Environ. Sci. Technol. 2001, 35, 560-567
Dynamic Behavior of Semivolatile Organic Compounds in Indoor Air. 2. Nicotine and Phenanthrene with Carpet and Wallboard M I C H A E L D . V A N L O Y , †,‡,§ W I L L I A M J . R I L E Y , †,‡ J O A N M . D A I S E Y , ‡,| A N D W I L L I A M W . N A Z A R O F F * ,†,‡ Department of Civil and Environmental Engineering, University of CaliforniasBerkeley, Berkeley, California 94720-1710, and Indoor Environment Department, Environmental Energy Technologies Division, E. O. Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720
The surface interactions of nicotine and phenanthrene with carpet, painted wallboard, and stainless steel were investigated in a room-sized environmental test chamber. Adsorption kinetics were tested by flash evaporating a known mass of each compound into a sealed 20 m3 chamber containing one or more of the tested sorbents. In each experiment, one or more emissions were performed after the gas-phase concentration had reached an apparent plateau. At the end of each experiment, the chamber was ventilated and resealed to monitor reemission of the compound from the sorbents. Kinetic sorption parameters were determined by fitting a mass-balance model to the experimental results. The sorption capacity of stainless steel was of similar magnitude for nicotine and phenanthrene. Sorption of nicotine on carpet and wallboard was much stronger, with equilibrium partitioning values 2-3 orders of magnitude higher. The sorption capacities of phenanthrene on carpet and wallboard were smaller, approximately 1020% of the stainless steel values. The rates of uptake are of similar magnitude for all sorbate-sorbent pairs and are consistent with the limit imposed by gas-phase boundarylayer mass transport. The rates of desorption are much faster for phenanthrene than for nicotine. Model simulations predict average nicotine levels in a typical smoking residence that are consistent with published data.
Introduction Organic compounds are important indoor air pollutants, and a significant body of research has focused on factors affecting their concentration and persistence. However, much of that research has been directed at low molecular weight species commonly known as volatile organic compounds (VOCs). Organic compounds with vapor pressures between 10-6 and 10 Pa at ambient temperatures are classified as semivolatile * Corresponding author phone: (510)642-1040; fax: (510)642-7463, e-mail:
[email protected]. † University of CaliforniasBerkeley. ‡ E. O. Lawrence Berkeley National Laboratory. § Present address: EPRI, 3412 Hillview Ave., Palo Alto, CA 943041395. | Deceased. 560
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organic compounds (SVOCs) (1). Relatively few studies have focused on these pollutants in indoor air. The importance of sorption on indoor materials has been demonstrated for many VOCs (2-8). Because of their lower vapor pressure, sorption and desorption processes may have an even greater impact on SVOCs. Only recently has research on the interactions of SVOCs with indoor surfaces begun to be reported (7, 9-12). Most buildings have a large surface area-to-volume ratio, so sorption can significantly affect the dynamic behavior of indoor air pollutants. Because reversibly sorbed compounds may reenter the gas phase through desorption from surfaces, occupant exposures may occur long after elimination of sources. Accurate knowledge about the surface interactions of SVOCs indoors is important for assessing and mitigating health risks from inhalation of indoor air as well as for improving perceived indoor air quality. Nicotine (C10H14N2, molecular mass ) 162.2 g mol-1) is the most prevalent organic constituent of environmental tobacco smoke (ETS). Nicotine’s vapor pressure at 298 K is 2.7 Pa (13), and it is present in ETS almost entirely in the gas phase (14, 15). Approximately 400 compounds have been identified and measured in ETS (15, 16). ETS has been identified as a human carcinogen (17, 18). There is now evidence that it may also be a cause of heart disease (19). Because of the complexity of ETS and because of its potentially serious adverse health effects, marker compounds are sought that can be used for measuring human exposure (20, 21). Nicotine has been widely used as a marker because (i) it is specific to and a major constituent of ETS; (ii) it is easy to detect; and (iii) it has similar emission rates for different cigarette types (22, 23). Typical time-averaged concentrations of nicotine in homes with smokers are ∼2-4 µg m-3 (24, 25). Also, cotinine, a metabolic byproduct found in body fluids of those exposed to nicotine, has been used as a biomarker of ETS exposure (26, 27). Because gas-phase nicotine exhibits different dynamic behavior than do many other ETS constituents, the suitability of nicotine as a marker for ETS has been questioned (28, 29). Elucidation of the factors affecting nicotine concentrations in indoor environments would improve the basis for using nicotine and cotinine to assess ETS exposures. Furthermore, because of nicotine’s polar functional groups, it may be a useful surrogate for other SVOCs with similar moieties, such as amines and carbonyls, which generally have lower odor and irritation thresholds than pure hydrocarbons (30). Phenanthrene (C14H10, molecular mass ) 178.2 g mol-1) is a three-ring polycyclic aromatic hydrocarbon (PAH) with a vapor pressure of approximately 0.02 Pa at room temperature (31). It is found in petroleum products and is emitted by incomplete combustion. Limited field data suggest that typical concentrations in indoor air are ∼20-50 ng m-3 (32, 33). Phenanthrene is not a known human carcinogen, but its behavior may be similar to that of other condensable, potentially carcinogenic PAHs and other nonpolar SVOCs with high molecular weights, such as polychlorinated biphenyls (PCBs), pesticides, and dioxins, whose surface interactions may depend more on physical sorption than on chemical interactions. In a previous paper (10), we described the interactions of nicotine with the interior surfaces of the 20 m3 stainless steel chamber used in the current study. Time-dependent gasphase concentrations were measured following pulsed releases of nicotine vapor into the unventilated chamber. A dynamic mass-balance model incorporating reversible sorption was fit to the data to extract kinetic parameters. 10.1021/es001372a CCC: $20.00
2001 American Chemical Society Published on Web 12/30/2000
Equilibrium partitioning coefficients were measured in independent experiments and used to reduce the number of fitted kinetic parameters. The results of that study show that nicotine interacts strongly with stainless steel, with a large proportion of the emitted mass sorbed at equilibrium. The time scale for sorptive uptake was on the order of 1 h, which is comparable to the ventilation time scale in occupied buildings. In the present paper, a similar experimental approach was applied in five experiments to investigate the sorption dynamics for each of the following sorbate-sorbent pairs: nicotine-carpet, nicotine-painted wallboard, phenanthrenestainless steel, phenanthrene-carpet, and phenanthrenepainted wallboard. The time-dependent gas-phase concentration of each SVOC with each sorbent was measured in a sealed environmental chamber with a very low air-exchange rate over periods ranging from 16 to 155 days. The gas-phase concentration was monitored following flash evaporations of the tested compound in the chamber. After multiple cycles of SVOC emission and uptake by the materials, the chamber was ventilated at a high air-exchange rate for a few days to reduce the gas-phase SVOC concentration. Finally, the chamber was sealed to observe reemission of sorbed mass. The gas-phase data were interpreted with a dynamic massbalance model to estimate sorption parameters for each sorbate-sorbent pair. To illustrate the significance of the results, the model and sorption parameters were then applied to predict the evolution of nicotine in a residence in which smoking habitually occurs.
Methods and Materials Adsorbents and Reagents. Reagent-grade nicotine and phenanthrene (CAS Registry Nos. 54-11-5 and 85-01-8, respectively; Aldrich Chemicals, Milwaukee, WI) were used in this study. Standard solutions used for calibration of analytical instruments and for sample internal standards were prepared with HPLC-grade methanol (Burdick and James) in glassware washed with a saturated solution of potassium hydroxide in ethanol and rinsed with deionized water. To prevent loss of nicotine onto glassware, all nicotine standard solutions were prepared with methanol modified with 0.01% v/v triethylamine (TEA) (34). This treatment was not used in phenanthrene solutions. Unused, 3-year-old carpet with an approximately 1-cmdeep pile of nylon fibers and backing consisting of a coarse polypropylene mesh bonded to the primary backing material with styrene-butadiene rubber latex adhesive was used in the experiments. The backing layer was 0.24 ( 0.03 cm thick. No stain resistance or other treatment was applied to the carpet. Gypsum wallboard panels (1.2 m × 2.4 m × 1 ( 0.1 cm) were covered on one side with approximately 700 mL of flat white indoor latex paint (Sherwin Williams Classic 99) applied with a roller. The average thickness of the applied paint layer was 0.02 cm based on wet volume. After being painted, the panels were stored in a warehouse for 0.5 yr prior to use in experiments. Stainless Steel Test Chamber. Experiments were conducted in the same 20 m3 environmental test chamber employed for our earlier studies (10). All internal surfaces (area ) 45.2 m2) were clad with Type 304 stainless steel. A schematic diagram of the chamber as configured for the present study is shown in Figure 1. As in the earlier experiments, six 8-cm-diameter wall-mounted fans aligned with the blade axes at a 45° angle to the wall surface and parallel to the floor created well-mixed conditions during the experiments. For experiments with carpet, a sample measuring 3.6 × 2.1 m covered most of the chamber floor. For experiments with wallboard, pairs of painted panels were bolted together
FIGURE 1. Schematic of environmental test chamber. with the painted sides facing outward. The edges were sealed with aluminized furnace tape so that each pair of panels had an exposed painted wallboard surface area of 5.7 m2. Two pairs of panels were exposed simultaneously (Figure 1). The panels were supported by a wood frame, which was covered with aluminum foil. The total exposed area of aluminum (tape and foil) in the wallboard experiments was approximately 0.015 m2 (compared to 45 m2 of stainless steel and 11.4 m2 of wallboard). In modeling, sorption on aluminum surfaces was assumed to be negligible. The temperature and relative humidity inside the chamber were uncontrolled but fairly consistent during the initial, sealed-chamber phase of each experiment at 23 ( 4 °C and 55 ( 12%, respectively. The air-exchange rate with the chamber sealed was determined during two of the chamber experiments by SF6 tracer gas decay and found to be approximately 0.008 h-1 in each test. The temperature and relative humidity did not vary by more than 2 °C and 6%, respectively, during the sealed-chamber period of each run. However, the temperature and relative humidity dropped to 14 ( 5 °C and 25 ( 15%, respectively, during the ventilation phases of the nicotine-carpet and nicotine-wallboard experiments, which were conducted in January during cold and dry weather conditions. After the chamber was resealed, the temperature and relative humidity stabilized at 20 ( 3 °C and 35 ( 5%, respectively, during the reemission phase. Temperature and relative humidity variations during the ventilation phases of the phenanthrene experiments were smaller because these experiments were conducted during April and September when the outdoor weather conditions were milder. Because the gas-phase concentrations during these phases of the experiments were very small, we do not believe that changes in temperature and relative humidity are important in interpreting these experiments. However, we note that water molecules may compete with SVOCs for sorption sites on the surfaces of the tested materials, and so changes in absolute humidity might have affected the results. Prior to each experiment, the chamber’s stainless steel surfaces were washed with a phosphoric acid-based detergent (Heavy Duty LC-30, EcoLab), followed by an alkaline detergent (Kart-Klenz) to remove residual sorbate from the stainless steel surfaces and to provide a consistent starting condition. After each detergent application, the walls were rinsed thoroughly with tap water and then with deionized water and dried. Before use, the chamber was closed and ventilated at 40 m3 h-1 for 2 days with HEPA-filtered and granulated activated-carbon-filtered outdoor air to allow equilibration with ambient humidity. After 2 days, the chamber was entered to install the tested sorbent and then VOL. 35, NO. 3, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Summary of Experimental Conditions parameter sorbate sorbenta duration of experiment (h) sealed air-exchange rate (h-1) sealed periods (h)b ventilated air-exchange rate (h-1) ventilated period (h)b total SVOC mass emitted (mg) no. of emission events amounts emitted in each event (mg) emission times (h)b
experiment 2
experiment 3
nicotine carpet 1300 0.008 0-1106, 1173-1300 1.0
experiment 1
nicotine wallboard 1700 0.008 0-1232, 1467-1700 1.0
phenanthrene stainless steel 380 0.008 0-215, 291-380 3.0
phenanthrene carpet 3700 0.008 0-3453, 3477-3700 1.0
phenanthrene wallboard 1300 0.008 0-1061, 1127-1300 1.0
1106-1173 250 5 50 each
1232-1467 301 2 192, 109
215-291 40 4 10 each
3453-3477 102 4 23, 23, 28, 28
1061-1127 60 2 30 each
0, 334, 408, 528, 647
0, 698
0, 48, 99, 144
0, 262, 574, 2734
0, 722
a
Stainless steel chamber walls present and exposed in each experiment. of experiment.
resealed and ventilated for 5 more days to condition the sorbent. Experimental Protocol. The experimental parameters are summarized in Table 1. During each experiment, 10-200 mg of the tested sorbate was vaporized in the sealed chamber on 2-5 occasions. In experiments 1, 2, 4, and 5, the gasphase concentration was monitored for at least 1 week following each SVOC emission. In experiment 3 (phenanthrene-stainless steel), the equilibration period following each emission was limited to 1 or 2 days, because we expected equilibrium to be achieved rapidly. Following the final emission and decay period in each experiment, the chamber was ventilated at a high air-exchange rate (1-3 h-1 for 1-10 days) to remove gas-phase SVOC and then resealed to monitor reemission from the sorbed phase. SVOC Emission Methods. Nicotine and phenanthrene were flash evaporated in the chamber using a slightly modified version of the evaporator unit described previously (10). A 0.53 cm i.d., 10-cm-long stainless steel tube was loosely packed with clean glass wool. Immediately prior to each emission, the unit was removed from the chamber, loaded with a measured quantity of SVOC from a clean syringe, and quickly reinserted into the chamber. For the nicotine experiments, we used pure liquid nicotine. Because phenanthrene is a solid at room temperature, an aliquot of a saturated solution of phenanthrene in methanol was used for the phenanthrene experiments. The loading process took less than 1 min, so evaporative losses outside the chamber were minimal. Once the evaporator unit was positioned, electrical current was supplied to a resistance heater that was coupled to the evaporator. Simultaneously, the evaporating SVOC was flushed into the chamber with a 20 cm3 min-1 flow of clean, dry nitrogen. Within 10 min, the heater temperature reached approximately 300 °C, as measured with a thermocouple, and remained fairly steady at that level until the current was shut off after approximately 30 min. The nitrogen gas flow remained on until the evaporator unit cooled to 35 °C. The evaporator unit effectively delivered the SVOC into the chamber. For confirmation, at the end of each experiment, the stainless steel tube was removed from the evaporator, thermally desorbed onto a Tenax sorbent tube, and analyzed by gas chromatography with flame ionization detection. Less than 0.5 µg of SVOC (out of a total of ∼100 mg injected) was recovered in this manner. Gas-Phase Sampling. Gas-phase SVOC samples were collected on reusable, commercially available sorbent samplers (Part ST032, Envirochem Inc.) packed with Tenax-TA (Aldrich Chemicals). Before each use, the samplers were cleaned and conditioned by heating them to 300 °C for 30 562
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b
experiment 4
experiment 5
Times referenced to first injection at t ) 0 h, corresponding to start
min with a helium counterflow of 100 cm3 min-1. During collection of chamber gas samples, each sampler was mounted on the end of a 45-cm stainless steel tube, which could be retracted from the chamber through a port in the wall to exchange an exposed sample tube for a clean one. The stainless steel tube was connected to a peristaltic pump outside of the chamber, which sampled at a flow rate of 90-110 cm3 min-1. Several duplicate samples were collected over the course of the experiment to verify measurement reproducibility. The lower limit of detection for this method was approximately 0.1 µg m-3 with a limit of quantification (LOQ) of 0.5 µg m-3. Below these limits, background interference and analytical system losses prevented accurate quantification of the collected SVOC mass. To ensure collection of adequate SVOC mass, the sampling duration was increased during periods when lower concentrations were anticipated. For samples that exceeded the LOQ by more than a factor of 3, the variability between duplicate samples was generally less than 15%. Sample Analysis. The analytical procedures for organic compounds collected on sorbent samplers have been described previously (35, 36). Briefly, the sample is thermally desorbed from the sampler, concentrated, and introduced into a capillary GC with a UNACON 810 sample concentrator. This instrument passes the sample through dual sequential traps before introducing it to the GC. Sample components are resolved with a GC (5890 Series II, Hewlett-Packard Co.) equipped with a 15 m × 0.53 mm i.d. fused-silica capillary column with a film thickness of 1.65 µm (Hewlett-Packard Co.). The GC is connected via a direct capillary interface to a flame ionization detector (FID). Calibration regression lines were generated by analyzing Tenax TA cartridges spiked with known volumes of nicotine in methanol containing 0.01% TEA (MeOH/TEA) or phenanthrene in methanol. The calibration curves for nicotine and phenanthrene were linear from 0 to >1 µg of total injected mass. However, both regression lines had negative intercepts indicating a possible loss of ∼30 ng of nicotine and ∼40 ng of phenanthrene per sample in the desorption system. To avoid contamination of sequential samples, the concentrator was cycled twice after each phenanthrene standard or sample run. This procedure kept the background smaller than 1 ng as measured by system blanks. Reagent-grade quinoline (CAS Registry No. 91-22-5; Aldrich) was used as an internal standard in this study, and was added to each nicotine sample tube as a 1-µL aliquot of a 109 ng µL-1 solution prepared in MeOH/TEA. No internal standard was used in analysis of the phenanthrene samples to reduce the risks of sample contamination during addition of the standard. Prior to analysis (and after application of the
TABLE 2. Equilibrium and Kinetic Parameters for Nicotine and Phenanthrene Sorption on Stainless Steel, Carpet, and Wallboard sorbate-sorbent combination nicotinestainless steela
parameter
nicotinecarpet
nicotinepainted wallboard
phenanthrenestainless steel
phenanthrene- phenanthrenecarpet painted wallboard
Equilibrium Parameters isotherm exponent, nij (-) isotherm coeff, Kij (m) partitioning coeff, Pij (m)
0.57 4.69b 45 c
1 (4.6 ( 0.7) × 104 4.6 × 104
sorption exponent, nSij (-) desorption exponent, ndij (-) sorption coeff, kSij (m h-1) desorption coeff, kdij (h-1)
1.32 ( 0.11 2.31 ( 0.19 1.5 ( 0.3 b 0.043 ( 0.007b
1 1 5.3 ( 0.5 (1.2 ( 0.1) × 10-4
1 (3.4 ( 0.7) × 103 3.3 × 103
1 160 ( 30 160
1 36 ( 41 28
1 21 ( 8 18
1 1 0.34 ( 0.05 (2.1 ( 0.3) × 10-3
1 1 6.3 ( 2.7 0.23 ( 0.10
1 1 5.1 ( 2.4 0.28 ( 0.17
Kinetic Parameters 1 1 1.4 ( 0.3 (4.2 ( 0.8) × 10-4
a Nicotine-stainless steel data are from Van Loy et al. (10), experiments 3-5. b For nicotine-stainless steel, units are as follows: K , mg0.43 m-0.29; ij kSij, mg-0.47 m2.41 h-1; kdij, mg-1.59 m3.18 h-1. c Partitioning coefficient for nicotine on stainless steel evaluated at a gas-phase concentration of 0.005 -3. mg m
internal standard for nicotine samples), each sorbent sample tube was conditioned to remove methanol and water collected during sampling by purging with clean, dry nitrogen flowing at 100 cm3 min-1 for 20 min in the same direction as sample gas collection. Loss of collected SVOC during this procedure could be neglected as demonstrated by the reproducible recovery of nicotine and phenanthrene from tubes spiked with standard solutions and conditioned for periods varying from 0 to >30 min. A nicotine calibration standard was run at least once per analysis day. Response of the FID to nicotine remained nearly constant over time. Some variability in the FID response to phenanthrene was observed. To correct for this variability, calibration standards were run approximately every three phenanthrene samples, and a time-dependent response factor was applied.
Data Analysis and Interpretation Modeling Framework. Data from the sorption experiments are interpreted with a dynamic mass-balance model to discern whether specific model forms are consistent with our observations in controlled, but realistic settings. We use the model-measurement comparison to extract empirical parameters describing the rates of sorption and desorption. Ultimately, models and empirical parameters that result from efforts such as this can serve as tools to better understand and manage SVOC behavior in real indoor environments. The model is based on the principle of mass conservation, as expressed by eqs 1 and 2:
dCi dt
)
Ei V
- λv(Ci - Cio) -
1
g
∑S J V
j ij
(1)
reflecting different assumptions about the rate-controlling processes. Dunn and Tichenor (37) proposed a model in which the rates of sorption and desorption were directly proportional to the gas-phase and sorbed-phase concentrations, respectively. This model implicitly assumes linear equilibrium partitioning between the gas and sorbed phases and requires two empirical rate constants. Tichenor et al. (3) and later Van Loy et al. (10) modified this description to allow for a nonlinear, Freundlich sorption isotherm. Axley and Lorenzetti (38) proposed two models: in one, the sorbed phase was assumed to equilibrate instantaneously with the gas phase; in the second, the rate of uptake was limited by mass transport through the gas-phase boundary layer adjacent to the sorbent. Several model formulations have been developed in which diffusion through the bulk sorbent influences the kinetics of the sorption-desorption process (39-42). Piade et al. (12) proposed a model that divided the sorbed phase into two compartments, one fully reversible and the other fully irreversible. Both compartments are governed by linear mass-transfer kinetics as in the Dunn and Tichenor model. We ran simulations for our experiments with several of these model formulations. We were not able to obtain excellent fits to the shape of our dynamic data with any of the models. For presenting results in this paper, we chose the simplest model that provides a good fit to the experimental data using a minimum number of empirical parameters. We assumed that nicotine sorption onto stainless steel followed nonlinear kinetics as described by eq 3. The parameters were obtained by fitting data from experiment 3 reported in Figure 4 of our previous study (10):
j)1
dMij ) Jij dt
Jij ) kSijCni Sij - kdijMnijdij
(3)
(2)
where the subscripts i and j specify parameters applicable to a given SVOC and sorbent, respectively; Ci and Cio are the gas-phase concentrations in the chamber and in the ventilation supply air, respectively (mg m-3); t is time (h); Ei is the mass emission rate (mg h-1); V is the interior volume (m3); λv is the chamber air-exchange rate (h-1); g is the number of sorbent materials (-); Sj is the sorbent projected surface area (m2); Jij is the net SVOC flux from the gas phase to the airsorbent interface (mg m-2 h-1); and Mij is the sorbed mass of compound i per unit area of the air-surface interface of sorbent j (mg m-2). Equations 1 and 2 are the multisorbent analogue of the single sorbent model described in our previous study (10). The model requires a mathematical expression for the net flux of SVOC from the gas to the sorbed phase, Jij. Many forms for Jij have been proposed, with differences generally
We refit the model to the data to obtain new kinetic parameters (see Table 2). The parameters were constrained by the equilibrium data shown in Figure 3 of our previous paper. For most other sorbate-sorbent combinations, we found that a linear kinetic model provided a good fit to the data:
Jij ) kSijCi - kdijMij
(4)
At equilibrium, the rates of sorption and desorption are equal. For the kinetic models employed in this study (eqs 3 and 4), the equilibrium isotherm is
Meij ) Kij(Cei )nij
(5)
where the superscript e denotes an equilibrium level of the parameter. The isotherm parameters Kij and nij are related VOL. 35, NO. 3, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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to the kinetic parameters by these expressions:
Kij )
() () kSij
1/ndij
kdij
nij )
nSij
ndij
(6)
(7)
The equilibrium partitioning coefficient, Pij, is defined to be the ratio of the sorbed mass per sorbent area to the gasphase concentration at equilibrium, Meij/Cei . From eq 5, the partitioning coefficient is seen to be
Pij ) Kij(Cei )nij-1
(8)
For a linear isotherm, nij ) 1, so that Pij ) Kij, independent of the gas-phase concentration. To summarize, from each experiment, we extracted two kinetic parameters. We solved eqs 1 and 2 numerically and iteratively, using a fourth-order Runge-Kutta method (43). For each sorbent in the chamber (the stainless steel chamber walls and carpet or wallboard), the net flux was described by either eq 3 or eq 4. Initially, guesses were made for the unknown kinetic parameters. A goodness-of-fit metric was computed by summing the square of the differences between the predicted and measured log-transformed gas-phase concentrations. An optimized fit was obtained by manually varying the kinetic parameters to minimize the goodnessof-fit metric. The optimization was terminated when the computed parameters changed fractionally by less than 10-4 over successive iterations. Uncertainty in the kinetic parameters was estimated by repeating the optimization process with synthetic data. The synthetic data were generated by adding a Gaussiandistributed error term to the measured variables: gas-phase SVOC concentrations, SVOC mass emitted, and chamber airexchange rate. The process was repeated 100 times for each experiment. The results are reported as means ( one standard deviation.
Results and Discussion Equilibrium Isotherms and Partitioning Coefficients. Table 2 lists isotherm parameters derived from experiments described in this paper and from the nicotine-stainless steel experiment in our earlier work (10). The isotherm coefficient is reported as the mean ( one standard deviation based on fits to the 100 sets of stochastic data. The reported partitioning coefficient is the ratio of mean sorption coefficient to mean desorption coefficient. Theoretically, for a linear isotherm, the isotherm coefficient and the partitioning coefficient should be the same. The results in Table 2, calculated using different methods, agree within experimental uncertainty. The results show that phenanthrene and nicotine sorb comparably to stainless steel, with partitioning coefficients of 160 and 45 m, respectively. (For nicotine, since the sorption isotherm is nonlinear, the partitioning coefficient is a function of the gas-phase concentration, assumed here to be 5 µg m-3.) To put these numbers into perspective, the partitioning coefficient of 160 m implies that, at equilibrium, 360 times as much phenanthrene is sorbed on the chamber’s stainless steel surfaces as is present in the gas phase (160 m × 45 m2 ÷ 20 m3 ) 360). The sorption capacity pattern among materials is very different for phenanthrene and nicotine. Nicotine sorbs strongly on carpet and wallboard, with partitioning coefficients of 46000 and 3300 m, respectively. On the other hand, phenanthrene sorbs only moderately on carpet and wallboard, with partition coefficients of 28 and 18 m, respectively. 564
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FIGURE 2. Experiment 1: Nicotine on stainless steel and carpet. (a) Measured and predicted gas-phase nicotine concentration. (b) Predicted stainless steel and carpet sorbed nicotine. (c and d) Measured and predicted gas-phase nicotine concentration for selected periods of the experiment. Sorption Kinetics. The gas-phase concentration measurements from selected experiments are shown in Figures 2-4 along with predictions based on the best-fit model parameters. Also shown in the figures are predictions of the sorbed mass on each surface. Complete experimental data are available (44). Qualitative SVOC behavior was similar in each experiment. Following a pulsed emission, the gas-phase SVOC concentration decreased over a time scale of hours to a concentration that was orders of magnitude lower than the peak value. The concentration then stabilized and remained roughly constant until the next perturbation. Because the chamber is ventilated at a very low rate, little of the released mass is lost from the chamber by ventilation during the experiments. The rapid decreases in concentration after each injection are a consequence of sorptive uptake on materials in the chamber. Because of the high partitioning coefficients, the periods of high ventilation at the end of each experiment did not completely eliminate the SVOCs from the indoor environment. For example, in the experiment with nicotine and carpet (Figure 2), the nicotine concentration was measured to be 4.4 µg m-3 at t ) 1055 h, 15 d after the final nicotine injection, and immediately before the high-ventilation period. The chamber was flushed with ∼70 vol of clean air over the next 3 days. After resealing the chamber, the nicotine concentration slowly rose back to a level of approximately 1 µg m-3, because of desorption from the carpet and stainless steel chamber surfaces. The experiment with phenanthrene on stainless steel and wallboard shows similar behavior (Figure 4). At t ) 1050 h, 2 weeks after the final injection, the airborne phenanthrene level is about 3 µg m-3. After ventilating at a rate of 1 h-1 for 66 h, the chamber was resealed, and the phenanthrene level stabilized at 1.5 µg m-3.
FIGURE 3. Experiment 4: Phenanthrene on stainless steel and carpet. (a) Measured and predicted gas-phase phenanthrene concentration. (b) Predicted stainless steel and carpet sorbed phenanthrene. (c and d) Measured and predicted gas-phase phenanthrene concentration for selected periods of the experiment.
FIGURE 4. Experiment 5: Phenanthrene on stainless steel and wallboard. (a) Measured and predicted gas-phase phenanthrene concentration. (b) Predicted stainless steel and wallboard sorbed phenanthrene. (c and d) Measured and predicted gas-phase phenanthrene concentration for selected periods of the experiment.
The model fits to the experimental data are good in most cases, although perhaps only fair for phenanthrene on carpet (Figure 3). The general model trends are consistent with the experimental data. However, we were not able to fit the model to the measurements precisely. The case of nicotine on carpet illustrates nicely the success and limitations of the model (Figure 2). On the scale presented in the figure, the model describes well the sharp rise and fall in the gaseous nicotine concentration following each pulsed injection. For the first half of the experiment, the lengthy decay periods between injections are also modeled reasonably well: deviations between the model and measurements are comparable to the variability in the measurements themselves. However, the deviation between model and measurement appears systematically more substantial during the second half of the experiment. The best model fit does not match the high plateau that is experimentally observed following the last injection, nor does it capture in detail the drop and subsequent rise associated with the high-ventilation period. The poorest fit between model and measurement occurs in the case of phenanthrene on carpet (Figure 3). The model does well in predicting the concentration versus time shortly after the pulsed releases (see panels c and d). However, it substantially overpredicts the plateau level that occurs after injections (panel a). The difficulty in achieving a good model fit stems from the uncertainty in phenanthrene sorption on stainless steel and the relatively weak sorption of phenanthrene on carpet. In fitting the model to experiment 4, we only adjusted the parameters for sorption on carpet, using the stainless steel results derived from experiment 3. Because stainless steel sorbs phenanthrene much more strongly than carpet, errors or variability in the sorption parameters for stainless steel prevent a better fit of the model to these data.
The weak sorption of phenanthrene on carpet also causes high uncertainty in model parameters (Table 2). Of course, given the complex nature of the materials involved, it is unrealistic to expect that a simple twoparameter model for sorptive uptake (eq 4) would accurately describe gas-phase concentrations, which vary with time over 3 orders of magnitude. Significant improvements require a more fundamental understanding of the nature of the interactions of volatile and semivolatile organic compounds with the complex materials found indoors. Comparison of the fitted parameters with literature data helps substantiate that the model predictions are reasonable approximate descriptions of reality. In our earlier study of nicotine in the stainless steel chamber (10), the masstransport-limited deposition velocity for nicotine under the same chamber airflow conditions was measured using filters coated with sodium bisulfate, which irreversibly reacts with deposited nicotine through acid-base chemistry. The experiment provided an upper bound estimate of 4 m h-1 for the sorption coefficient, ks. This value is approximately consistent with the sorption coefficients estimated for nicotine-carpet, phenanthrene-carpet, and phenanthrenepainted wallboard. Note that roughness may enhance mass transfer to carpet as compared to the bisulfate-coated filters. SVOC uptake for the other sorbate-sorbent pairs appears somewhat slower but of similar magnitude. The rate of uptake on surfaces is very important in determining long-term persistence of sorbing pollutants. When these species are released into indoor air, they may either be removed by ventilation (at a rate proportional to the air-exchange rate, λv), or be taken up by sorption (at a rate proportional to ks × S/V). At a typical value of the air-exchange rate of 0.5 h-1 and a typical surface-to-volume ratio of 3 m-1, a sorption VOL. 35, NO. 3, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Predicted concentration of nicotine in a residence in which smoking occurs habitually. The plotted concentrations are averaged over the whole day (solid line), over the 6 h during which smoking occurs (dashed line), or over the 18 nonsmoking hours (dot-dashed line). See text for simulation conditions. coefficient of 4 m h-1 implies that only 4% (0.5 ÷ [0.5 + 3 × 4]) of what is released would be removed initially by ventilation. The remainder would sorb, and be available for later desorption. Desorption coefficients govern the long-term persistence of sorbing compounds. The reciprocal of the desorption coefficient is a crude estimate of the time scale of persistence. The model results are markedly different for nicotine and phenanthrene. The values of 0.2-0.3 h-1 for phenanthrene on carpet and wallboard indicate that this species would persist only for hours after release to ventilated indoor environments. On the other, with desorption coefficients on the order of 10-4 h-1, nicotine could persist in indoor air for months and possibly even years after emissions ceased. Implications for Exposure. To examine the impact of sorption and desorption on nicotine’s persistence in a building, we applied the model and parameter values obtained in this study to predict time-dependent nicotine concentrations in a home with habitual smoking. The building parameters were selected to be typical of a twobedroom single-family dwelling: volume, 270 m3; airexchange rate, 0.5 h-1; area of carpet, 96 m2; area of painted wallboard, 326 m2. We assumed that 12 cigarettes were smoked indoors each day, at a rate of two per hour for 6 h. Each cigarette was assumed to emit 5 mg of nicotine, a value that is consistent with published sidestream smoke emission factors (23). The surface interactions are modeled using eq 4 with the mean parameter values reported in Table 2. Equations 1 and 2 are solved numerically, starting with initial conditions of no nicotine in the indoor environment. Average airborne nicotine concentrations are predicted on a daily basis for three time intervals: the 6 h of smoking, the 18 h during which no smoking occurs, and the entire 24-h day. Figure 5 presents the results. Focusing initially on the period between 10 and 300 d, we see that the daily average nicotine concentration is predicted to be in the range of 2-5 µg m-3, consistent with values reported in the literature for smoking households (24, 25). The average level during nonsmoking periods is much lower than the average level during smoking, as would be expected. But because of desorption, the average level is not zero even without smoking. Owing to the very low rates of desorption, the model predicts that a steady-state cycle is not achieved in this system on time scales of 1000 d (3 yr). Rather, desorption from surfaces is predicted to cause average airborne concentrations to continue to rise as the surfaces become saturated with nicotine. On a percentage basis, this growth in nicotine levels 566
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with increased duration of habitual smoking is especially important for the nonsmoking periods. At 1000 d, the model predicts that the nicotine level during the nonsmoking periods would exceed 5 µg m-3. Because of the large reservoir sorbed to surfaces, nicotine concentrations of this order are predicted to persist for very long periods after smoking ceases. We caution that these predictions extrapolate from the available evidence. A better mechanistic understanding of sorption phenomena is needed to engender confidence in the accuracy of such predictions. Human exposure that leads to an increased risk of adverse health outcomes is a major cause for concern about air pollution. The surfaces of the buildings we occupy can interact with air pollutants in ways that may have an enormous effect on exposure. The research presented in this paper quantifies the interactions for a few important compounds and a few common types of indoor materials. More studies such as this are needed to explore the interactions of a broader range of compounds and materials under a variety of environmental conditions, including variable humidity and temperature. Given the vast number of combinations of interest, an even greater need is for systematic studies aimed at elucidating the mechanisms of interaction. Only with this understanding will it be possible to thoroughly understand human exposure to air pollutants.
Acknowledgments This research was supported by a U.S. Environmental Protection Agency Graduate Fellowship and an S. C. Johnson Wax Research and Development Fellowship. Additional support came from funds provided by the Cigarette and Tobacco Surtax Fund of the State of California through the Tobacco-Related Disease Research Program of the University of California, Award 7RT-0099. Support was also provided by Grant RO1-HL42490 from the National Heart, Lung, and Blood Institute through the U.S. Department of Energy under Contract DE-AC03-76SF00098. The authors thank B. C. Singer and A. T. Hodgson for their contributions.
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Received for review June 13, 2000. Revised manuscript received November 14, 2000. Accepted November 15, 2000. ES001372A
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