Rainfall characteristics significantly affect the scavenging of water-soluble ions attached to leaves

Precipitation is considered the most effective way to remove particulate matter from the leaves of plants. Changes in rainfall characteristics can affect the scavenging processes of particulate matter from leaves. In order to better understand the dynamics of PM scavenging during rainfall, especially the water-soluble ions components, leaves from the 11 plant species (trees, shrubs, terrestrial herbs, wetland plants) from the Olympic park were sampled and used in indoor experiments. During the experiments, the rainfall intensity was set at 30 mm/h, 45 mm/h, and 60 mm/h, and the duration was divided into 0-20 min, 20-40 min, and 40-60 min. The sampled plant leaves were set in the experiments at 1 m and 3 m height from the ground. Concentrations and compositions of nine water-soluble ions of rainfall samples were analyzed in this experiment. The results revealed that SO₄^2-, Ca²⁺, and Na⁺ were the most abundant ionic species removed from the leaves, and NO₃^- ranked fourth, followed by Cl^-, Mg²⁺ K⁺, NH₄⁺, and F^-. The ions concentration of rainfall samples decreased when the rain intensity increased from 30 to 45 mm/h and when the rain intensity increased to 60 mm/h. The efficiency of scavenging during different rainfall durations depends on the ionic species. Na⁺, Mg²⁺, Ca²⁺, and SO₄^2- concentrations increased with the increase in rainfall duration, whereas those of NH₄⁺, K⁺, and Cl^- decreased. The effect of leaf height on ions concentration of rainfall samples was also different among the ionic species: Na⁺, Mg²⁺, Ca²⁺, NO₃^-, and F^- concentrations were significantly higher at 1 m compared with 3 m. The principal component analysis of ions in rainfall samples revealed two main sources of particulate matter in our study. One is from vehicle exhaust and industrial and agricultural pollution. The other is agricultural combustion and ground dust sources. The results of the above study can provide a basis and theoretical support for the establishment of urban cleaning systems and the prevention of air pollution.