Regulating Atmospheric Particulates By Urban Trees: Rainfall And Wind Effects

Particulate matter(PM)is defined as various solid or liquid particles suspended in an atmospheric aerosol system,and it makes a significant contribution to air pollution.PM has the potential to exert negative impacts on human health.Urban trees are widely used as biological filters to combat the PM.Rainfall and wind are the key factors affecting the accumulation of atmospheric particulate matter by urban trees.Understanding rainfall and wind effect is necessary to evaluate the potential long-term air purification ability and efficiency of urban trees and to carry out greening design of urban blocks.In this paper,taking urban typical tree species as the research object,rainfall effect on particulate matter of the leaf surface was quantified by means of artificial simulation of rainfall.The results were as follows:(1)significant differences(P < 0.05)were found in PM accumulation amount and in leaf coarseness;there was a significant correlation between PM retention and leaf coarseness of different tree species(P < 0.05);(2)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.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 and this indicates that the traditional water washing method underestimates the PM retention capacity on trees;(3)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).PM removal rates were significantly related with rainfall patterns(P < 0.01);In this paper,taking urban street trees and ideal street canyon as research objects,numerical simulation method of the computational fluid dynamics(CFD)was used to quantify the influence of wind on the regulation of atmospheric particles by trees in street canyon.The results were as follows:(1)the transport and distribution patterns of traffic pollutants were different under different wind directions and different wind speeds;trees effect on the fine particles concentration depends on different tree configurations;the total effect of all scenarios ranged from-88.3% to 25.5%;(2)from the view of tree species,the total effect of cypress was superior to that of the other two tree species;from the point of view of only tree configuration,all unilateral planting was superior to bilateral planting;the study found that the optimal tree configuration strategy of each wind direction,the parallel wind and the oblique wind were suitable for bilateral shrub planting as well as the vertical wind is suitable for tree-shrub combination of bilateral planting with increasing the tree volume of street center;(3)tree parameters have little influence on tree effect,and wind speed may be an important parameter affecting PM diffusion under parallel wind;tree effect on leeward wall depends on the aerodynamic parameter(P<0.05),while the windward wall and the pedestrian level depend on the tree deposition parameter under the vertical wind direction(P<0.05);tree effect of leeward wall,pedestrian level and the total all decreased significantly with the tree aerodynamic parameter under the oblique wind direction(P<0.05).