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Abstract:
The abundance of airborne iron-bearing particles in anthropogenic settings makes an ideal use case for applying environmental magnetism techniques. The sensitivity of magnetic techniques to mineral composition, oxidation state, morphology, and grain size in the range 10–104 nm means that, in principle, it is possible to unmix and distinguish different sources of air pollution. The magnetic properties of particles change fundamentally across this size range and, given the known health risks associated with exposure to iron oxide particulate matter (PM), magnetic methods offer a potentially powerful way to detect and monitor the presence of iron-oxide nanoparticles that may be poorly characterised by traditional methods. Here we show the utility of a range of magnetic techniques—both experimental and modelling. High-resolution room-temperature (RT) and low-temperature (LT) first-order reversal curves (FORCs) are sensitive to grain size, mineralogy, and domain state. We also show the application of thermal fluctuation tomography (TFT) to characterise the grain size distribution in the superparamagnetic (SP) and single-domain (SD) range and the use micromagnetic simulations on PM electron tomography data to ground truth experimental results. We believe complementing environmental magnetism techniques with microscopy (TEM, SEM, and tomography) can give us a greater insight into the characteristics of iron oxide PM. We illustrate these methods using recent case studies from various urban microenvironments — traffic related pollution in Lahore, Pakistan, abrasion related pollution in the London Underground, and the potential use of green infrastructure to reduce child exposure to particulate matter pollution in a school playground in Manchester.
