Magnetic Characterization of α-Fe Oxidation Kinetics: Implications for Source Attribution in Urban Pollution.

Iron-bearing nano- and microparticles are ubiquitous in urban environments, often emitted by anthropogenic pollution sources. However, the occurrence of metallic iron (α-Fe) in non-reducing, near-surface environments is anomalous given the oxidizing nature of Earth’s atmosphere. Quantifying the magnetic changes associated with the oxidation kinetics of these particles allows for the determination of their atmospheric residence times. We conducted hysteresis measurements on synthetic iron powders of varying grain sizes (1-149 µm) at specific intervals following exposure to temperatures ranging from 200°C to 500°C. By plotting saturation magnetization (Ms), saturation remanent magnetization (Mrs), and coercive force (Hc) against oxidation time, we observed that oxidation is most likely diffusion-limited: it proceeds rapidly at the grain surface and decelerates as oxygen penetration becomes restricted by the oxide shell. Using M_S decay as a proxy for oxidation progress, we constructed an Arrhenius plot to determine activation energies. This allows for the extrapolation of reaction rates to room temperature. Consequently, we present a method to estimate the time elapsed since particle emission. When combined with meteorological data, we can backtrack trajectories to pinpoint specific anthropogenic sources of α-Fe emission.