## 1 Introduction  
Efficient management of particulate matter (PM) is required to mitigate high PM concentrations and increased high PM warnings in Korea [1, 2]. PM is an air pollutant containing nitrogen oxides, sulfur dioxide, lead, and ozone, and is mainly generated from fossil fuel combustion and the operation of factories and vehicles [3, 4]. High PM concentrations can reduce visibility in the environment, exacerbate the atmospheric environment, and cause various respiratory diseases in humans [5, 6, 7]. Thus, the government has strengthened regulations on large PM emission sources since 2017 by establishing a comprehensive PM management plan [8]. However, since this regulation is implemented locally, such as in industrial complexes, thermoelectric power plants, and diesel vehicles, other measures are needed to directly reduce human exposure to PM in normal neighborhoods.  
Urban greenery is highly utilized in urban air quality management because of its high accessibility to citizens and its effects on reducing PM [9, 10, 11]. The PM reduction mechanism of trees consists of the absorption of air pollutants through stomata [12, 13, 14], adsorption of PM by tree structures such as leaves and branches [15, 16], and blocking and deposition of PM owing to microclimate changes within forests [17]. However, during high PM episodes, tree PM reduction is limited depending on the characteristics of the leaves, and PM can be absorbed and adsorbed through their leaves by precipitation or strong winds [18, 19, 20]. In addition, as the damage caused by climate change (dry and heat) has increased, trees have restricted their functions, such as moisture stress and inset damage [21], and tree growth and resilience to extreme drought [22].  
A mist generator called the 'cooling fog' is a structure that can reduce PM to less than 0.5 \(\upmu\)m by combining PM and the mist sprayed from spray nozzles [2]. Mist generators are widely implemented as PM reduction systems in urban areas because of their applicability to various urban living spaces, such as residential playgrounds and bus stops [2]. In particular, mist is often used in high PM episodes because it can reduce PM by condensing PM growth [23], mitigating PM damage to trees. Additionally, spraying mist may reduce temperature and increase relative humidity, preventing leaf injuries by insects and fungi [23]. It also enhances the leaf defense against PM damage such as on leaf shading, increased leaf temperature, stomatal plugging, and interference with stomatal closure [24].  
However, previous conventional mist devices can induce the detrimental effect of leaf burn by intense, focused sunlight in summer. Also, most studies on mist are related to crop cultivation [25; 26], and the effects of thermal comfort and temperature reduction [2; 27]. There is little study on the PM reduction effects of mist generators under trees' canopies during high PM episodes. Therefore, this study was conducted at two urban parks in Seoul, Republic of Korea (Yangjae Citizen's Forest--YCF, Cheongdam Road Park--CRP), where mist generators were installed, and the PM concentration and weather conditions before, during, and after spraying mist were measured to identify the PM reduction effects of mist from May 6 to 8, 2020. The research objectives were to determine the effects of PM reduction and microclimate conditions through mist generators during high PM episodes.