Assisted Deposition of PM2.5 from Indoor Air by Ornamental Potted Plants

sustainability

Article Assisted Deposition of PM2.5 from Indoor Air by Ornamental Potted Plants

Yanxiao Cao 1,2,* , Fei Li 1,2 , Yanan Wang 2, Yu Yu 3, Zhibiao Wang 2, Xiaolei Liu 2 and Ke Ding 2

1 Research Center for Environment and Health, Zhongnan University of Economics and Law, Wuhan 430073, China; [email protected] 2 School of Information and Safety Engineering, Zhongnan University of Economics and Law, Wuhan 430073, China; [email protected] (Y.W.); [email protected] (Z.W.); [email protected] (X.L.); [email protected] (K.D.) 3 Low Carbon Water Research Center, School of Environment and Nature Resources, Renmin University of China, Beijing 10087, China; [email protected] * Correspondence: [email protected]

Received: 28 March 2019; Accepted: 29 April 2019; Published: 2 May 2019

Abstract: This study clarifies whether vegetation can promote the decrease of indoor PM2.5 concentration. The indoor PM2.5 concentration in two periods of 2013 in Wuhan city was simulated by cigarette burning in a series of sealed chambers. Six common indoor potted plants were selected as samples to investigate the effect of plants on PM2.5 decline. The effects of potted plants on PM2.5 decline were analyzed from three aspects: plant species, leaf characteristics and relative humidity. The results show that the presence of potted plants accelerated the decline of PM2.5. The additional removal rates (excluding gravity sedimentation of PM2.5 itself) for Aloe vera and Epipremnum aureum 3 were 5.2% and 30% respectively, when the initial PM2.5 concentration was around 200 µg/m . The 3 corresponding values were 0% and 17.2%, respectively, when the initial PM2.5 was around 300 µg/m . Epipremnum aureum was the optimum potted plant for PM2.5 sedimentation, due to its rough and groove leaf surface, highest LAI (leaf area index, 2.27), and strong humidifying capacity (i.e., can promote chamber humidity to 65% in 30–60 minutes.). Actual indoor studies have also confirmed that a certain amount of Epipremnum aureum can promote the decrease of indoor PM2.5. This paper provides insights on reducing the concentration of fine particulate matter by indoor greening efforts.

Keywords: potted plant; PM2.5; indoor; Epipremnum aureum

1. Introduction Over the last two decades, the impact of PM on human health have drawn increasing attention [1]. In particular, particulate matter with diameter of 2.5 µm or less (PM2.5), owned larger speciﬁc surface area and bigger adsorption ability, along with various toxic compounds, e.g., heavy metals, acid oxides, organic pollutants and pathogenic microorganisms [2,3] accumulated in human bronchi and lungs [4], cause premature mortality, pulmonary inﬂammation, accelerated atherosclerosis, and altered cardiac functions [5]. Besides, continuous exposure to high levels of PM2.5 may trigger cardiovascular diseases [6]. Therefore, PM2.5 has become a major concern for air pollution control because of its harmful impact on human health. Urban residents spend 80% or more of their time indoors—at home, work, school or in leisure activities [7,8], and long-term exposure to indoor PM2.5 particles is a major threat to public health. Researchers have carried out extensive researches on the sources and control strategies of indoor PM2.5. The high pollution levels that are found in most large cities are transported into the indoor

Sustainability 2019, 11, 2546; doi:10.3390/su11092546 www.mdpi.com/journal/sustainability Sustainability 2019, 11, 2546 2 of 10

atmosphere by ventilation and infiltration airflows [1]. Solutions to reduce indoor PM2.5 are categorized into three aspects: reducing outdoor PM2.5 sources, reducing indoor PM2.5 sources and reducing the transport of outdoor PM2.5 into the indoor atmosphere. The most widespread approach in commercial buildings is the use of cloth filters, which are integrated in mechanical ventilation systems [1]. Portable, high-efficiency particulate air cleaners help remove residential indoor PM2.5, although the cost is unaffordable for most families. In the search for eco-friendly and sustainable control measures, the ability of plants to capture and remove atmospheric PM particles has been widely investigated [9–13]. Scholars from various countries constructedSustainability 2019, 11 fields, x FOR andPEER REVIEW simulations (i.e., wind tunnels [14] and models [152]) of on 10 different scales to investigatein commercial PM buildings deposition is the byuse plants.of cloth filters, They which focused are onintegrated the following in mechanical aspects: ventilation (i) vegetation factors, suchsystems as forest [1]. Portable, coverage, high-efficiency forest structure, particulate air vegetation cleaners help species remove [ 16residential,17], tree indoor structure PM2.5, and leaf characteristicsalthough [18 ];the (ii) cost meteorological is unaffordable for conditions, most families. such as precipitation, wind speed and boundary In the search for eco-friendly and sustainable control measures, the ability of plants to capture layer heights,and remove air temperature, atmospheric PM as wellparticles as has relative been widely humidity investigated (RH) [9–13]. [14]; andScholars (iii) from PM various characteristics, e.g., particlecountries size distribution, constructed fiel chemicalds and simulations components, (i.e., wind etc. tunnels [19]. These [14] and studies models were [15]) on usually different carried out outdoors, somescales to valuable investigate conclusions PM deposition had by beenplants. obtained,They focused which on the have following important aspects: (i) guiding vegetation significance for urban greening.factors, such as forest coverage, forest structure, vegetation species [16,17], tree structure and leaf characteristics [18]; (ii) meteorological conditions, such as precipitation, wind speed and boundary Overlayer the heights, last three air temperature, decades, as researches well as relative have humidity demonstrated (RH) [14]; and that(iii) PM indoor characteristics, potted e.g., plants can significantlyparticle reduce size thedistribution, concentrations chemical ofcomponents, most urban etc. [19]. air pollutants,These studies i.e. were NO usually2, CO carried2, VOC, out especially benzene andoutdoors, formaldehyde some valuable [20 conclusions–25]. While had been it was obtained, established which have that important urban guiding vegetation significance can decrease for urban greening. outdoor PM2.5 levels, the specific role of potted plants in elimination of indoor PM2.5 particles is Over the last three decades, researches have demonstrated that indoor potted plants can unknown. Thus, this study aimed at investigating the specific role of potted plants in indoor PM significantly reduce the concentrations of most urban air pollutants, i.e. NO2, CO2, VOC, especially 2.5 reduction.benzene In particular, and formaldehyde this work [20–25]. endeavors While it (i)was to established explore thethat eurbanffect vegetation of different can potteddecrease plants on PM2.5 removal,outdoor (ii) PM to2.5 investigate levels, the specific the influence role of potted of potted plants plantsin elimination on PM of2.5 indoorunder PM di2.5ff erentparticles indoor is PM2.5 unknown. Thus, this study aimed at investigating the specific role of potted plants in indoor PM2.5 concentrations, (iii) to further illustrate the difference in PM2.5 deposition caused by potted plants, reduction. In particular, this work endeavors (i) to explore the effect of different potted plants on (iv) and toPM provide2.5 removal, reference (ii) to investigate suggestions the influence for indoor of potted greening plants on eff PMorts.2.5 under different indoor PM2.5 concentrations, (iii) to further illustrate the difference in PM2.5 deposition caused by potted plants, 2. Materials(iv) and and to Methods provide reference suggestions for indoor greening efforts.

2.1. Material2. Materials and Equipment and Methods

2.1.1. Potted-Plants2.1. Material Selectionand Equipment Six potted2.1.1. Potted-Plants plants bought Selection from local markets were chosen as samples: Chlorophytum comosum, Spathiphyllum ﬂoribundumSix potted plants, Epipremnum bought from aureumlocal markets, Ficus were elastica chosen, Sansevieria as samples: Chlorophytumand Aloe vera comosum(Figure, 1). We chose plantsSpathiphyllum of similar floribundum age and,growth Epipremnum status, aureum with, Ficus plant elastica height, Sansevieria of 25–35 and Aloe cm. vera (Figure 1). We chose plants of similar age and growth status, with plant height of 25–35 cm.

Figure 1. Picture of potted plants: (a) Chlorophytum comosum,(b) Spathiphyllum ﬂoribundum, Figure 1. Picture of potted plants: (a) Chlorophytum comosum, (b) Spathiphyllum floribundum, (c) (c) EpipremnumEpipremnum aureum aureum,(d), Ficus(d) Ficus elastica elastica,(, (e)e) AloeSansevieria vera,,( (f)f) AloeSansevieria vera. . Sustainability 2019, 11, 2546 3 of 10

2.1.2. Testing Instrument Blounton HOL-1209 air quality detector (Brenton medical technology (Beijing) Co., Ltd., China) 3 was employedSustainability as 2019 a, 11 PM, x FOR2.5 PEERdetector, REVIEW the measuring range of the instrument was 0–9993 of 10 µg/m , and its precision was 10%. This instrument can simultaneously measure PM concentration and RH. ± 2.5 A blank2.1.2. chamber Testing was Instrument used to calibrate the detector. Mean values were obtained from two replicates of sample testsBlounton to reduce HOL-1209 uncertainties. air quality detector (Brenton medical technology (Beijing) Co., Ltd., China) was employed as a PM2.5 detector, the measuring range of the instrument was 0–999 μg/m3, and its 2.1.3. Experimentprecision was Equipment ±10%. This instrument can simultaneously measure PM2.5 concentration and RH. A blank chamber was used to calibrate the detector. Mean values were obtained from two replicates of Particulatesample tests matter to reduce produced uncertainties. from cigarette smoke was mainly below 10.0 µm, and particulate matter below 2.5 µm in size accounted for more than 80% [26], therefore, cigarette smoke was chosen 2.1.3. Experiment Equipment as the source of PM2.5 in this study. Experiments were conducted using a series of closed chambers Particulate matter produced from cigarette smoke was mainly below 10.0 μm, and particulate with a volume of 60 L (40 cm length 30 cm width 50 cm height), as seen in Figure2. To eliminate matter below 2.5 μm in size accounted× for more than 80%× [26], therefore, cigarette smoke was chosen the eﬀectasof the electrostatic source of PM2.5 force, in this the study. test Experiments chamber waswere madeconducted of glass using panelsa series of with closed thickness chambers of 6 mm [27]. A 1 cm roundwith a volume hole withof 60 L a (40 piston cm length was × made30 cm width in the × 50 middle cm height), of the as seen front in Figure face, used2. To eliminate to insert the probe of the instrument.the effect of electrostatic To prevent force, any the leak test chambe duringr was the made tests, of the glass chamber panels with was thickness sealed of by6 mm both glue and [27]. A 1 cm round hole with a piston was made in the middle of the front face, used to insert the adhesiveprobe foam-rubber of the instrument. insulation To prevent tape, anyand leak during the test the holetests, the was chamber sealed was with sealed a rubberby both glue stopper unless measuring.and adhesive A small foam-rubber fan ﬁxed insulation inside each tape, and chamber the test tohole provide was sealed complete with a rubber mixing stopper of unless the smoke and a thermometermeasuring. was A used small tofan check fixed inside temperature each chamber and to calibrate provide complete RH. The mixing laboratory of the smoke was illuminatedand a over the experimentalthermometer periods. was used to check temperature and calibrate RH. The laboratory was illuminated over the experimental periods.

FigureFigure 2. 2. TheThe experimental experimental setup. setup.

2.2. Experimental2.2. Experimental Method Method

2.2.1. Removal2.2.1. Removal Eﬃciency Efficiency of PMof PM2.52.5by by Potted-Plants and and Its Influencing Its Inﬂuencing Factors Factors

According to the historical air quality data of Wuhan in 2013, the average concentration of PM2.5 According to the historical air quality data of Wuhan in 2013, the average concentration of PM2.5 ranged from 11 μg/m3 to 284 μg/m3. The monitoring data of indoor PM2.5 in Wuhan in 2013 [28] was µ 3 µ 3 ranged fromtaken as 11 a referenceg/m to (40–340 284 μgg/m/m3).. Taking The monitoring into account the data impact of indoor of the most PM disadvantageous2.5 in Wuhan in 2013 [28] 3 was takenconditions, as a reference the initial (40–340 PM2.5 concentrationµg/m ). Taking in this experiment into account were the set impactto around of 200 the μg/m most3 and disadvantageous 300 3 3 conditions,μg/m the, respectively. initial PM 2.5 concentration in this experiment were set to around 200 µg/m and 300 µg/m3, respectively.The leaves of the potted plants were first washed and naturally dried, and the pots were wrapped in plastic bags. Each potted plant was placed inside the test chamber, and an empty Thechamber leaves served of the as potted blank control. plants wereAll the first test chambe washedrs were and naturallyfilled with eq dried,ual amounts and the of potscigarette were wrapped in plasticsmoke bags. (cigarette Each potted burning planttime inside was was placed contro insidelled and the a fan test was chamber, used to mix and the an smoke). empty The chamber top served as blankdoor control. of each All chamber the test was chambersclosed until PM were2.5 concentration filled with was equal in the amounts test range. of PM cigarette2.5 and RHsmoke were (cigarette burningrecorded time inside every was10 minutes controlled for 180 andminutes. a fan The was experiment used to was mix carried the smoke).out in a temperature- The top door of each controlled laboratory at 20 ± 3 ℃. All experiments were repeated twice. chamber wasAfter closed the experiment, until PM2.5 sixconcentration kinds of plant leaves was we inre the picked, test range.and nail PMpolish2.5 wasand applied RH were to them recorded every 10 minutesto obtain for 180 the epidermis minutes. [29]. The Fine experiment structure of wasthe leaf carried surface out was in observed a temperature-controlled by inverted fluorescence laboratory at 20 3 °C. All experiments were repeated twice. ± After the experiment, six kinds of plant leaves were picked, and nail polish was applied to them to obtain the epidermis [29]. Fine structure of the leaf surface was observed by inverted fluorescence microscope (BDS 200-FL, Chongqing Optec Instrument CO., LTD, Chongqing, China). Sustainability 2019, 11, 2546 4 of 10

For determination of the leaf area index (LAI, leaf area/ground area, dimensionless), three pieces of large, medium and small leaves were taken from the six potted plants. After scanning, the area of each leaf was measured according to the principle that the pixel ratio in Photoshop software was equal to the area ratio, and the corresponding average value was recorded. Total plant area is the product of average leaf area and leaf number. The maximum vertical projected area of the plant was taken and the method of area measurement was the same as above. The leaf area index is the ratio of the two areas [30–32].

2.2.2. Actual Indoor Study Two rooms with the same internal structure and external conditions, adjacent to each other with 2 an area of 16 m were selected as test rooms. After closing for a week, the indoor PM2.5 concentrations were basically the same. Eight pots of Epipremnum aureum were placed in one room, and there was no potted plant in the other room. After a cigarette was burned in both rooms, PM2.5 concentration values were recorded every 10 minutes and continuously monitored for 180 minutes. Since the test room area was less than 50 m2, one measuring point was set according to the requirements of the indoor air monitoring technical guidelines in the Indoor Air Quality Standard (GB/T 18883–2002) [33]. The measuring point was located in the center of the room, 120 cm above the ground. The experiment was repeated twice under similar conditions and the experimental data were averaged.

3. Results and Discussion

3.1. Deposition Eﬀect of PM2.5 by Potted Plants under Diﬀerent PM2.5 Concentrations

The variation of PM2.5 particles in the chamber with potted plant exhibited a downtrend similar to those in the empty chamber under different PM2.5 concentrations (Figure2). Moreover, the test chamber with potted plants showed superior performance on PM2.5 removal than the empty chamber in both conditions, except that Aloe vera exhibited poor PM2.5 removal efficiency at high PM2.5 concentrations. Besides, for most potted plants, PM2.5 curves declined more sharply at low PM2.5 concentrations (Sansevieria was an exception), while it was better under high concentration conditions in terms of total PM2.5 removal, suggesting that the density of the particles settling upon the leaf surface increased with increasing PM2.5 concentration [34]. Additionally, the maximum PM2.5 removal rate for the six potted plants, as well as the empty chamber, occurred in the first hour (Figure3), and total PM 2.5 removal rate in the empty chamber exceeded 40% under both conditions, indicating that gravitational sedimentation was an effective way in decreasing the concentration of solid particles [27]. However, the removal rate of PM2.5 in test chambers were higher than that in the empty chamber under both conditions, suggesting that the presence of potted plants promoted the deposition of PM2.5. The most intuitive reason can be attributed to the presence of potted plants occupying a certain volume in the chamber, thereby shortening the effective distance between PM particles and collectible surfaces [27]. Sustainability 2019, 11, x FOR PEER REVIEW 4 of 10

microscope (BDS 200-FL, Chongqing Optec Instrument CO., LTD, China). For determination of the leaf area index (LAI, leaf area/ground area, dimensionless), three pieces of large, medium and small leaves were taken from the six potted plants. After scanning, the area of each leaf was measured according to the principle that the pixel ratio in Photoshop software was equal to the area ratio, and the corresponding average value was recorded. Total plant area is the product of average leaf area and leaf number. The maximum vertical projected area of the plant was taken and the method of area measurement was the same as above. The leaf area index is the ratio of the two areas [30–32].

2.2.2. Actual Indoor Study Two rooms with the same internal structure and external conditions, adjacent to each other with an area of 16 m2 were selected as test rooms. After closing for a week, the indoor PM2.5 concentrations were basically the same. Eight pots of Epipremnum aureum were placed in one room, and there was no potted plant in the other room. After a cigarette was burned in both rooms, PM2.5 concentration values were recorded every 10 minutes and continuously monitored for 180 minutes. Since the test room area was less than 50 m2, one measuring point was set according to the requirements of the indoor air monitoring technical guidelines in the Indoor Air Quality Standard (GB/T 18883–2002) [33]. The measuring point was located in the center of the room, 120 cm above the ground. The experiment was repeated twice under similar conditions and the experimental data were averaged.

3. Results and Discussion

3.1. Deposition Effect of PM2.5 by Potted Plants under Different PM2.5 Concentrations

The variation of PM2.5 particles in the chamber with potted plant exhibited a downtrend similar to those in the empty chamber under different PM2.5 concentrations (Figure 2). Moreover, the test chamber with potted plants showed superior performance on PM2.5 removal than the empty chamber in both conditions, except that Aloe vera exhibited poor PM2.5 removal efficiency at high PM2.5 concentrations. Besides, for most potted plants, PM2.5 curves declined more sharply at low PM2.5 concentrations (Sansevieria was an exception), while it was better under high concentration conditions in terms of total PM2.5 removal, suggesting that the density of the particles settling upon the leaf surface increased with increasing PM2.5 concentration [34]. Additionally, the maximum PM2.5 removal rate for the six potted plants, as well as the empty chamber, occurred in the first hour (Figure 3), and total PM2.5 removal rate in the empty chamber exceeded 40% under both conditions, indicating that gravitational sedimentation was an effective way in decreasing the concentration of solid particles [27]. However, the removal rate of PM2.5 in test chambers were higher than that in the empty chamber under both conditions, suggesting that the presence of potted plants promoted the deposition of PM2.5. The most intuitive reason can be Sustainabilityattributed2019, 11, 2546 to the presence of potted plants occupying a certain volume in the chamber, thereby 5 of 10 shortening the effective distance between PM particles and collectible surfaces [27].

(a) 220 200 180 160 ) 3 140 Sustainability 2019, 11, x FOR PEER REVIEW 5 of 10 120 (ug/m

2.5 100 Figure80 3. Temporal variation curves of PM2.5 particle concentrations under different initial PM2.5, (a) PM when60 initial PM2.5 was around 200 μg/m3, (b) when initial PM2.5 was around 300 μg/m3. The upper and 40lower limits of the concentration error bar represent standard deviation of PM2.5 concentrations. 20 0 20 40 60 80 100 120 140 160 180 200 3.2. Influencing Factors of PM2.5 Deposition by Potted Plants Time (min)

To investigate the dust settlement capability of different potted plants, influencing factors Figureincluded 3. Temporal plant species, variation leaf characterist curves ofics PM and2.5 RHparticle were concentrationsinvestigated as follows. under diﬀerent initial PM2.5, 3 3 (a) when initial PM2.5 was around 200 µg/m ,(b) when initial PM2.5 was around 300 µg/m . The upper and3.2.1. lower Influence limits of of the Plant concentration Species error bar represent standard deviation of PM2.5 concentrations.

As seen in Figures 3 and 4, the decline rates of PM2.5 particles in chambers with different potted 3.2. Influencing Factors of PM2.5 Deposition by Potted Plants plants were considerably different. Significant removal efficiency (71.5% at high PM2.5 and 59.2% at To investigatelow PM2.5) was the observed dust settlement in the chamber capability with Epipremnum of different aureum potted, while plants, the influencingchamber with factors Aloe vera included plant species,showed leaf a slight characteristics advantage over and th RHe empty were chamber. investigated Relatively as follows.stable removal rates were observed in the three chambers with Sansevieria, Spathiphyllum floribundum and Ficus elastica, while for 3.2.1. InfluenceSansevieria of, the Plant PM2.5 Species removal rate increased at high PM2.5. Figure 4 showed removal rates of potted plants at different initial PM2.5 concentrations for every Ashour. seen PM in Figures2.5 removal3 andrates4 ,in the the decline test chamber rates with of PM potted2.5 plantsparticles were in higher chambers than that with of the diff emptyerent potted plants werechamber considerably in the first hour. different. It decreased Significant with time removal in the next effi twociency hours, (71.5% except at for high Sansevieria PM2.5, andwhich 59.2% at exhibited a higher removal rate in the third hour than that in the second hour. low PM2.5) was observed in the chamber with Epipremnum aureum, while the chamber with Aloe vera When ignoring the gravity sedimentation of PM2.5 itself, the additional removal rates for Aloe showed a slight advantage over the empty chamber. Relatively stable removal rates were observed in vera and Epipremnum aureum were 5.2% and 30% respectively, when the initial PM2.5 concentration the threewas chambers around 200 with μg/mSansevieria3. The corresponding, Spathiphyllum values floribundumwere 0% and 17.2%,and Ficus respectively, elastica, when while the for initialSansevieria , the PM2.5PMremoval2.5 was around rate 300 increased μg/m3. This at high indicated PM 2.5that. there were other reasons accounting for these results.

Empty chamber (a) (b) Chlorophytumcomosum 40 30 Spathiphyllum floribundum 35 Epipremnum aureum 25 30 Ficuselastica 25 Sansevieria 20 Aloe vera 20 15 15 removal rate (%) rate removal 10 removal rate (%) 2.5

2.5 10

PM 5 PM 5 0 0 123 123 Time (h) Time (h)

Figure 4.FigureRemoval 4. Removal rate perrate hourper hour for for PM PM2.52.5under under didifferentﬀerent initial initial PM PM2.5 2.5concentrations:concentrations: (a) initial (a) initialPM2.5 PM2.5 was around 200 3μg/m3, (b) initial PM2.5 was around 300 μg/m3. 3 was around 200 µg/m ,(b) initial PM2.5 was around 300 µg/m .

3.2.2. Leaf Surface Characteristics and LAI Figure4 showed removal rates of potted plants at di ﬀerent initial PM2.5 concentrations for every hour. PM2.5Theremoval removal rates of PM in pollutants the test chamber from the air with by pottedplants was plants typically were due higher to deposition than that [35]. of The the empty chambermorphological in the ﬁrst hour.features It decreasedof a leaf are withconsidered time into thebe an next important two hours, factor exceptaffecting for PMSansevieria deposition , which [18,36]. Studies have shown that leaves with fine groove-like tissues, fluff, sticky, waxes and exhibited a higher removal rate in the third hour than that in the second hour. roughened surfaces was conducive to the adsorption of PM [18,37,38]. Moreover, leaf surface Whenroughness ignoring was the closely gravity related sedimentation to the number of PM of2.5 retaineditself, theparticulates, additional with removal rougher rates surfaces for Aloe vera and Epipremnumcorresponding aureum to morewere rugged 5.2% folds and and 30% grooves respectively, and a stronger when theretention initial ability PM2.5 [38].concentration However, was 3 aroundleaves 200 µ gwith/m .relatively The corresponding smooth leaf surfaces, values and were grooves 0% and that 17.2%, are shallow respectively, and sparse when have therelatively initial PM2.5 was aroundweak 300dustµ retentiong/m3. This abilities indicated [39]. that there were other reasons accounting for these results. All six potted plants have a waxy layer on the leaf surface. Images from inverted fluorescence microscope observation showed that there were roughened surfaces with grooves on the leaves of Chlorophytum comosum, Spathiphyllum floribundum, and Epipremnum aureum; Chlorophytum comosum Sustainability 2019, 11, 2546 6 of 10

3.2.2. Leaf Surface Characteristics and LAI The removal of PM pollutants from the air by plants was typically due to deposition [35]. The morphological features of a leaf are considered to be an important factor affecting PM deposition [18,36]. Studies have shown that leaves with fine groove-like tissues, fluff, sticky, waxes and roughened surfaces was conducive to the adsorption of PM [18,37,38]. Moreover, leaf surface roughness was closely related to the number of retained particulates, with rougher surfaces corresponding to more rugged folds and grooves and a stronger retention ability [38]. However, leaves with relatively smooth leaf surfaces, and grooves that are shallow and sparse have relatively weak dust retention abilities [39]. All six potted plants have a waxy layer on the leaf surface. Images from inverted fluorescence microscope observation showed that there were roughened surfaces with grooves on the leaves of Sustainability 2019, 11, x FOR PEER REVIEW 6 of 10 Chlorophytum comosum, Spathiphyllum floribundum, and Epipremnum aureum; Chlorophytum comosum and andEpipremnum Epipremnum aureum aureumhad obvious had obvious ridges onridges their on leaf their surface, leaf assurface, shown as in Figureshown5 .in By Figure comparison, 5. By comparison,Ficus elastica andFicusAloe elastica vera showedand Aloe less vera rough showed and flat less surfaces, rough whichand flat were surfaces, not conducive which forwere dust not to conducivesettle. A distinctive for dust roughto settle. and A tumorous distinctive leaf rough surface and structure tumorous appeared leaf surface in Sansevieria structure. In appeared general, thein Sansevieriahigh removal. In general, rate of PM the in high test removal chambers rate with of PMChlorophytum in test chambers comosum withand ChlorophytumEpipremnum comosum aureum wasand Epipremnumcorrelated with aureum their was leaf correlated characteristics. with their leaf characteristics.

Figure 5. Leaf morphologies of six potted plants (10 10): (a) Chlorophytum comosum,(b) Spathiphyllum × Figurefloribundum 5. Leaf,(c morphologies) Epipremnum aureumof six potted,(d) Ficus plants elastica (10 ×,( 10):e) Sansevieria (a) Chlorophytum,(f) Aloe comosum vera. , (b) Spathiphyllum floribundum, (c) Epipremnum aureum, (d) Ficus elastica, (e) Sansevieria, (f) Aloe vera. Other studies have shown that the LAI of a plant is positively correlated with the dust retention capacityOther of studies the plant have [40 shown]. After that measurement the LAI of a and plant calculation, is positively the correlated order of LAI with for the six dust potted retention plants capacityused in thisof the study plant was [40]. as After follows: measurementEpipremnum and aureum calculation,(2.27) >theFicus order elastica of LAI(1.41) for six> Spathiphyllumpotted plants usedfloribundum in this (1.09)study> wasAloe as vera follows:(0.83) Epipremnum> Chlorophytum aureum comosum (2.27)(0.51) > Ficus> Sansevieria elastica (1.41)(0.26). > Spathiphyllum Considering floribundumthe leaf surface (1.09) structure > Aloe vera presented (0.83) > above,ChlorophytumEpipremnum comosum aureum (0.51)had > Sansevieria favorable leaf(0.26). characteristics Considering andthe leafthe highestsurface LAIstructure value, presented which was above, consistent Epipremnum with the aureum highest had PM favorable2.5 removal leaf rate characteristics in the test chamber. and the highestHowever, LAI the value, lowest which PM2.5 wasremoval consistent rate occurredwith the inhighest the test PM chamber2.5 removal with rateAloe in vera the, nottestSansevieria chamber. However,with the lowest the lowest LAI value. PM2.5 Theseremoval findings rate occurred suggest in that the PM test2.5 chamberremoval ratewith was Aloe not vera always, not Sansevieria positively withcorrelated the lowest with theLAI LAI value. value. These findings suggest that PM2.5 removal rate was not always positively correlated with the LAI value. 3.2.3. Influence of RH 3.2.3.Due Influence to the diofff RHerence among season, location, and meteorological variables, as well as characteristics of PMDue2.5, theto correlationthe difference between among RH andseason, PM2.5 locationdiffered, inand most meteorological studies [41–43 ].variables, However, as there well were as characteristicssome consistent of conclusions: PM2.5, the correlation RH was positively between correlated RH and with PM2.5 sulfate differed and in nitrate most concentration, studies [41–43]. but However,negatively there associated were some with organicconsiste carbonnt conclusions: (OC) and RH elemental was positively carbon (EC)correlated concentration with sulfate [1,44 and,45]. nitrateAccording concentration, to a study, totalbut negative carbon (TC)ly associated accounts forwith about organic 66% carbon of the total(OC) mass and elemental of PM2.5 after carbon cigarette (EC) concentration [1,44,45]. According to a study, total carbon (TC) accounts for about 66% of the total mass of PM2.5 after cigarette burning, and OC accounts for the majority of TC [46]. In this study, cigarette smoke was used as the source of PM2.5, which resulted in a high carbon content in PM2.5 components. The hygroscopicity of PM particles increased with the increase of RH, especially when the humidity was above 65%, which promoted the aggregation of fine particles into larger particles [47]. Figure 6 showed that the trend of RH curve for the same plant under different PM2.5 conditions was almost the same. Moreover, the six potted plants can be classified into two groups. The first was group A, which included Epipremnum aureum, Chlorophytum comosum and Spathiphyllum floribundum, showing a fast-rising trend in RH in early stage of the experiment, and RH in the group A chambers increased to 65% in a very short time, i.e., 30 min for (a) and 60 min for (b). The other three plants were assigned to group B. RH in the group B chambers increased slowly and finally remained around 70%. The time that group B spent to attain the same RH (65%) was twice as long as group A. The RH in the empty chamber remained stable throughout the experiment. Sustainability 2019, 11, 2546 7 of 10 burning, and OC accounts for the majority of TC [46]. In this study, cigarette smoke was used as the source of PM2.5, which resulted in a high carbon content in PM2.5 components. The hygroscopicity of PM particles increased with the increase of RH, especially when the humidity was above 65%, which promoted the aggregation of fine particles into larger particles [47]. Figure6 showed that the trend of RH curve for the same plant under different PM2.5 conditions was almost the same. Moreover, the six potted plants can be classified into two groups. The first was group A, which included Epipremnum aureum, Chlorophytum comosum and Spathiphyllum floribundum, showing a fast-rising trend in RH in early stage of the experiment, and RH in the group A chambers increased to 65% in a very short time, i.e., 30 min for (a) and 60 min for (b). The other three plants were assigned to group B. RH in the group B chambers increased slowly and finally remained around 70%. The time that group B spent to attain the same RH (65%) was twice as long as group A. The RH in the empty chamber remained stable throughout the experiment. Sustainability 2019, 11, x FOR PEER REVIEW 7 of 10 Combining the above PM2.5 data, the three potted plants that caused the fastest decline of PM2.5 belonged to group A when initial PM was around 200 ug/m3, which confirmed that the PM Combining the above PM2.5 data,2.5 the three potted plants that caused the fastest decline of PM2.5 2.5 depositionbelonged was positivelyto group A correlated when initial with PM the2.5 was increase around of 200 RH ug/m in this3, which study. confirmed However, that the the corresponding PM2.5 relationshipdeposition changed was withpositively the increasecorrelated of with PM 2.5the; theincrease order of of RHSpathiphyllum in this study. floribundum However, inthe group A was replacedcorresponding by Sansevieria relationshipin groupchanged B with (Figure the increase6(b)), theof PM most2.5; the likely order reasonof Spathiphyllum being the floribundum inadaptability of Spathiphyllumin group A floribundumwas replaced toby highSansevieria PMin in itsgroup flowering B (Figure period. 6(b)), the The most underlying likely reason causes being needthe to be inadaptability of Spathiphyllum floribundum to high PM in its flowering period. The underlying causes further explored. need to be further explored.

(a) 100 (b) 100 Empty chamber Chlorophytum comosum 95 95 Spathiphyllum floribundum 90 90 85 Epipremnum aureum 85 Ficus elastica Sansevieria Aloe vera 80 80 75 75 70 70 65 65 RH (%) RH (%) 60 60 55 55 50 50 45 45 40 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 Time (min) Time (min)

Figure 6.FigureVariations 6. Variations of RH of inRH the in the test test chamber chamber during during PM 2.52.5 monitoring:monitoring: (a) initial (a) initial PM2.5 PMwas2.5 aroundwas around 3 3 33 200 µg/m200 ,(μg/mb) initial, (b) initial PM2.5 PMwas2.5 was around around 300 300 µμgg/m/m. .

3.3. Actual3.3. Actual Indoor Indoor Study Study

On the basis of the PM2.5 removal performance of the six potted plants, Epipremnum aureum was On the basis of the PM2.5 removal performance of the six potted plants, Epipremnum aureum was selected for the actual study. The initial PM2.5 concentrations in both rooms were around 250 μg/m³ 3 selected for the actual study. The initial PM2.5 concentrations in both rooms were around 250 µg/m (Figure 7). At 20 min, PM2.5 concentration in the room with Epipremnum aureum started lower than (Figure7). At 20 min, PM concentration in the room with Epipremnum aureum started lower than the the empty room, in the2.5 entire evaluation period. Final PM2.5 concentrations of the two rooms were empty room,148 (empty in the room) entire and evaluation 135 μg/m³ period. (room Final with PMEpipremnum2.5 concentrations aureum), respectively, of the two roomsand the were 148 (emptycorresponding room) and 135 removalµg/m rates3 (room were with40.8%Epipremnum (empty room) aureumand 46.0%), respectively,(room with Epipremnum and the aureum corresponding), removalrespectively. rates were Results 40.8% from (empty the actual room) study and 46.0%indicated (room that the with presenceEpipremnum of Epipremnum aureum ),aureum respectively. Resultsenhanced from the actualthe deposition study indicatedof PM2.5, although that the the presence performance of Epipremnum was not soaureum positiveenhanced as that in the test deposition chamber. The possible reasons were as follows: (i) particulate pollutants were transported to leaf of PM , although the performance was not so positive as that in the test chamber. The possible reasons 2.5surfaces by gravity sedimentation or impaction [31], while the Epipremnum aureum were too small were as(about follows: 30 cm) (i) in particulate comparison pollutants with indoor were height transported (280 cm). They to cannot leaf surfaces provide more by gravity contact sedimentationarea for or impactionPM2.5 deposition, [31], while (ii) theeightEpipremnum pots of Epipremnum aureum aureumwere were too not small enough (about to significantly 30 cm) in increase comparison RH with 2 indoor heightof the room (280 cm).with They16 m , cannot resulting provide in poor more aggregation contact settlement, area for PM and2.5 (iii)deposition, the experimenter's (ii) eight pots of Epipremnummovement aureum maywere cause notresuspen enoughsion of to fine signiﬁcantly particles. increase RH of the room with 16 m2, resulting in poor aggregation settlement,260 and (iii) the experimenter’s movement may cause resuspension of Epipremnum aureum ﬁne particles. 240 Empty room 220 ) 3 200

ug/m 180 (

2.5 160 PM 140 120 0 20 40 60 80 100 120 140 160 180 200 Time (min) Sustainability 2019, 11, x FOR PEER REVIEW 7 of 10

Combining the above PM2.5 data, the three potted plants that caused the fastest decline of PM2.5 belonged to group A when initial PM2.5 was around 200 ug/m3, which confirmed that the PM2.5 deposition was positively correlated with the increase of RH in this study. However, the corresponding relationship changed with the increase of PM2.5; the order of Spathiphyllum floribundum in group A was replaced by Sansevieria in group B (Figure 6(b)), the most likely reason being the inadaptability of Spathiphyllum floribundum to high PM in its flowering period. The underlying causes need to be further explored.

Figure 6. Variations of RH in the test chamber during PM2.5 monitoring: (a) initial PM2.5 was around

200 μg/m3, (b) initial PM2.5 was around 300 μg/m3.

3.3. Actual Indoor Study

On the basis of the PM2.5 removal performance of the six potted plants, Epipremnum aureum was selected for the actual study. The initial PM2.5 concentrations in both rooms were around 250 μg/m³ (Figure 7). At 20 min, PM2.5 concentration in the room with Epipremnum aureum started lower than the empty room, in the entire evaluation period. Final PM2.5 concentrations of the two rooms were 148 (empty room) and 135 μg/m³ (room with Epipremnum aureum), respectively, and the corresponding removal rates were 40.8% (empty room) and 46.0% (room with Epipremnum aureum), respectively. Results from the actual study indicated that the presence of Epipremnum aureum enhanced the deposition of PM2.5, although the performance was not so positive as that in the test chamber. The possible reasons were as follows: (i) particulate pollutants were transported to leaf surfaces by gravity sedimentation or impaction [31], while the Epipremnum aureum were too small (about 30 cm) in comparison with indoor height (280 cm). They cannot provide more contact area for PM2.5 deposition, (ii) eight pots of Epipremnum aureum were not enough to significantly increase RH Sustainability 2019, 11, 2546 8 of 10 of the room with 16 m2, resulting in poor aggregation settlement, and (iii) the experimenter's movement may cause resuspension of fine particles.

260 Epipremnum aureum 240 Empty room 220 ) 3 200

ug/m 180 (

2.5 160 PM 140 120 0 20 40 60 80 100 120 140 160 180 200 Time (min) Figure 7. Temporal variation curves of PM2.5 concentrations in an actual indoor environment.

4. Conclusions

Potted plants in a sealed chamber have the potential to improve PM2.5 deposition in this study, especially those with rough and groove leaf surfaces, as well as high LAI, like Epipremnum aureum. The presence of Epipremnum aureum in the test chamber promoted the removal rates of PM2.5 to 71.46% 3 3 (when initial PM2.5 was around 200 µg/m ) and 59.25% (when initial PM2.5 was around 300 µg/m ) in three hours, respectively, while the removal rate of PM2.5 in the empty chamber was around 42.0% under both conditions. In the actual indoor study, the effect of Epipremnum aureum on deposition was not as significant as the test chamber. Other factors, such as plant height and density, should be taken into account in terms of indoor applications. In general, a certain number of potted plants like Epipremnum aureum are beneficial in reducing indoor PM2.5, while in practical application, considering the limitations of indoor spaces, indoor vertical greenery will be an optimum method to promote PM2.5 settlement. In addition, there exist inadequate aspects in this research. A quantitative study on the contribution of each factor need to be further undertaken.

Author Contributions: Y.C. organized this study, conducted the study design, and drafted the manuscript. F.L. contributed to the study design, and revision of the manuscript. Y.W. contributed to the conduct of the experiment. Y.Y. contributed to the conduct of the experiment. Z.W. contributed to purchase of the plants and participated in the actual study. X.L. contributed to calculation of LAI. K.D. contributed to observation of leaf characteristics. Funding: This study was financially supported by the Humanities and Social Sciences Foundation of Ministry of Education of China (Youth Fund: 17YJCZH081), the Nature Science Foundation of Hubei Province (2018CFB722), and Fundamental Research Funds for the Central Universities from Zhongnan University of Economics and Law (2722019JCG073). Conflicts of Interest: The authors declare no conflict of interest.

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