Species-specific dynamics in magnetic PM accumulation and immobilization for six deciduous and evergreen broadleaves

Numerous studies have demonstrated the effectiveness of plants in accumulating airborne particulate matter (PM) on their leaf surfaces. However, it is not fully clear whether leaf traits or atmospheric conditions influence the accumulation and immobilization dynamics of magnetic PM on leaf surfaces. In this study, leaves of two deciduous broadleaf trees (Quercus petraea, Quercus robur), two deciduous broadleaf shrubs (Prunus padus, Sambucus nigra) and two evergreen broadleaf shrubs (Prunus laurocerasus, Rhododendron sp.) were sampled from a common garden every 48 to 72-h to determine the dynamics of magnetic PM accumulation and immobilization. The mass of water-insoluble removable PM was estimated using gravimetric analysis. The ferro-magnetic and magnetizable component of leaf surface accumulated PM, leaf immobilized PM and water-insoluble removable PM in three size fractions (PM > 10, 3-10, and 0.2-3 mu m) was determined using Saturated Isothermal Remanent Magnetization (SIRM). Leaf SIRM for both surface accumulated PM and leaf immobilized PM differed in the following order Q. robur < Q. petraea < S. nigra < P. laurocerasus < P. padus < Rhododendron sp. indicating that PM immobilization on leaves of plant species is a function of net accumulated PM. In proportion to the SIRM signal of leaf surface accumulated PM, on average 4 % was recovered in the SIRM of the water-insoluble removable PM and 63 % was found in the SIRM signal of the immobilized PM while 33 % of the SIRM signal of the leaf surface accumulated PM could not be recovered in the immobilzed or water-insoluble removable PM. The leaf surface accumulated SIRM related with leaf wettability. The mass and SIRM of water-insoluble removable PM were significantly affected by leaf traits and meteorological conditions i.e., (i) negatively by leaf wettability, precipitation and wind speed and (ii) positively by relative humidity and ambient PM2.5 concentrations. These results indicate that magnetic PM accumulation is influenced by both the atmospheric conditions as well as by the micro-morphological leaf traits of plant species. Leaves of Rhododendron sp. followed by Q. robur showed the highest median net deposition velocities for both coarse and fine-particles. Based on the results of this study we recommend S. nigra a deciduous broadleaf shrub species, as a preferred choice to mitigate PM pollution in urban environments. As it accumulated the highest proportion of ferro-magnetic particles in the mass of water-insoluble removable PM considering the harmful effects of these particles on human health. In addition, S. nigra is known to have low biogenic volatile organic compound (BVOC) emissions and provides a good provision for birds and insects.