# Seasonal Characteristics of Particulate Matter by Pollution Source Type and Urban Forest Type
## 1 Introduction
Meanwhile, the reduction in PM by plants can be classified into two processes: external (adsorption and accumulation on leaves) and internal (absorption and storage in plant tissues). The external process is influenced by various factors, including leaf microstructure (trichomes, leaf structure, stomatal density, etc. ), macrostructure (total leaf area, leaf arrangement, leaf shape, etc. ), canopy size, tree age, and physiological factors (such as transpiration and boundary layer conductance). In addition, physicochemical properties (such as wax layer composition) and interactions between leaves and the surrounding environment play a critical role in PM reduction. PM accumulated on leaf surfaces can be washed off by rain or resuspended by wind. It can also settle and disperse under the influence of environmental factors, such as humidity and temperature. These processes are further categorized into dry deposition, which includes the settling, diffusion, and resuspension of PM on leaves, trees, and forest surfaces, and wet deposition, where PM accumulated on leaves is washed off by precipitation [11; 12; 13; 14; 15; 16; 17; 18; 19]. The internal process refers to the uptake of PM through stomata or cuticles. PM can be absorbed through stomatal openings, cell membranes, and roots, where it accumulates on the surface or inside the roots [20; 21]. Researchers are actively developing methods to utilize forests in reducing PM concentrations. These studies can be categorized into micro-, mezzo-, and macro-research approaches based on the scope, duration, and complexity of this study. Micro-research focuses on the mechanisms of PM absorption and removal through the biological properties of trees [22; 23; 24], such as the differences in accumulation among various vegetative organs and identifying species that efficiently absorb PM. Mezzo-research expands the focus to include the trees, their surrounding environments, and human interactions [25; 26; 27; 28; 29; 30], examining the interplay of ecological factors such as forest structure, meteorological and spatial factors, and anthropogenic activities. Macro-research takes a broader perspective, encompassing longer study periods and wider areas, including the previously mentioned subjects [31; 32; 33; 34]. These studies explore PM concentration differences among observation stations, estimating PM removal and accumulation by urban forests and converting these estimates into annual values. Furthermore, they analyze the positive impacts of these forests on air quality. This study analyzed the concentration of particulate matter in urban forests where both non-point source pollution (e.g., roads) and point source pollution (e.g., industrial complexes) are present using long-term monitoring data from the AICAN (Asian Initiative for Clean Air Networks, Korea) system operated by the National Institute of Forest Science in Korea. This research aims to identify the common characteristics of particulate matter (PM) concentrations in urban forests located near different pollution sources, examine the differences in PM concentrations between pollution sources and forest plots, and explore the significance of these differences to better understand the role of urban forests in reducing PM.