## 1 Introduction  
Heavy metal pollution has received increasing attention in recent years because it can threaten human health to different degrees [1, 2]. Heavy metals can penetrate the tissues of plants and animals as well as humans, and their ability to penetrate is determined by the size and shape of the particles [3]. In recent years, many studies have been conducted on the sources and contents of heavy metals in soil, the atmosphere, and water [4, 5, 6]. The main sources of heavy metals in the atmosphere are automobile exhaust emissions, fossil fuel combustion, and heavy industrial emissions [7, 8]. A method for directly detecting the degree of atmospheric heavy metal pollution is to detect heavy metals in different particle matters (PM). However, the use of this method to indicate environmental pollution has not been comprehensively analyzed. This is because the level of heavy metals in the environment may be high, but only a small proportion belongs to the bioavailable fraction. Therefore, many studies have used plants as monitors to reveal the relationship between atmospheric heavy metal pollution and living organisms [9]. Plants can be used not only to monitor the level of heavy metal pollution in the atmosphere, but also to reduce the heavy metal content in the environment via root uptake and leaf retention, thus reducing heavy metal pollution.  
Woodly plants have huge root, stem, and leaf areas that are able to act on the environment [10]. Their leaves are less hydrophobic than herbs [11], and rainfall washing has less of an effect on their surface heavy metal content. Therefore, the leaves of woody plants are a good choice for the study of the retention of heavy metals in the atmosphere. The leaves of evergreen plants rarely fall off, and therefore evergreen plants can be used as indicator plants to monitor heavy metals in the air [12]. Coniferous trees are more capable of absorbing heavy metals than deciduous trees. Among common landsc spacing species, Chinese juniper (_Juniperus chinensis_ L.), dragon juniper (_Juniperus chinensis_ L. '[PERSON]'), and cedar (_Cedrus deodara_ (Roxb. ex D.Don) [PERSON]) show a strong heavy metal uptake capacity [13].  
The difference in elemental content between washed and unwashed leaves was studied by [PERSON] et al. [14]. They used the fraction washed off with water to represent the heavy metals deposited by the atmosphere on the surface of the leaves, and many subsequent studies have used this method. However, it has been suggested that care should be taken when using the rinsing method to explore atmospheric contamination and that washing leaves with water is not sufficient [15]. Studies have found that the effect of water washing depends on the chemical nature and material characteristics of heavy metals [16]. In urban fine particulate matter, Mn, Cu, and Cd are more often present in their organic bound states [17], and water washing does not perform well for removal of this fraction of metal content. Several studies have investigated the effect of different detergents, such as HCI and Na\({}_{2}\)EDTA, on the elution of heavy metals [15]. However, these detergents may make the experimental operation more complicated. Ethanol, a common organic solvent, is widely used in various fields, given its low price, low toxicity, and low pollution. In this study, we selected Chinese juniper, dragon juniper, and cedar on the main roads in a central city. We used ethanol detergent to elute heavy metals from needles and compared the elution effects of it and water, from which we tried to explain the following: (a) the interaction between several heavy metal elements, (b) the degree of atmospheric pollution, (c) the effect of alternative washing methods on the needles.