Use of Trees as Biomonitors to Detect Spatial Patterns in Metal Air Pollution

Abstract

In the US, combustion of fossil fuels, industrial production, and other anthropogenic activities contribute to increasing levels of atmospheric heavy metal pollution, which in high quantities can negatively affect human health. Although tree plantings are often recommended as a remedy for poor air quality, in-situ studies have been limited to quantifying the deposition of air pollutants onto the foliage of individual trees. No studies have investigated heavy metal deposition across multiple canopies within a city. Because air pollution can vary substantially over small (<100m) distances, it is likely that different neighborhoods may experience pollution differently. To understand the use of trees as biomonitors, this study collected tree leaf samples before and after rain periods, as well as throughfall samples to detect heavy metal pollution across seventeen forest patches in Baltimore City. Leaves were rinsed, and the rinsate was filtered using gravimetric methods. The rinsate was then analyzed for metal concentration using mass spectrometry. Values from the pre-rain and post-rain samples were compared to investigate pollutant rinse-off, which appeared to only occur for chromium under heavier rain (<0.36mm) conditions, whereas iron and nickel appeared to be wet deposited during these rain events. Chromium tended to be more abundant in dry deposition, whereas nickel was more abundant in wet deposition. Iron appeared to be common in both. Comparisons of metal concentrations across the three sample types showed that iron tended to be most abundant overall, though nickel concentrations increased at the end of summer. Chromium concentrations decreased through the summer. Average metal concentrations sometimes showed significant differences among sites, with some sites showing very high concentrations compared to others. However, the relationship between these values and known sources of air pollution are still unclear.