Evergreen needle magnetization as a proxy for particulate matter pollution in Salt Lake City, UT: spatial and temporal trends

Abstract:

Winter-time temperature inversion in Salt Lake City, Utah causes extreme degradation in air quality and represents a major health risk for urban populations in the region. We test the use of magnetic measurement of evergreen needles as a proxy for particulate matter pollution during these events. Measurements of saturation isothermal remanent magnetization as well as low temperature magnetic analysis 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. 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. Results are consistent with previous studies that found needle magnetization is a fast, cost-effective measure of particulate matter pollution which can provide high spatial resolution maps of particulate matter in polluted urban environments year-round. Initial results from a more recent time series analysis of needle magnetism indicates a mixed response to specific pollution events through time and underlines potential complexities in needles as pollution proxies during these events. A second environmental magnetic data set comprised of needles collected every two weeks over a four month period indicates that needles undergo rapid changes in their magnetic properties. Independent measurements of particulate matter, however, collected by in situ continuously-monitoring particle-size analyzers installed at the sampling sites, suggest that these changes do not always reflect short term changes in air quality. Additional environmental conditions, such as precipitation, may influence needle retention of particulate matter over time and modify magnetic parameters used to track air quality.