A comparative study of foliar particulate matter wash-off from plants under natural and simulated rain conditions.

Urban greening can reduce concentrations of particulate matter (PM). However, most of the PM is deposited temporarily on foliage and can be removed by precipitation. Due to the unpredictability and randomness of natural rain and the complexity of environmental conditions, the effect of rainfall on PM removal from foliage is usually examined using simulated rain. The aim of this study was to compare the effects of natural and simulated rain on washing off PM deposited on the foliage of seventeen plant species (evergreen/deciduous trees and shrubs, herbaceous plants). Regardless of the PM fraction studied, simulated rain washed off a larger fraction of the PM than natural rain. A novel finding is the differential role played by various plant characteristics in determining the effectiveness of PM wash off under natural and simulated rain conditions, with their influence being more pronounced under natural rainfall. The type of rainfall and PM size fraction influenced the effectiveness of the PM wash-off from different plant groups. Natural rain was more effective at removing PM from evergreen trees, while simulated rainfall was more effective at removing PM from deciduous shrubs and herbaceous plants. The results obtained from this study suggest that simulated rain does not reflect the effects of natural precipitation on the processes removing PM from leaves, and therefore should not be used to explain phenomena that occur in realistic conditions.