Introduction
Following the emergence of the unprecedented global pandemic COVID-19, indoor air quality (IAQ) has garnered heightened attention due to its significant impact on human health (Deng and Deng, 2018). Poor IAQ has been associated with detrimental health consequences, leading to growing concerns that deteriorating IAQ can negatively affect well-being (Kim et al., 2015). Common indoor air pollutants include particulate matter (PM), carbon dioxide (CO2), toxic gases, and volatile organic compounds (VOCs).
Plants play a crucial role in reducing air pollution by acting as natural filters. Numerous studies have demonstrated their ability to accumulate particulate matter (PM) on their leaves and wax surfaces (Liu et al., 2022; Panda et al., 2018). However, the extent of PM accumulation is influenced by various factors, including plant species, PM types, and environmental conditions (Bui et al., 2022b; Kwon et al., 2020; Räsänen et al., 2013). Plants with high leaf surface area-roughness achieve greater PM reduction than those with lower values (Popek et al., 2022). Additionally, external factors such as rain and wind can impact the amount of PM accumulated on leaf surfaces (Bui et al., 2022a). Furthermore, the concentration of PM itself also influences the rate of PM accumulation (Ter et al., 2020).
The amount of PM accumulation in plants increases with the increase of PM concentration in the atmosphere (Przybysz et al., 2014). In the indoor environment, plants’ ability to accumulate PM can be impacted by other factors, such as humidity. Another study showed that the humidity of the environment also impacts the amount of PM accumulation on the leaves of plants (Cummings and Waring, 2020). Moreover, the leaf area also impacted the amount of PM accumulation on the leaves of plants. Additionally, plants can reduce CO2 from the environment (Torpy et al., 2014). Many studies showed that plants removed CO2 through photosynthetic processing (Han and Ruan, 2020; Shannigrahi et al., 2004; Torpy et al., 2017). However, the ability to remove CO2 depends on the photosynthesis of plants. Sevik et al. (2017) showed that plants with high photosynthesis were more effective in reducing CO2 than plants with low photosynthesis. Light conditions strongly influenced the efficacy of the photosynthesis of plants (Paradiso and Proietti, 2022). For their aesthetic appeal, plants are often placed in low-light areas like hallways and conference rooms with light intensities far below the optimal range for photosynthesis (500–1000 lux). These indoor environments have limited natural sunlight for photosynthesis (Jang et al., 2021). The light intensity in these areas was significantly lower than that required for the photosynthetic needs of many plants (Rehman et al., 2017). However, the light absorption and effectiveness of photosynthesis depended on the interaction between plants and the light environment conditions (Liu et al., 2022). Plants adapt to low light conditions by altering leaf orientation to maximize light capture at the leaf surface. Moreover, this photosynthetic process acts as a crucial regulator of temperature and humidity balance in the atmosphere (Dominici et al., 2021). Clear Air Delivery Rate (CADR) is often used to evaluate indoor air purifying systems (Rai, 2022). It shows the indoor air-cleaning potential of a standalone device. However, many studies use CADR as an index to determine the effect of a single potted plant on clearing air (Budaniya and Rai, 2022; Mata et al., 2022).
This study aims to demonstrate the efficacy of PM and CO2 reduction in four broad-leaved evergreen plants under indoor lighting conditions. The findings will provide a scientific basis for designing effective greening strategies to improve indoor air quality. Because the burning of mosquito coils releases a significant amount of PM, including PM10 and PM2.5, mosquito coils were often used as a PM source in many experiments concerning PM reduction in indoor environments or chambers (Khandelwal et al., 2019; Kwon et al., 2021). This study used a mosquito coil and CO2 cylinder as a CO2 and PM source. The initial concentration was set at 300 μg m−3 (three times the amount of PM in the stipulated limit for PM Short-Term Exposure) and 1000 ppm (the 1000 ppm CO2 limit in ASHRAE Standard 62.1) (Simanic et al., 2019; Zhang et al., 2021). The plants’ photosynthesis parameters were also analyzed to determine the influence of these parameters on the ability to remove air pollution in the studied plant species.
Research Methods
Study plant species
We selected four plant species: three trees (Neolitsea sericea (Blume) Koidz., Coffea arabica L., Photinia glabra (Thunb.) Franch. & Sav.,) and one ornamental plant (Farfugium japonicum (L.) Kitam). Neolitsea sericea (Blume) Koidz. is an evergreen, broad-leaved large tree that often grows in China, Taiwan, Japan, Korea (Jeollanam-do island region, Chungcheongnam-do island, Jeju Special Self-Governing Province, and Ulleungdo). It was used as a windbreak, landscaping tree, and for timber. Coffea arabica L. is well known as the most valuable and widely traded commodity crop in several countries. They are shrubs or small trees native to tropical and southern Africa and tropical Asia. In Korea, it is also used as an ornamental plant. Photinia glabra (Thunb.) Franch. & Sav. is an evergreen broad-leaved tree that grows in China, Japan, and the southern region of Korea. Farfugium japonicum (L.) Kitam is a shrub that often grows in environments with sufficient moisture, in semi-shade or sunlight. It grows well at 10 to 21 degrees Celsius and has yellow flowers in September–October.
Before the experiment, all plants were cultivated in the laboratory at 25°C and light intensities (50 μmol m−2 s−1) for three months. Thus, white LED was used to provide the light intensities (50 μmol m−2 s−1) during the cultivation period and the experiment. The photosynthetic rate of each plant species was determined using a portable photosynthesis system (LCpro+, ADC BioScientific Ltd., UK). The leaf area of each plant species was measured using a leaf area meter (LI-3000A, LI-COR, USA) after the experiment.
PM and CO2 test chamber setting
Following the method of Jang et al. (2021), an acrylic chamber (800 mm × 800 mm × 1000 mm, L × W × H) was used to model the indoor environment. In order to provide light for the plants, an LED white light source panel emitting a light intensity of 50 μmol m−2 s−1 was placed on top of the test chamber. Using air conditioning, the chamber’s temperature was precisely maintained at 25°C. All tests were conducted five times in both a treatment chamber (with plants) and a control chamber (without plants). For the treatment chamber, one potted plant was put in the center of the chamber, and all potted plant were watered twice a week to ensure they received the necessary amount of water. A mosquito coil was used to create the PM source. The coil was burned in a closed chamber connected to the treatment chamber by a valve to channel the smoke into the chamber as a PM source. The mosquito coil burned for 10 minutes. After the burning process, PM was injected into the chamber until the initial concentration of PM was 300 μg m−3. The concentrations of PM1, PM2.5, and PM10 inside the chamber were recorded every minute for five hours by a PM counter (GT-531S, MET One Instruments, Inc., USA), which was placed in the center of the chamber. The same method was performed for the chamber without plants to determine the PM concentration in the control chamber (Fig. 1).
The same chamber configuration was used for the CO2 removal experiments. For this study, a CO2 cylinder was used as the source of CO2. The CO2 was injected directly into both a treatment chamber (with plants) and a control chamber (without plants) until the initial concentration of CO2 reached 1000 ppm. Each set of experiments for both the treatment and control chambers was conducted five times to ensure consistency. The CO2 concentration inside the chamber was recorded every minute for five hours by Indoor Air Quality Meters (IAQ-CALC Indoor Air Quality Meter model 7545, TSI Inc., USA) placed at the center of the chamber. During this test, the temperature and humidity were recorded to assess the plants’ impact on controlling the chamber’s temperature and humidity (Fig. 1).
Clean air delivery rate analysis (CADR) of plants
Following the modified method of Armijos-Moya et al. (2022), we analyzed the CADR of CO2 for the four plant species in this study. The amounts of CO2 depleted inside the chamber were evaluated using the following formula:
where λ = decay rate (h−1), N(t) = amount of pollutant after time t (μg/m3) or (mg/m3), and N(0) = initial amount of pollutant at t = 0 h (μg/m3) or (mg/m3).
The CADR was used to calculate the rates of contaminant reduction in the test chamber using the following formula:
where λe = total decay rate (h−1); λn = natural decay rate, which is the reduction of the contaminant in the control chamber (h−1); V = volume of the chamber (m3); and A = leaf area (m2). To calculate the reduction efficiency under the different test conditions, the following formula was used:
where η = efficiency (%), N(t) = amount of pollutant after time t (μg/m3) or (mg/m3), and N(0) = initial amount of pollutant at t = 0 h (μg/m3) or (mg/m3).
Data collection and statistical analysis
All data were analyzed using SAS software version 9.4 (SAS Institute, NC, USA) with analyses of variance (ANOVA) and Duncan’s Multiple Range Test (DMRT). The significance level was set at 5%. The concentrations of PM1, PM2.5, and PM10 during the study were converted to percentages of the initial value, which was set at 100%. The percent reduction rate of PM was calculated as the reduction in PM concentration per unit leaf area at 1 hour, 2 hours, and 3 hours, by comparing the differences in PM concentrations at these times with the initial value.
Results and Discussion
CADR of CO2 in four plant species
CADR is well known as a measure of the indoor air-cleaning potential of a standalone system. It was used to determine the effective volumetric flow rate at which ‘clean’ air is supplied to the environment. In this study, we found that the CADR ranged from 0.48 to 1.15 m3/h/leaf area (p < .0001) (Fig. 2). CADR was significantly different among the four plant species. Among four plants, C. arabica showed the highest CADR, followed by P. glabra and F. japonicum. Conversely, the plant species that showed the lowest CADR was N. sericea. Comparing the average temperature, we found that the average temperature of the treatment chamber was higher than that of the control chamber. The average temperature of the four treatment chambers ranged from 25.7°C to 25.8°C, while the control chamber was 26.3°C (Fig. 2). Moreover, the average humidity of the control chamber was significantly lower than the average humidity of the treatment chamber (Fig. 3).
Through photosynthesis, plants use light as an energy source to fix CO2 in the form of carbohydrates and release O2 into the atmosphere (Sowa et al., 2019). In the same environment, plants with a high photosynthetic rate showed more effective CO2 removal than plants with a low photosynthetic rate. In this study, C. arabica and P. glabra, with high photosynthetic rates, showed significantly higher CADR than other plant species. The CADR showed a significant correlation with the photosynthetic rate of plants (Fig. 4). However, the photosynthesis varies with the CO2 concentration in the cell of plants and the light intensity. The effective removal of CO2 increases with the increasing CO2 in the environment. Moreover, temperature and humidity affected photosynthesis in plants (Medlyn et al., 2002). Temperature and water stress strongly influence photosynthesis, affecting the growth and survival of plants (Wei et al., 2021). Therefore, optimizing environmental conditions for plants could substantially boost CO2 removal capabilities. Additionally, while transpiring, plants significantly enhance natural cooling systems by lowering their own temperature, subsequently cooling the surrounding air (Mangone et al., 2014). Our experiment results also demonstrated that evapotranspiration by plants in the treatment chamber increased air moisture content by 10–20%, resulting in a lower temperature compared to the control chamber.
PM reduction rate of four plants
In this study, the reduction rate of PM increased over time and differed among plant species in 1 hour. We found that the reduction rate by plants differed among PM1, PM2.5, and PM10. For all plant species, the reduction rate of PM10 and PM2.5 was higher than that of PM1. After five hours, the reduction rate of PM1, PM2.5, and PM10 ranged from 71.36% to 74.26%, 80.07 to 82.66%, and 80.87% to 83.49%, respectively. Among the four plant species, C. arabica had the highest reduction rate, followed by P. glabra, while F. japonicum showed the lowest PM reduction in PM1, PM2.5, and PM10 (Fig. 5).
The ability of plants to accumulate PM varies among different plant species. These differences are determined by the leaf structure of plants. Plants with large leaf areas exhibited greater than those with smaller leaf areas. Among the four plant species, C. arabica and P. glabra had the highest leaf area (Table 1). This may cause an increase in the PM reduction rate of these plant species compared to others. During photosynthesis, the photoelectric effect produces many negative air ions (NAIs) (Yan et al., 2015). The NAIs not only improve people’s health but also play an essential role in absorbing dust in the air (Perez et al., 2013). For this study, the highest photosynthetic rate also showed the highest PM reduction rate. The NAIs can be one of the factors that impact the ability of plants to accumulate PM. Moreover, Jang et al. (2021) indicated that humidity affected the ability pf plants to accumulate PM. In the future, the correlation between the reduction rate of PM and factors such as NAIs, humidity, and temperature needs to be studied to maximize the effectiveness of plants in improving air quality.
Conclusion
All study plants effectively removed PM and CO2, although their effectiveness varied among the four plant species. Plants with high photosynthetic rates demonstrated more effective CO2 removal than those with lower rates. A strong correlation was found between the CADR and the photosynthetic rate of plants. Additionally, plants played important roles in controlling temperature and humidity, with the chamber containing plants showing lower temperature and higher humidity compared to the chamber without plants. For the PM experiment, the PM reduction rate differed among the four plant species. The leaf area of plants could impact the PM reduction rate. Among the four plant species, C. arabica and P. glabra showed more effective removal of PM and CO2 than the other species. Difference in leaf structure and photosynthesis rates may be the factor impacting the PM reduction capabilities of plants. Future research should investigate the correlations between PM removal capacity, optimal environmental conditions like temperature and humidity, and photosynthetic rate to optimize the air quality improvement potential of plants.
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