Analysis of fine particulate matter blocking effect by air curtain at school entrance through experimental measurement

. Fine particulate matter is classified as a first-class carcinogen and is receiving global attention. In particular, particulate matter (PM2.5, and PM10) are more fatal in students at a growing age while their respiratory system develops than in ordinary adults. During school hours, the main entrance is often left open for the convenience of students and staff. In this situation, the inflow of PM2.5 through the main entrance increases significantly. Thus, air curtains are widely used at the main entrance to block the inflow of PM2.5. However, few experimental studies have examined the effect of blocking PM2.5 through such air curtains. This study analyzed the effectiveness of reducing PM2.5 inflow by installing an air curtain at the main entrance of a school. The efficiency in blocking PM2.5 was determined by analyzing the average I/O ratio of PM2.5 at the respiratory height through experimental measurements. The average I/O ratio of PM2.5 in the school entrance was 0.84 before the installation of the air curtain, which was reduced to 0.46 after installing the air curtain. The blockage efficiency of the air curtain was 42%. This study quantitatively confirms that air curtains effectively reduce the inflow of PM2.5 in schools.


Introduction
In 2013, fine particulate matter was classified as a first-class carcinogen by the International Agency for Research on Cancer (IARC), a research institute affiliated with the International Health Organization (WHO), and subsequently has received global attention. According to the 2019 World Air Quality Report, Korea has the highest concentration of fine particulate matter (PM2.5) in the air among all OECD member countries. Since 2015, the number of days per year with poor PM2.5 air quality (defined as a 24-hour average registering more than 35 μg/m 3 ) was always above 60 days, and the number of days with high concentrations of daily average PM2.5 (exceeding 50 μg/m 3 ) is gradually increasing [1,2].
To address these issues, the Korean government enacted and amended several laws, including the "Special Act on the Reduction and Management of Fine particulate matter," in April 2020. Article 22 of the Special Act on the Reduction and Management of Fine particulate matter stipulates that those facilities used by the vulnerable, such as children, students, and the elderly, are among the areas considered to be seriously polluted by fine particulate matter. These are designated as fine * Corresponding author: tkim@yonsei.ac.kr particulate matter concentration control areas and are continuously managed [3].
Schools, academies, day care centers, kindergartens and hospitals are mainly used by the vulnerable, who are greatly affected by fine particulate matter. Students, whose bodies are still developing, have an immature respiratory system and weaker immunity than adults, while they inhale a high concentration of air when compared to their body weight. The bodies of students are at the stage of developing their lung function, and if during this period they are exposed to hazardous substances, such as fine particulate matter, the effects can last into adulthood [5]. In this study, the objective is to target a school within which students, at a growing age, spend most of their time [4].
In general, the main entrance of school is kept open for the convenience of students and teachers in the morning. In this case, fine particulate matter in the outdoor air is easily introduced into the building through the main entrance. Particularly, PM2.5, which is in the air, is introduced into the building through the main entrance [6]. To prevent this, an air curtain can be installed at the main entrance of the building [7].
Many studies have been conducted to test the performance of air curtains [7,8]. At present, when interest in fine particulate matter is increasing, these have focused mainly in simulation and scaledown experiments on the performance of blocking fine particulate matter from the outside air using air curtains. However, quantitative research on the performance of air curtains in blocking fine particulate matter from entering an indoor space through field experiments is insufficient. This paper quantitatively analyzed the particle size removal efficiency (PSE) of PM2.5 through experiments based on air curtains generally installed at the main entrance of schools where students at a growing age spend most of their time [4,9].

Experimental overview
This experiment was conducted during the summer of July 2022 at P Middle School, located in Namyangju-si, Gyeonggi-do, Korea. Figure 1 shows the panorama view and floor plan of the school where the experiment was conducted. In order to measure the inflow of PM2.5 through the school's main entrance, the experiment was conducted after closing all the inner door gaps in the windbreak room except for the main entrance where air curtains were installed. Experiments refer to the method of Shao et.al [10].
A diagram of the experimental setup is shown in Figure 2.  Figure 3 shows the experimental setup. Figure 3-(a) shows the installation of the experimental setup while Figure 3-(b) shows the airtight sealing of all door gaps except for the main entrance where the experiments took place. Table 1 shows the detailed conditions of the experimental setup. The experiment was conducted in two different conditions: Case1 and Case2. For both cases, the door was open, while in Case 1 the air curtain was not operated and in Case 2 it was operated. According to a study by Chen et.al, when the average surface wind speed reaches 1 m/s at the entrance, then the higher the wind speed of the air curtain, the higher the fine particulate matter blocking efficiency [7,8].
To obtain an average surface wind speed reaching the entrance of 1 m/s, the average wind speed of the fan, which migrates fine particulate matter from outdoor to indoor, was set to 3.5 m/s. Moreover, the air curtain was set to a maximum wind speed of 12 m/s to confirm its performance in blocking the fine particulate matter. Since both cases cannot be undertaken at the same time, the measurements were performed sequentially.
The time of the experiments was limited to one hour, the same as the time period during which doors are kept open in the morning for students to come into the school. The experiments were also conducted immediately after lighting the incense and generating fine particulate matter [10]. The experiments were performed by setting both Case 1 and Case 2 in similar conditions apart from whether the air curtain was in operation or not. In order to make the similar experimental condition for both cases, after conducting Case 1, an air purifier was used to bring down the concentration of fine particulate matter in the building to make the similar value as before running the experiment, therefore Case 2 was then conducted in similar condition.

Measurement
In this experiment, four particle size ranges were analyzed: 0.3-0.5, 0.5-0.7, 0.7-1.0, and 1.0-2.5 m. In addition, all particles with a diameter below 2.5 m (PM2.5), which flow with air, were selected as the measurement target. Fine particulate matter with diameters above 2.5 m was measured as 0.02% of the total number of particles and was therefore excluded from the analysis range. The particle number (PN) concentration in the air collected both indoor and outdoor was measured using an optical particle counter (OPS3330, TSI Inc., Shoreview, MN, USA). Both indoor and outdoor air was measured at the same time based on the same particle size range. The blockage efficiency of each air curtain was determined by comparing the ratio of particles of the four different particle size ranges flowing from the outside to the inside. Finally, the efficiency of an air curtain in blocking PM2.5 from entering the building from the outside was analyzed [6].

Data analysis
This study analyzed the efficiency of air curtains in blocking PM2.5 from flowing into a building while the entrance door is open for students and staff to enter school during the morning. Indoor particle concentration was analyzed using Eq (1), which represents the mass balance equation [11]. In Eq (1), when there is no indoor emission rate S, the I/O ratio is defined as in Eq (2). As shown in Eq (2), the I/O ratio represents the concentration ratio of particles measured in the indoor and the outdoor samples. The I/O ratio for each particle size range was calculated using Eq (2), which measures the ratio between the downstream particle concentration (i.e., the particle number concentration indoors) and the upstream particle concentration (i.e., the particle number concentration outdoors). The removal efficiency for each particle size range is defined by Eq (3). The PSE of the air curtain for fine particulate matter was obtained based on the calculated PN I/O ratio for each case and Eq (3), as defined in ASHREA standard 52.2 [9].  Figure 4 shows the indoor and outdoor PN concentrations. Figure 4-(a) shows the indoor and outdoor PN concentration by particle size range in Case 1 in which the door was opened and the air curtain was not operated. Figure 4-(b) shows the indoor and outdoor PN concentration by particle size range in Case2 in which the door was opened, and the air curtain was in operation. Table2 summarizes the average PN concentration measured for each particle size range for each case. By analyzing the ratio of fine particulate matter introduced from the outside to the inside as a function of the particle size range, the composition of fine particulate matter by particle size in both Case1 and Case2 was similar. For both cases, the particle size composition of PM2.5 introduced remained the same:  Figure 5 shows the I/O ratio for each case based on the PN concentration data in Figure 4. Based on the calculated I/O ratio, the PSE for each particle size range was obtained [9]. Table 3 shows the measured I/O ratio and PSE for each particle size range in each case. The PSE was determined to be 41% for 0.3-0.5 m, 47% for 0.5-0.7 m, 51% for 0.7-1.0 m, and 36% for 1.0-2.5 m. It is evident that the PSE for each particle size range is slightly different, with the particle diameter of 0.7-1.0 m being measured the highest, at 51%. Overall, the blockage efficiency of the air curtain for PM2.5 is about 42%.

Conclusion
This study conducted the method of blocking PM2.5 inflow through the opened main entrance during school hours to reduce the intake of carcinogen (PM2.5) into a place where students of growing age spend most of their time. The main entrance of the school is kept open in the morning to facilitate the entry of students and teachers. During this period PM2.5 rapidly enters the building through the opened main entrance [6]. In order to prevent the sudden inflow of PM2.5 into the building, an air curtain was installed at the main entrance of the school, while its efficiency at blocking PM2.5 from entering the building (or PSE) was analyzed based on experimental data [9]. The PSE of the air curtain under this study for PM2.5 was found to be 42%. This is in agreement with the blocking efficiency found in previous study of about 40% and an error of 2% determined by CFD simulation of Chen et.al [7].
Through this study, it was proven that an air curtain installed at the main entrance of a school is an effective method for blocking PM2.5 fine particulate matter particles from entering the building from the outside. It is expected that this study will serve as the basis for highlighting the need to install air curtains in the main entrances of schools, reducing the exposure of students to PM2.5.

Acknowledgement
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT, MOE) and (No. 2019M3E7A1113095). The funding agency had no role in the study design, the collection, analysis, or interpretation of data, the writing of the report, or the decision to submit the article for publication.