Seasonal variation in foliar particulate matter deposition and associated metals on urban tree leaves

Particulate matter (PM) deposited on leaves reflects the quantity and chemical composition of atmospheric pollutants. The study investigated the seasonal variation in PM accumulated on the leaves of different roadside tree species (Chionanthus retusus, Ligustrum obtusifolium, Zelkova serrata, Acer palmatum, and Pinus densiflora) and the associated metal concentrations in different sizes of PM. The amount of PM accumulated on leaf surface was the lowest in summer (August) and increased in autumn (October), which was related to atmospheric PM concentration. The PM accumulation on leaves was influenced by rainfall and the duration of leaf exposure to the atmosphere. Pinus densiflora showed the highest PM accumulation across all seasons, with little seasonal variation in total amount of PM. Nickel, Zn, Cu, and Pb concentrations in PM_>10 ranged from 23 to 215, 495 to 2694, 99 to 462, and 29 to 115 mg/kg, respectively, while those in PM_2.5-10 ranged from 14 to 95, 210 to 817, 65 to 214, and 5.3 to 54 mg/kg. Metal concentrations in leaves varied, with Zn (26-929 mg/kg) being the most abundant metal, followed by Cu (6.1-100 mg/kg), Ni (0.95-58 mg/kg), and Pb (0.86-20 mg/kg). Metal concentrations in leaves were the lowest in spring and increased in summer and autumn. Adjacent soil samples beneath the tree canopy showed significant differences in metal concentrations despite their spatial proximity. The highest metal concentrations in soil were detected beneath Pinus densiflora, which may be influenced by inputs from PM accumulated on leaves via litterfall or wash-off. Strong positive correlations were found between metal concentrations in PM, soil and leaves, indicating potential bidirectional interactions and PM-associated leaf metal uptake.