Impacts of rainfall on leaf PM (airborne particulate matter) detention in five commonly used urban tree species in northern China

It is widely acknowledged that urban trees and forests can significantly improve air quality by removing airborne particulate matter（PM）and thus safeguard the health of urban residents.However,it is very difficult to accurately assess the ability of tree species to capture PMs from the air as such processes are governed by several biotic and abiotic factors.Here,we present a first ever study that explores the effects of rainfall on the detention of PMs by leaves in five commonly used urban greening species in northern China,including Ginkgo biloba,Acer mono Maxim,Euonymus japonicus,Koelreuteria paniculata,and Eucommia ulmoides.Our specific objectives were to:1)quantify how much PMs detained on the leaves of five different tree species can be washed off by natural and simulated rainfall events,2)compare the difference in PM removal between tree species under different amounts of rainfall,and 3)analyze the factors that might contribute to the differences in PM removal under different rainfall conditions.Leaves were uniformly collected from top,middle,and bottom layers of the tree canopy at four cardinal directions from the sampling trees after several minutes of rainfall and12 days after rainfall.Each sampled tree leaf was sealed in a plastic bag immediately after collection.To measure the quantity of PMs trapped on leaves,the samples were washed and water used for washing was collected.Thereafter,10%of the water was dried to determine the weight of total suspended particulates（TSP）,while the remaining 90%of the water was passed through a 10μm mesh filter and then through a 2.5μm mesh filter.These filters were then dried to determine the weight of PM>10μm(PM>10)and PMs that were between 2.5 and 10μm(PM10-2.5).Then,20%of the filtered water was dried to determine the weight of PMs that were