Rapid Measurement of Magnetic Particle Concentrations in Wildland–Urban Interface Fire Ashes and Runoff Using Compact NMR

Wildfires are increasing in size, frequency, and intensity, releasing increased amounts of contaminants, including magnetic particles (MPs), into the surrounding environment. The aim of this article is to develop a sensing method for the detection and quantification of MPs in fire ash and fire runoff using a compact time-domain nuclear magnetic resonance (TD-NMR) system. The system is made up of custom nuclear magnetic resonance (NMR) electronics with a compact and rugged permanent magnet array designed to enable future deployment as an in situ sensor. A signal-to-noise ratio (SNR) of 25 dB was measured for a single scan, and sufficient data can be acquired in one minute. A linear relationship with an
${R}^{{2}}$
value of 0.9699 was established between transverse relaxation rates and MP concentrations in ash samples. This was validated by testing known dilutions of pure magnetite particles and showing that they fit within the same linear curve. The developed approach was then applied to detect MPs in surface water, where changes in the relaxation rates as high as 400% were observed before and after a wildfire event. MPs were removed from the surface water using a MP separator to confirm that observed changes were solely due to the presence of MPs. The compact NMR system can be used as a simple and rapid approach to track and quantify the concentrations of MPs released from fire ashes and also from other sources such as discharges from coal ash and other combustion ashes.