Introduction
Recently, ongoing urbanization and increasing population density have led to a gradual rise in the use of multi-use facilities. Among these, underground shopping malls—originally developed in the 1970s and 1980s as air-raid shelters and pedestrian passageways—have since evolved into complex cultural spaces that integrate a variety of commercial functions, including retail and food and beverage services (Lee et al., 2020). Similarly, underground shopping malls connected to subway systems offer not only convenient mobility for users but also serve as complex cultural spaces. These facilities, originally constructed to maximize space utilization in densely populated urban centers, have contributed to the three-dimensional use of urban space (Lee et al., 2008). On the other hand, due to their underground location, such spaces are often characterized by enclosed structures and limited ventilation, which can lead to the re-suspension of particulate matter (PM), thereby deteriorating indoor air quality and posing health risks to users. In response, the Ministry of Environment (MoE) of the Republic of Korea has implemented a comprehensive PM Seasonal Management System since 2019 (MoE, 2022). Furthermore, under the Indoor Air Quality Control Act, the government has established indoor air quality recommendations and maintenance standards for multi-use facilities since 2016, and these regulations continue to be updated from a public health perspective.
Underground shopping malls emit MP with diverse chemical compositions originating from various sources. (Kim et al., 2022). Notably, ultrafine particles smaller than PM2.5 were classified as Group 1 carcinogens by the International Agency for Research on Cancer (IARC) under the World Health Organization (WHO) in 2013, highlighting the need for caution (IARC, 2016). In malls connected to outdoor environments, elements such as Cd, Cr, Cu, Ni, Pb, and Zn—typically associated with road traffic pollution sources—have been detected (Bae et a l., 2011). In malls connected to subway systems, PM containing transition metals such as Al and Fe, generated from the friction between train rails and wheels, has been reported to be dispersed inside through train-induced airflow (Jung et al., 2010). Additionally, as underground shopping malls accommodate various commercial facilities, fiber particles containing C, N, and O—likely originating from clothing stores and customers’ garments—can be detected. Relatively elevated concentrations of V, Mn, and Ni, which are associated with indoor combustion, have also been reported near restaurants within these malls (Maskey et al., 2011).
Once deposited, PM can become re-suspended into the atmosphere, and upon inhalation, may lead to a range of health issues, including respiratory and cardiovascular diseases. Previous studies have reported that long-term exposure to PM originating from road traffic pollutants increases the risk of cardiovascular diseases and cancer (Alemayehu et al., 2020). Moreover, substances such as V and Ni, which are produced by indoor combustion in restaurants, are known to negatively affect cardiopulmonary function (Fazakas et al., 2023). Additionally, transition metals such as Al and Fe, found in PM from subway systems, may cause neurodegenerative diseases such as Alzheimer’s and Parkinson’s. This is likely due to iron accumulation and oxidative stress in neural cells (Kim et al., 2022).
Furthermore, the composition of PM chemical species can vary depending on the particles’ physical form, and their emission source can be inferred through morphological analysis. One study classified PM particles emitted from the open-air incineration of agricultural plastic waste into solid and cotton-like particles, revealing that the chemical composition of the particles differed according to their size and shape (Kim et al., 2022). Additionally, PM particles with high concentrations of metallic chemical species, typically collected from downtown areas heavily affected by road pollution, exhibit distinct morphological characteristics compared to particles containing soil-derived chemical species (Ramirez-Leal et al., 2014).
Therefore, this study aims to determine the physicochemical characteristics of PM in underground shopping areas and analyze their morphological features to assess the potential health risks associated with PM exposure in such environments. To this end, foundational research was conducted to infer emission sources based on the physicochemical properties of PM, utilizing chemical speciation analysis infrastructure integrated with SEM-EDS, which was established in previous studies, along with air quality monitoring data from Indoor Air Quality (IAQ) stations.
Research Methods
Research site environment
This study was conducted in an underground shopping mall located in Gangnam-gu, Seoul (hereinafter referred to as Gangnam underground shopping mall). The facility was completed in July 2011 and is connected to a transfer station where Seoul Subway Line 2 and the Shinbundang Line intersect. As of 2023, it accommodates approximately 147,450 daily passengers, making it the second-largest multi-use facility among 647 subway stations in the Seoul metropolitan area (Seoul Metro, 2023; Fig. 1). The shopping mall has a total area of 12,315 m2, ranking third in size among the underground shopping malls managed by the Seoul Facilities Corporation. It contains 212 retail units, including clothing, cosmetics, and grocery stores. Typically, underground shopping malls rely on a central air conditioning system for air circulation. About one-third of the indoor air is discharged through ventilation shafts connected to the ground level (Lee et al., 2008). However, due to the structural limitations inherent to underground spaces, natural ventilation is significantly restricted, leading to a high potential for the accumulation of PM. However, due to the structural limitations inherent to underground spaces, natural ventilation is significantly restricted, leading to a high potential for PM accumulation (Samad et al., 2022).
Indoor air quality monitoring
A time-series analysis of PM concentrations was conducted to assess PM levels in the underground shopping mall. Monitoring took place over a seven-day period from April 15 to April 21, 2024, a period selected based on data from the Automatic Weather System (AWS) nearest to Gangnam Station during which time air quality was found to be at its worst. To accurately reflect the confined atmospheric conditions characteristic of the research site, a previously established monitoring method was employed (Kim et al., 2023). This involved the use of IAQ stations (SA-IL2, Aircok, Korea) certified at Performance Level 1 for PM measurement by the Korea Conformity Laboratories (KCL). The IAQ stations used in this study are compact devices measuring 180 × 130 × 30 mm and are equipped with sensors for PM2.5-10, volatile organic compounds (VOCs), carbon monoxide (CO), carbon dioxide (CO2), and other pollutants. Among these, PM levels are measured using a light scattering method, which employs laser diodes to convert voltage signals into frequencies and classify particles by size. (Lee et al., 2018). To monitor air quality by zone within the research site, two monitoring devices were installed in the North Plaza—an area with the highest pedestrian traffic—and one additional device was placed near the exit staircase landing to observe changes in air quality due to external inflow (Fig. 2).
PM sample collection and chemical speciation analysis
PM samples at the research site were collected either directly from suspended or deposited dust, or via filtration. Glass fiber filters with a diameter of 47 mm, capable of capturing particulate contaminants ≥ 0.4 μm, were used for sample collection. The filtration-based sampling followed a method employing portable, low-volume air samplers equipped with glass fiber filters having pore sizes ≤ 0.4 μm (Lee, 2004), to determine the status of PM in multi-use facilities.
The collected samples were cut into 5 mm × 5 mm sections for electron microscopy analysis and stored in a desiccator maintained at controlled temperature and humidity conditions (23–25°C, 0.1% RH). Prior to analysis, the samples underwent pretreatment using an ion sputter coater (G20, GSEM, Korea), during which a thin gold (Au) film was deposited via plasma sputtering for 180 seconds to enhance electron conductivity.
A large-area chemical speciation analysis was conducted using a combination of scanning electron microscopy (SEM; CUBE III, Emcrafts, Korea) and energy-dispersive X-ray spectroscopy (EDS; QUANTAX-610X, Bruker, Germany). To determine the relative weight ratios and spatial distributions of chemical species comprising particulate pollutants, wide-field SEM images were acquired at 100× magnification, followed by EDS mapping and spectral analysis. Subsequently, high-magnification imaging was performed according to particle size, and chemical speciation were analyzed by specific zones. PM particles were analyzed using a SEM-EDS (Phenom XL G2, Thermo Scientific, Netherlands), which allows for high-magnification analysis with ease. Individual particles were categorized based on size/diameter into PM10 (≤10 μm) and PM2.5 (≤2.5 μm) groups. For each category, EDS analysis was conducted three times by imaging three representative particles at magnifications of 25,000× or higher. The methodology for analyzing particle morphology and size was adapted from a previous study that examined the shape of PM particles generated from agricultural plastic waste incineration (Kim et al., 2022).
Results and Discussion
Time series analysis of indoor air quality
A time series analysis of monitoring data collected from IAQ stations revealed that the maximum PM10 concentration in the Gangnam underground shopping mall reached 83.5 μg/m3 at the North Plaza and 87.5 μg/m3 at the staircase landing connected to the outside. For PM2.5, the maximum concentrations recorded were 66.2 μg/m3 at the North Plaza and 97.9 μg/m3 at the staircase landing. Under the Indoor Air Quality Control Act, the recommended concentration standard for PM10 in multi-use facilities such as underground shopping malls has been strengthened from 150 μg/m3 to 100 μg/m3 since December 2024. For PM2.5, the previous recommended guideline has been upgraded to a maintenance standard, with the current limit set at 50 μg/m3 and scheduled to be further reduced to 40 μg/m3 in January 2026 (Ministry of Environment, 2024).
The PM10 concentration in the Gangnam underground shopping mall peaked at 83.5 μg/m3, which remained below the recommended standard. However, PM2.5 levels occasionally exceeded the current maintenance standard depending on the measurement date (Fig. 2 and Fig. 3). PM10 and PM2.5 concentrations recorded at the North Plaza of the underground mall exhibited similar trends to those measured at the staircase landing, a location directly influenced by outdoor air. Enclosed underground or indoor spaces have been reported to exhibit pollution levels more than twice those of outdoor environments, primarily due to the continuous accumulation of air pollutants infiltrating from outside (Gonzalez-Martin et al., 2021). Furthermore, research analyzing the effect of outdoor air on indoor air quality has shown that up to 31% of indoor air can be replaced by outdoor air (Shim et al., 2018), suggesting that the inflow of outdoor pollutants may contribute to the degradation of indoor air quality.
Large-area chemical speciation analysis of deposited and suspended dust
A large-area analysis of suspended dust samples collected from the research site, which was conducted using SEM-EDS at 100× magnification, revealed particles of various sizes and morphologies. Energy-dispersive X-ray spectroscopy (EDS) detected the presence of nonmetals (C, N, O), halogens (Cl), alkali metals (Na, K), alkaline earth metals (Mg, Ca), metalloids (Si), transition metals (Ti, Fe), and post-transition metals (Al) in the suspended dust samples collected from the North Plaza. In contrast, the deposited dust contained nonmetals (C, O), halogens (Cl), alkali metals (Na, K), alkaline earth metals (Mg, Ca, Ba), metalloids (Si), transition metals (Fe, Zn), and post-transition metals (Al) (Fig. 4 and Fig. 5). Based on previous studies on the chemical composition of PM, these results can provide insights into potential emission sources and formation environments.
A previous study simulating the open-air incineration of agricultural plastic waste reported the detection of Cr and Sr—originating from fertilizers and soil—as well as Mo, Cr, Nb, and Ni—attributed to agricultural equipment alloys—in PM samples (Kim et al., 2022). These findings confirmed a distinct chemical composition compared to the PM collected in this study. Conversely, a SEM-EDX-based chemical speciation analysis of PM10 collected from roadsides in suburban Spain identified chemical species such as S, Na, Fe, Ca, Mg, K, Al, and Zn (Megido et al., 2016). These elements closely matched those detected in this study, suggesting that road-derived PM may have been introduced to the study site, which was linked to road traffic environments.
Large-area chemical speciation analysis of PM particles
A chemical speciation analysis of PM particles in the samples, based on SEM-EDS, revealed variations in chemical species and their weight ratios depending on particle type and size. In deposited dust, the weight ratios of other metals, such as Al (4.6 wt%) and transition metals Fe (1.52 wt%) and Zn (1.06 wt%), were relatively higher in PM2.5 than in PM10. Additionally, alkaline earth metal Ba (0.72 wt%) and nonmetal S (0.47 wt%), which were not detected in PM10, were identified in PM2.5 (Fig. 6). Conversely, in suspended dust, higher weight ratios of transition metals such as Fe (2.25 wt%) and Ti (0.44 wt%) were observed in PM10 compared to PM2.5. Furthermore, the other metal Al (0.54 wt%), which was not detected in PM2.5, was identified in PM10 (Fig. 7).
Metallic fine particles such as Al, Fe, Zn, and Ti, which have been detected in both deposited and suspended dust, are known to pose health risks to humans. When Zn is inhaled while adsorbed onto ultrafine particles, it can reach the alveoli, leading to pulmonary edema and respiratory diseases (Cooper, 2008). Al, Fe, and Ti have also been reported to contribute to neurodegenerative diseases such as Alzheimer’s and Parkinson’s, potentially due to iron accumulation and oxidative stress in nerve cells (Kim et al., 2023). Moreover, Ba detected in PM2.5 from deposited dust, can stimulate cardiac and skeletal muscles when combined with atmospheric sulfuric acid, potentially resulting in paralysis in severe cases. The toxicity of Ba has been registered with the U.S. Environmental Protection Agency (EPA), the World Health Organization (WHO), and the U.S. Agency for Toxic Substances and Disease Registry (ATSDR; Oskarsson, 2022). These substances are particularly prone to forming compounds with elements such as the alkaline earth metals Na and Ca, the alkali metal K, and the halogen Cl. Previous studies analyzing the chemical composition of PM particle surfaces have suggested that metallic chemical species such as Fe, Al, and Ti are often associated with Na, Cl, K, and Ca, indicating similar compound formations (Wang et al., 2022).
Morphology by particle size
In this study, chemical speciation and morphological analyses were conducted on particulate matter (PM) from each sample. The emission sources of PM can be inferred by identifying morphological characteristics unique to specific sources. Based on SEM-EDS analysis, the physical morphology of PM in both samples was found to be irregular for both PM10 and PM2.5 (Fig. 8 and Fig. 9). These PM samples were collected from deposited dust originating from external road pollution, as well as from suspended dust generated by human activities. Therefore, the morphological features of the particles identified in this study may correspond to amorphous aggregates associated with metallic chemical species, similar to those reported in a previous study on the chemical composition of PM10 collected from urban areas heavily affected by road pollution sources (Ramirez-Leal et al., 2014). In contrast, a study analyzing PM captured from simulated open-air incineration of agricultural plastic waste detected cotton-like particles, which differ from the morphological characteristics observed in this study (Kim et al., 2022). Furthermore, distinct morphological features were reported in soot particles from wood combustion, which appeared as cotton-like aggregates composed of C, Al, Si, and S (Feng et al., 2009). These findings suggest that cotton-like particles are characteristic of PM produced during high-temperature combustion processes and, therefore, can be distinguished from the particle morphologies of PM collected in urban atmospheric environments.
Comprehensive analysis
A particle size-based chemical speciation analysis of deposited and suspended dust was conducted using SEM-EDS, with the results summarized as follows. In the case of PM2.5, metallic chemical species such as Al, Fe, Zn, and Ti—known to pose potential health risks—were found in relatively higher concentrations in deposited dust (Fig. 10). Conversely, PM10 exhibited higher concentrations in suspended dust. In the case of ultrafine dust with an aerodynamic diameter of 2.5 μm or less, previous studies have reported that its resuspension rate is higher than that of PM10 in indoor environments, thereby increasing the likelihood of absorption into the human body and potential health risks (Rasmussen et al., 2018). When comparing the composition of metallic chemical species in deposited and suspended dust by particle size, PM10 showed significantly higher weight ratios of Fe (181.4%), Al (618.5%), and Ti (213.6%) in deposited dust compared to suspended dust. Additionally, Zn—undetected in suspended dust—was found in the deposited dust (Fig. 11). Similarly, for PM2.5, the weight ratios of Fe (163.4%) and Ti (1,037%) were higher in deposited dust, and Al and Zn, which were not detected in suspended dust, were also confirmed in the deposited dust. These findings suggest that metallic chemical species present in deposited dust may pose a greater health risk compared to those in suspended dust.
Conclusion
Underground shopping malls, which are representative multi-use facilities, are large-scale environments frequented by numerous users. Concerns have been raised about potential health risks associated with deteriorating indoor air quality. In particular, the limited ventilation typical of underground spaces may lead to the deposition, and re-suspension of PM originating from outdoor sources. Therefore, it is necessary to investigate the chemical characteristics and potential health impacts of PM in these environments. This study derived the following results through time-series monitoring of an underground shopping mall and chemical speciation analysis and morphological examination of collected PM.
Time-series monitoring revealed that the concentration of PM2.5 relative to PM10 exceeded the maintenance standards set by the Indoor Air Quality Control Act and followed a trend similar to that of outdoor air quality. Large-area analysis using SEM-EDS of both deposited and suspended dust identified the presence of C, O, Cl, Na, K, Mg, Ca, Ba, Si, Fe, and Zn in deposited dust, and C, N, O, Cl, Na, K, Mg, Ca, Si, Ti, Fe, and Al in suspended dust. These differences in chemical composition between the two types of PM can provide insights into their emission sources and formation environments, as inferred from related previous studies. On the other hand, a chemical speciation analysis further revealed that deposited dust contained relatively higher concentrations of metallic elements such as Fe, Al, Ti, and Zn—substances known to be harmful to human health—compared to suspended dust. The particle morphology also resembled forms typically associated with chemical species found in urban environments. Additionally, the weight ratio of metallic chemical species in PM2.5 was higher in deposited dust than in suspended dust, depending on particle size. Based on previous studies, these findings suggest that deposited dust may pose a greater health risk than suspended dust.
However, since this study’s findings offer limited assessment of the harmfulness of deposited dust, further research is needed. Future studies should quantitatively analyze the total concentration of hazardous metallic substances in deposited dust and investigate their spatial behavior, especially concerning human exposure in multi-use facilities.
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