Assessing Air Quality Through Tree Bark Biomonitoring of Praseodymium in Leicestershire, UK

Praseodymium (Pr) has been detected in the air of the city of Leicester (England). To further biomonitor air quality for Pr, thin layers of bark were collected from 96 trees in Leicester (n=55) and surrounding rural/suburban areas (41). Pr was monitored by ICP-MS [LoD=0.157 ng/g dry weight (dw)]. Levels were slightly higher in the samples collected from trees growing in urban areas (median and ranges, in ng/g dw): 2.611 (0.714-47.603) and 2.450 (0.757-14.839). Pr content varied between bark samples collected across the city (medians, in ng/g dw): 11.374 (SE) > 4.183 (SW) > 2.471 (NE) > 1.967 (NW), and between the three quadrants into which the rural areas were divided: 7.348 (NW) > 2.244 (NE) > 0.881 (SE). Finding a hypothesis that could explain the differences found is challenging as the atmospheric transport of this metal and other rare earth elements (REEs) is poorly understood. However, our results could suggest that the airborne Pr content is little influenced by its presence in the topsoils, as the patterns found were different for the two main areas: NE>SW>SE>NW and SE>NE>SW>NW. Levels were much higher than the range reported in bark samples (1.85-2.69 ng/g dw) collected in an area of eastern Washington (US) with little anthropic pollution, suggesting that the air quality of Leicestershire would have some anthropic input. However, the enrichment factors (0.436 and 0.487), calculated in relation to the average Pr concentration in the Earth’s continental crust and the scandium content, were very low in both areas, suggesting no discernible enrichment. These values were also lower than those observed for other REEs monitored in the same bark samples, reinforcing the minimal anthropogenic input of Pr into the air of Leicester city and surrounding areas, as REEs are known to behave as a coherent group of elements in plants.