# Seasonal Characteristics of Particulate Matter by Pollution Source Type and Urban Forest Type
## 2 Materials and Methods  
### Study Sites  
The National Institute of Forest Science strategically established from 2019 to 2023 a total of 132 plots across 44 sites for AICAN in the Korean peninsula to investigate the role of forests in mitigating or obstructing PM pollution. To identify suitable sites, urban forests were categorized into four main construction backgrounds. The selection process also involved analyzing three years of meteorological data, conducting atmospheric modeling, and performing on-site surveys and evaluations.  
To address pollutant reduction at the source, this study is on urban forests and proximal pollution sources. The pollution sources were divided into industrial complexes (point-source pollutants) and road areas (non-point-source pollutants). Several AICAN plots were installed near these pollution sources. When selecting the research sites, two primary criteria were considered. First, the installation timeline of each region, and the time to finalize confirmation of the collected data, had to be accounted for based on site selection. Second, each site needed to provide a full year of data for seasonal comparisons, with missing values comprising no more than 5% of the total data. Within the AICAN pollutant groups, the road areas included Gomae, Yangjae, and Gwanak, while the industrial complexes covered Goyang, Sihwa, and Gijang (Table 1). Data were collected throughout 2021.  
At the Gomae site, plots were installed 50 m and 150 m away from the highway (source), with broadleaved forests occupying the area. The Yangjae site, located along the same highway, included plots at distances of 200 m and 300 m and featured mixed forests. Both sites are situated near the Gyeongbu Expressway, which had an average traffic volume of 192,000 vehicles per day in 2021 (Seoul Metropolitan Facilities Management Corporation, 2022). At the Gwanak site, measurement points were installed on the rooftop of the Seoul National University Graduate School of Public Health, SNUGPI observation station, with the traffic island near the university's main gate serving as the pollution source. This site features both coniferous and mixed forests and is located near the Gangnam Circle City Expressway, which had a daily traffic volume of 90,451 vehicles in 2021 (Seoul Metropolitan Government and Seoul Metropolitan Facilities Management Corporation, 2022). The Gijangsite is situated near the Jeonggwan General Industrial Complex (source), with measurement plots in both forested and residential areas. The forest plots contain mixed forests, while the residential area plot consists of broadleaved forests. The Goyang site, located near the IlSan Urban High-Tech Industrial Complex, includes source plots near LNG facilities. Forest plots in this area feature mixed forests, while the residential area contains broadleaved forests. Lastly, the Sihwa site, positioned near the Sihwa Industrial Complex, has plots in barrier forests and residential areas. The forest plot contains a mixed forest, while both the forest and residential plots include coniferous forests.