposits were analyzed. Speciation was determined primarily using synchrotron-based X-ray absorption spectroscopy, complemented by XRD, SEM-EDS, and total elemental analysis. Significant differences in terms of elemental composition, crystalline phases, and Zn chemical forms were observed between fine- and coarse FA particles. Fine particles were dominated by Cl, K, and Na with Zn almost exclusively present as potassium zinc chlorides. Coarse particles were heterogeneous, with Zn occurring in stable forms such as aluminate, ferrite, and silicates (e.g., gehlenite). The major elemental constituents were Ca, Si, and Al. Although coarse particles constitute the major mass of the FA, about 50% of the Zn was found in the fine fraction. These findings support strategies for efficient secondary use and recycling of FA, such as targeted Zn extraction from fine particles and potential utilization of the Ca-rich coarse particles in construction, reducing the reliance on virgin materials.