Evergreen Needle Magnetization as a Proxy for Particulate Matter Pollution in Urban Environments

We test the use of magnetic measurements of evergreen needles as a proxy for particulate matter pollution in Salt Lake City, Utah. Measurements of saturation isothermal remanent magnetization indicate needle magnetization increases with increased air pollution. Needle magnetization shows a high degree of spatial variability with the largest increases in magnetization near roadways. Results from our magnetic measurements are corroborated by scanning electron microscopy of needle surfaces and by inductively coupled plasma mass spectrometry of metal concentrations in residues collected from sampled needles. Low-temperature magnetic analysis suggests the presence of small (<20 nm) partially oxidized magnetite particles on needles collected adjacent to a major roadway. Magnetization may be a low-cost proxy for certain metal concentrations (including lead) during periods of increased particulate pollution. The spatial resolution of our method appears capable of resolving changes in ambient particulate matter pollution on the scale of tens to hundreds of meters. Questions remain regarding the timescales over which evergreen needles retain particulate matter accumulated during atmospheric inversion events in Salt Lake City. Results presented here corroborate previous studies that found needle magnetization is a fast, cost-effective measure of particulate matter pollution. This method has the potential to provide high spatial resolution maps of biomagnetically monitored particulate matter in polluted urban environments year-round.