An investigation on the leaf accumulation-removal efficiency of atmospheric particulate matter for five urban plant species under different rainfall regimes
Urban trees and forests are widely used as biological filters to combat the airborne particulate matter (PM). Precipitation washing PM off from plants is regarded as filter cleaning, which is a key factor for recovering the function of foliar PM filtering. However, it is uncertain on how much PM can be total filtered by urban trees due to lack of understanding about how PM deposition, removal, resuspension and redeposition interact with species and rainfall variability. For this reason, we developed a study to determine foliar PM removal amount and rate of different sizes for five plant species commonly used for urban greening by simulated different rainfall regimes. Our specific objectives were to: (1) explore the difference in PM removal between different plant species and different rainfall patterns; (2) understand the response of foliar PM removal as a function of rainfall characteristics; and (3) quantify the relationship between foliar PM removal rate and leaf coarseness. Results showed that significant differences (P < 0.05) in PM removal amount and rate were found not only between different species within the same rainfall pattern, but also between different rainfall patterns for the same species. PM removal rates from the leaf surface were significantly correlated with rainfall intensity (P < 0.01). Different size PM cumulative removal rate exhibited an exponential loss with rainfall duration (P < 0.01). For smooth leaf surfaces, long duration-low intensity rainfall could increase PM removal rate while for rough leaf surfaces, short duration-high intensity rainfall could achieve a larger removal rate using the same amount of total rainfall. Additionally, more PM was removed by rainfall than that by water washing. The findings from this study have implications for better estimating long-term air purification potential of urban plants, and for air phytoremediation planning in urban areas.