Environ. Sci. Technol. 2009, 43, 8338–8343
Uptake of Aldehydes and Ketones at Typical Indoor Concentrations by Houseplants A K I R A T A N I * ,† A N D C. NICHOLAS HEWITT‡ Institute for Environmental Sciences, University of Shizuoka, 52-1 Yada, Shizuoka 422-8526, Japan, and Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, United Kingdom
Received July 13, 2009. Revised manuscript received September 18, 2009. Accepted September 22, 2009.
The uptake rates of low-molecular weight aldehydes and ketones by peace lily (Spathiphyllum clevelandii) and golden pothos (Epipremnum aureum) leaves at typical indoor ambient concentrations (101-102 ppbv) were determined. The C3-C6 aldehydes and C4-C6 ketones were taken up by the plant leaves, but the C3 ketone acetone was not. The uptake rate normalized to the ambient concentration Ca ranged from 7 to 19 mmol m-2 s-1 and from 2 to 7 mmol m-2 s-1 for the aldehydes and ketones, respectively. Longer-term fumigation results revealed that the total uptake amounts were 30-100 times as much as the amounts dissolved in the leaf, suggesting that volatile organic carbons are metabolized in the leaf and/ or translocated through the petiole. The ratio of the intercellular concentration to the external (ambient) concentration (Ci/Ca) was significantly lower for most aldehydes than for most ketones. In particular, a linear unsaturated aldehyde, crotonaldehyde, had a Ci/Ca ratio of ∼0, probably because of its highest solubility in water.
Introduction Large amounts of anthropogenic volatile organic compounds (VOCs) are continuously emitted to the atmosphere. Many of these compounds, including aromatic hydrocarbons, aldehydes, and ketones, are toxic to humans because of their carcinogenic and mutagenic properties and their capacity to form toxic and phytotoxic radical intermediates (1). In addition to being toxic, some are irritants. Nonoccupational human exposure to these compounds is largely determined by their concentrations in indoor air because on average urban dwellers spend up to 90% of their time indoors (2). Many of these compounds are emitted from materials that are common indoors, including fabrics, carpets, glues, and building materials (3), and there is therefore considerable interest in developing low-cost and reliable methods for reducing the concentrations of these compounds in indoor air. It is well-known that plants can remove gases from the atmosphere by absorption through the stomatal apertures and by adsorption on the cuticle or outer surface of the leaf. It has been shown that some plants can remove aldehydes * Corresponding author phone +81-54-264-5788; fax +81-54-2645788; e-mail;
[email protected]. † University of Shizuoka. ‡ Lancaster University. 8338
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(4) and ketones (5) from the atmosphere, and it has even been suggested that plants may be used to cleanse indoor air of pollutant VOCs (6), although the experimental evidence for this is rather tenuous. However, few quantitative and reliable measurements have been made of the uptake rates of low-molecular weight aldehydes and ketones by indoor house plants. To measure VOC uptake rates by plants, two methods have been applied. The first is the static chamber method, in which VOC concentrations in the chamber are periodically measured and the uptake rate determined from the rate of change in VOC concentrations in the chamber (7, 8). Loss of soluble compounds to condensed water, soil, and other surfaces is likely to be major problem with this method. The second method uses a flow-through chamber (4, 9, 10) in which the VOC uptake rate is determined from the concentration difference between the inflowing and outflowing air (4, 10). Condensation is easier to avoid with this method. Most previous plant uptake studies have been conducted at unrealistically high concentrations of VOCs (mixing ratios of 10-6) because of the sensitivity limitations of, for example, gas chromatography. However, a highly sensitive and rapid analytical method, proton transfer reaction mass spectrometry (PTR-MS), allows the determination of VOC concentrations at mixing ratios of around 10-8 (10 ppbv) with a precision of 1-1.5% in humid air (10, 11). In order to investigate the uptake capacity of house plants for low-molecular weight ketones and aldehydes at typical indoor mixing ratios of 10-8 to 10-7 (10-100 ppbv), we used PTR-MS combined with a flow through chamber method.
Experimental Section Plant Materials. Four plants (50-70 cm height) were obtained of each of the common houseplant species peace lily (Spathiphyllum clevelandii) and golden pothos (Epipremnum aureum). The plants were acclimated to indoor air for one month prior to the experiments. VOC Exposure and Measurement System. The VOC exposure and measurement system has been described previously (10), and therefore only a brief description is given here. The system consisted of three parts: preparation of an air flow containing known and constant concentrations of specific VOC species, a fumigation chamber, and an analytical system (see Figure S1 in Supporting Information or Figure 1 in the related paper by Tani et al. (10)). An attached leaf of the test plant, of which projected leaf area was 60-150 cm2, was enclosed in a transparent PFA bag (20-40 L volume). The open side of the bag was tightly closed with a cable tie, and the enclosed volume was less than 20 L. VOC-rich air was introduced into the bag via the inlet port at a flow rate of 1.3 L min-1. A 99% equilibrium time for VOC concentration was 98%. Most of the impurities had molecular weights different from those of the target VOC and, therefore, did not interfere with the PTR-MS measurement of the target VOC. The VOC uptake measurement was conducted 4-7 times for each compound and for each plant species. The fumigation period was 6-8 h or, occasionally, 24-30 h, including an 8 h dark period. Determination of VOC Uptake Rate. The calculation of the VOC uptake rate A (mol m-2 s-1) by an exposed leaf was described previously (10), and therefore only a brief description is given here. A is determined by mass balance in the enclosure bag and can be calculated using A × LA + B ) Massin - Massout
(1)
where LA is the leaf area (m2), and B is VOC loss in the enclosure bag (mol s-1). Massin and Massout are VOC influx and efflux, respectively, per second (mol s-1) and determined from Massin ) Vin × Cin
(2A)
Massout ) Vout × Cout
(2B)
where Vin and Vout are flow rates of the inlet and outlet air respectively (mol s-1) and Cin and Cout are the humiditycorrected VOC concentrations of the inlet and outlet air respectively (mol mol-1). VOC concentrations were corrected
number of carbon
molecular weight
rational formula
concentration range (ppbv)
3 3 4 4 4 4 4 5 5 5 5 5 6 7
58 58 72 72 70 70 72 86 86 86 86 86 100 106
CH3CH2CHO (CH3)2CO CH3(CH2)2CHO (CH3)2CHCHO CH3CHdCHCHO CH2dC(CH3)CHO CH3CH2(CH3)CO CH3(CH2)3CHO (CH3)2CHCH2CHO (CH3CH2)2CO CH3CH2CH2(CH3)CO (CH3)2CH(CH3)CO (CH3)2CHCH2(CH3)CO C6H5CHO
14-150 45-600 80-320 70-300 70-130 22-210 40-400 80-180 80-280 24-240 68-650 650 80-820 50-310
for humidity as described in detail previously (12, 13). A brief description is also given in the Supporting Information. To estimate the minimum detectable relative uptake rate, relative error (A) is calculated from Relative error (A) )
EMassin - Massout
(3)
Massin
where EMassin-Massout represents the total error in calculating Massin - Massout and relative error (A) represents the overall relative error for the whole measurement system (10). This value represents the minimum significant difference between influx and efflux concentrations of the target VOC. VOC Loss in the Enclosure Bag. In order to determine if VOCs are lost to the enclosure system, two blank measurements were carried out, first using the experimental system without plant material being present and second measuring the outflowing air stream from the bag to which the outflows from the permeator and dew point generator were individually introduced. This measurement was conducted in order to investigate the humidity dependence of the blank concentration of the target VOCs. The details of the two measurement protocols are described previously (10). Determination of Stomatal Conductance and Intercellular VOC Concentration. To calculate stomatal conductance gs and intercellular VOC concentration Ci, leaf boundary layer conductance gb was measured using leaf-shaped boards covered with thin paper. The board was wetted before measurements and was enclosed by the bag to measure evaporation rate and surface temperature. Six boards with different areas, three shaped like peace lily leaves and three like golden pothos leaves, were used for the measurement to obtain the relationship between leaf area and gb. Ci and gs were determined following the calculation procedure for CO2 described in appendix 2 of the paper by Caemmerer and Farquhar (14). If the calculated Ci is below 0, the Ci is assumed to be 0 because a negative value for the intercellular concentration is unrealistic.
Results Blank Measurements. No loss of any of the target VOC was observed to the bag surface in the two blank measurements. The water vapor concentration was raised to 0.02 mol/mol in the bag in the second blank measurement, but because it was lower than the saturated concentration of the outside air, we did not observe any visible water condensation onto the bag surface or any measurable VOC losses inside. When a plant leaf was present in the bag in clean air, a small difference between the inlet and outlet concentrations was observed. The difference was less than 0.2 ppbv for mass VOL. 43, NO. 21, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Changes in propionaldehyde (A,B) concentrations in the inlet and outlet air streams of the bag enclosing a peace lily leaf. Blank measurement data indicate the concentrations of inlet and outlet VOCs in the bag without plant material present, measured every other 5 min. 59 and negligibly small (
