## 4 Discussion  
The heavy metal content in the unwashed leaves (i.e., the total amount of heavy metals in the leaves) had several sources; it derived partly from the transport of heavy metals from the soil to the leaves by the root system through transpiration [23], and partly from atmospheric particulate matter deposition. PM containing heavy metals can be captured by leaf epidermal waves and diffused into the leaves via lipophilic or hydrophilic channels [24], or it may enter the leaves directly through the stomata and become integrated into components within the leaves. The relative contents of Cu, Mn, and Zn were higher than those of Cd in all three plant species, partly because Cu, Mn, and Zn are essential elements for plants and are transferred more effectively from roots to leaves [25; 26; 27], and partly because the atmosphere also showed this trend in content [21]. The higher _MAI_ values of the dragon juniper leaves compared to the leaves of the other two plants may have been due to this plant's complex crown [28] and leaf structure [13]. These factors allow it to retain more dust particles, thus leading to a greater heavy metal accumulation effect, which has a positive effect on the selection of greening species.  
Combined with the eluted heavy metal content and correlation analysis, the leaf surface had high Cu, Zn, and Cd contents. The Cu, Zn, and Cd in PM\({}_{2.5}\) in Tianjin are mainly derived from anthropogenic sources [29]. Most of the Cd settles on the leaf surface in a residue state [30], and less enters the leaf interior. The Cd and Cu on the blade surfaces may have come from traffic sources, such as motor vehicle exhaust and mechanical wear caused by tires and engines; the Zn and Mn may have come from steel smelting and the alloy industry [31]. Since the sampling sites were located on the main roads in the urban center, this provides some guidance for the further management of pollution caused by  
Figure 4: A heavy metal correlation analysis heat map of the unwashed leaves for \(C_{w}\) and \(C_{e}\). “U” presents the content of the unwashed needles. ***, **, and * represent significant differences at 0.001, 0.01, and 0.05, respectively.  
traffic. Studies have shown that there is a significant correlation between urban PM\({}_{10}\), PM\({}_{2.5}\), and mortality [32; 33]. As for Tianjin, the urban population density is high and pollution is a serious issue [34]. Selecting appropriate plants, diversifying forest structure, and providing reasonable green space management can effectively improve the efficiency of atmospheric cleaning [35], which will provide assistance in reducing urban pollution.  
For copper, cadmium, and zinc, cleaning, especially ethanol cleaning, showed a good removal effect. In comparison, the two washing methods were not so effective for manganese, probably because not much of it was retained on the surface of the blade itself, as the Mn mainly existed in the interior of the blade. The observed variation in the trend of the heavy metal content of the leaves suggests that water washing has a limited effect on the removal of heavy metal particles from leaf surfaces and is influenced by the plant species and the particular heavy metal element, which may mask real differences between the heavy metal contents of the leaves of different species. Water-insoluble substances in atmospheric particles include polycyclic aromatic hydrocarbons, aromatic hydrocarbons, olefins, paraffins, and phthalates [36], and some of these compounds may bind some heavy metals. Ethanol may have a better elution effect on some organically bound heavy metals, which is a possibility that needs to be further explored. The tree species and heavy metal elements studied should be varied in subsequent research to further our knowledge of the treatment effect of ethanol washing.