The Rate of Ozone Uptake on Carpets: Experimental Studies

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Environ. Sci. Technol. 2000, 34, 4963-4968

The Rate of Ozone Uptake on Carpets: Experimental Studies GLENN C. MORRISON† AND WILLIAM W NAZAROFF* Environmental Engineering Program, Department of Civil and Environmental Engineering, 633 Davis Hall, University of California, Berkeley, California 94720-1710

Ozone can react with surfaces, reducing indoor concentrations. Carpets may be important ozone sinks because of their high surface area. We conducted laboratory experiments to measure ozone uptake on four samples of whole carpet and on the corresponding carpet fibers and carpet backing. Results were parametrized in terms of reaction probability, defined as the rate of ozone loss on a surface normalized by the rate of ozone-surface collisions. For whole carpet and carpet-backing samples, we found the apparent reaction probability to be of magnitude 10-5 to 10-4. These results are referenced to the floor area that would be covered by the carpet, rather than to the total surface area of the carpet and its fibers. Reaction probabilities of the order of 10-7 to 10-6 were measured on carpet fibers, referenced to total estimated fiber area. The results indicate that carpet is of comparable significance to painted walls in scavenging ozone from indoor air. All samples tested exhibited aging, such that the rate of ozone uptake diminished with increasing cumulative exposure. Although reactions on carpeting can reduce human exposure to ozone, we caution that the reaction products may include volatile carbonyls that have low odor or irritation thresholds.

Introduction Indoor air pollutant concentrations are influenced by the large amount of surface area associated with building materials and furnishings. Ozone is a strong oxidant, which can react with many surfaces, making indoor ozone concentrations particularly sensitive to the high surface-tovolume (S/V) ratio present indoors. In the absence of strong indoor sources (1-3), surface reactivity causes indoor concentrations of ozone to be lower than outdoors (4). Carpet covers a large fraction of the total area of indoor surfaces. Furthermore, with about 10 million fibers per square meter woven tightly into a textile backing, the presence of carpet can increase the indoor S/V ratio by more than an order of magnitude compared with hard-surface floors. Carpet may, therefore, be an important sink for ozone and could have a big influence on human exposures to this air pollutant. Field monitoring yields conflicting results regarding the importance of carpet in residences in reducing indoor ozone concentrations. Lee et al. (5) found that indoor ozone decay rates were statistically higher in homes with floor space that was 100% carpeted, compared to homes with less than 100% * Corresponding author phone: (510)642-1040; fax: (510)642-7463; e-mail: [email protected]. † Present address: National Oceanic and Atmospheric Administration, R/AL2, Boulder, CO 80303. 10.1021/es001361h CCC: $19.00 Published on Web 11/01/2000

 2000 American Chemical Society

carpet. Avol et al. (6) found no correlation between the ratio of indoor to outdoor ozone concentrations and the presence of carpet. Laboratory studies confirm that ozone is scavenged by carpet (7-9). However, the data on which to base quantitative assessments of the effect are sparse. Recognizing that ozone reactions on carpets may strongly influence indoor ozone concentrations and that existing studies were suggestive but not conclusive, we undertook an experimental investigation of ozone uptake on carpets. We exposed samples of common carpet types to ozone in an environmental chamber. We also separated the fibers from the backing and measured the ozone uptake on these components separately. This paper reports the results of these experiments along with an evaluation of their significance.

Materials and Methods Deposition Velocity and Reaction Probability. Building spaces are typically modeled as continuously mixed flow reactors, with source and sink terms contributing to the timedependent concentration of pollutants. Loss by mass transfer to surfaces is commonly parametrized by the deposition velocity, vd, a mass-transfer coefficient defined by this expression

vd )

J C

(1)

where J is the mass flux to a surface (µg m-2 h-1) and C is the pollutant concentration in indoor air (µg m-3). For a reactive gas such as ozone, the deposition velocity to a surface depends on the rate of mass transfer through a concentration boundary layer and on the rate of reaction with the surface (10-12). The surface reactivity can be parametrized by the reaction probability, γ, which is also known as the uptake coefficient, or the (mass) accommodation coefficient. Reaction probabilities have been reported for ozone interactions with many surfaces of interest in indoor air and range from ∼10-8-10-7 for glass to ∼10-4 for bricks (10). The ozone reaction probability has been found to diminish with ozone exposure, a phenomenon known as “aging” (13, 14). The reaction probability has also been found to vary with relative humidity on some surfaces (12, 13, 15). For the present purposes, the reaction probability can be defined as the rate at which ozone molecules are irreversibly consumed at a specified boundary, divided by the rate at which ozone strikes that boundary. For a flat, nonporous material, the boundary coincides with the interface between air and the surface. The surfaces we studied are porous and are not flat. Thus, the boundary upon which the reaction probability is defined may not coincide with the gas/solid interface. Consider a carpet, for example. At the tips of the fibers, pollutants are exchanged between a region of freemoving air above the fibers and a more stagnant region of interfiber air. For this research, we defined an apparent reaction probability for whole carpet, γo. This parameter relates the measured rate of ozone scavenging by carpet to that which would occur to a highly reactive surface that covers the same floor area. The apparent reaction probability of the carpet backing, γb, is similarly defined. Carpet fibers are approximately cylindrical but are not smooth. For this study, we defined the fiber reaction probability, γf, based on a comparison between the measured rate of uptake and the rate that would occur to highly reactive cylinders with a diameter equal to the average carpet fiber diameter, df. VOL. 34, NO. 23, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Characteristics of Carpet Samplesa sample

df (µm)

Rf

intended usage

style

fiber type

fiber treatment

CP1 CP2 CP3 CP4

60 70 80 70

66 33 46 30

residential commercial residential commercial

cut pile loop cut pile loop

nylon olefin, stain resistant nylon olefin, stain resistant

3M Scotchgard Stain Release unknown Monsanto Wear-Dated no treatment

a

Symbols: df, fiber diameter; Rf, estimated total area of carpet materials per unit floor area covered.

Carpet Samples. Four carpets were chosen for this study to represent commonly installed carpets in California. Carpets CP1 and CP3 are residential, cut pile carpets; CP2 and CP4 are commercial, loop carpets. Carpet characteristics, such as fiber composition and stain-resist treatments, are shown in Table 1. Carpets samples were prepared by cutting squares (232 cm2) from newly manufactured rolls that were subsequently stored in 19 L ventilated chambers in August 1997. Using a scalpel, fibers were trimmed from some samples to expose carpet backing. Fibers obtained from this operation were separated from each other using wool carders. The diameter of carpet fibers, df, was measured under a light microscope. The packing density of fibers was determined by trimming fiber bundles from a measured area, weighing the total fiber mass, weighing 5-10 representative bundles, and counting the number of individual fibers in those bundles. To parametrize the relationship between superficial area and the additional area associated with fibers, we determined the normalized carpet area, Rf ) 1 + (fiber surface area)(carpet area)-1. For example, a value of Rf of 50 implies that the carpet contains about 50 times more potential area for reaction than the floor area that the carpet would cover. Note that Rf is a lower-bound estimate of the carpet’s specific intrinsic surface area, as it does not take into account the internal porosity of the fibers or backing. Values of df and Rf for each carpet are shown in Table 1. Measuring the Reaction Probability for Whole Carpet and Carpet Backing. The experimental configuration and operational methods for determining the reaction probability on samples of whole carpet and carpet backing are identical to those described in the “ozone deposition” section of Morrison et al. (16). Briefly, a sample of whole carpet or carpet backing was mounted in a Teflon frame and then placed in a 10.5-L stainless steel chamber. The chamber was ventilated at an airflow rate of 1.2 L min-1. The reactor was operated at ∼7 air changes per hour, a ventilation rate much higher than measured in a typical residence. We believe that comparability in the mass-transport-limited deposition velocity, vt, is the most appropriate scaling goal for making measurements of the reaction probability. The masstransport-limited deposition velocity estimated for ozone deposition in a typical residence is 0.02-0.2 cm s-1 (17). The mass-transport-limited deposition velocity measured for carpet in our experiments was about 0.17 cm s-1. The moisture content of the air was controlled to a relative humidity of 50%. Ozone was generated at a controlled rate in the supply air by ultraviolet radiation. A feedback control loop ensured that the level in the chamber was maintained at 100 ppb. A typical experiment continued for 48 h, during which the inlet and outlet ozone concentrations were monitored and archived for analysis. The emission rates of volatile organic compounds were also measured in these experiments (17) but are not reported here. The reaction probability cannot be measured directly at ordinary environmental temperature and pressure. CanoRuiz et al. (10) showed that the reaction probability can be determined for a material by comparing two measured loss rates in a continuously mixed flow reactor (CMFR): the surface-specific deposition velocity, vd, and the surface4964

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specific mass-transport-limited deposition velocity, vt. The overall deposition velocity can be approximated by an areaweighted sum of the surface-specific deposition velocities from all available surfaces. The mass-transport-limited deposition velocity applies under conditions of fast surface kinetics, such that ozone is lost at a surface as rapidly as transport permits. To ensure that the reactor operated as a CMFR, we performed several “residence-time distribution” experiments (18) using ozone as the inert tracer after the inner surfaces had been quenched. These experiments confirmed that the reactor had a well-mixed volume of between 10 and 11 L. For the laboratory reactor, modeled as a CMFR at steady state, the ratio of the outlet to inlet level of ozone is given by

C Q ) Cinlet Q + Scvdc + Srvdr

(2)

where Q is the chamber ventilation rate, Cinlet is the pollutant level in the supply air, C is the outlet (and internal) concentration of the pollutant, Sc is the area covered by the carpet, Sr is the area of all other accessible surfaces in the reactor, vdc is the deposition velocity of the pollutant to the carpet, and vdr is the area-averaged deposition velocity of the pollutant to all other inner reactor surfaces. In our experiments, C/Cinlet and Q were measured. Equation 2 is then used to solve for Sc vdc + Srvdr. The term, Srvdr, was made negligibly small by exposing the empty chamber to a high level of ozone (∼4000 ppbv) in advance of each carpet experiment. This process quenched ozone-reactive sites on chamber surfaces as was confirmed by observing that C/Cinlet ) 1 in the absence of a carpet sample. Since the nominal carpet surface area, Sc, is known, the ozone deposition velocity, vdc, can be computed from these experiments. The mass-transport-limited deposition velocity, vt, was obtained by conducting the same type of experiment using a carpet sample that was coated with potassium iodide, which is considered a perfect sink for ozone (19). We assume that there is no surface resistance to ozone removal at the KIcoated carpet surface. Thus the deposition velocity, vdc, in eq 2 represents the mass-transport-limited value, vt, for that experiment. Given vdc and vt, the apparent reaction probability for ozone on carpet can be calculated (10)

γ)

[ (

)]

〈v〉 1 1 4 vdc vt

-1

(3)

where 〈v〉 is the Boltzmann velocity (3.62 × 104 cm s-1 for O3 at 296 K). Equation 3 was used to calculate both the reaction probability of whole carpet, γo, and of carpet backing, γb. The mass-transport-limited deposition velocity, vt, is always greater than vdc, because vt represents the maximum mass transfer coefficient possible under the given flow conditions. Note that for vdc , vt, the reaction probability, equals 4vdc 〈v〉-1, independent of the rate of external transport. In this case, slow surface reactivity controls the overall rate of uptake.

Measuring the Reaction Probability for Carpet Fibers. Approximately 2 g of fibers from each carpet sample was exposed to ozone in a 15-cm long, Teflon tubular reactor with an inner diameter of 1.75 cm. The inlet ozone mole fraction was initially set to ∼100 ppb but was not controlled. Ozone was measured upstream and downstream of the reactor throughout each exposure experiment. Ozone scavenging by the reactor walls was made negligibly small by exposing the empty reactor to a high ozone level in advance of each experiment. To facilitate the determination of the reaction probability on fiber surfaces, we designed the reactor to operate in plugflow fashion. For plug-flow assumptions to be valid, the Bodenstein number, NBo, should be greater than 100 (20), where

NBo )

UeL D

(4)

Here, L is the total length of the packed section of the reactor, D is the molecular diffusivity of O3 in air, and Ue is the effective gas velocity, i.e., the volumetric flow rate divided by the product of the cross-sectional area of the reactor and the bed porosity. For a typical carpet fiber experiment, the Bodenstein number was approximately 800. Assuming this system acts as a perfect plug-flow reactor (PFR), the differential change in concentration with respect to axial reactor distance, z, is given by

γf〈v〉(1 - p) dC )C dz Uedf

(5)

where p is the bed porosity. The reaction probability of individual fiber surfaces diminishes with time in the presence of ozone. With a continuous flow of ozone through the PFR, a reaction probability gradient exists along the reactor axis. Thus, a length-averaged reaction probability, γ j f, for the entire fiber bed was calculated by solving eq 5

γ jf )

1 L

df Q

∫ (γ )dz ) 〈v〉p(1 - p)V ‚ ln L

0

f

( ) Cinlet CL

(6)

where CL is the ozone level at the reactor outlet (z ) L), Q is the volumetric flow rate of gas through the reactor, and V is the total volume of the fiber-filled portion of the reactor. For eq 6 to be valid, the ozone concentration may only vary axially. Also, surface resistance to ozone deposition must be the rate-limiting process, relative to gas-phase masstransfer resistance. For the conditions of our experiments, surface resistance dominates provided that γf is less than ∼10-3. Owing to limitations in the precision of ozone measurements, the highest measurable fiber surface reaction probability is about 10-5.

Results and Discussion Two phenomena were observed for every material tested, whether whole carpet, carpet fibers, or carpet backing. First, every sample reacted with ozone, removing some of it from the reaction chamber. Second, the reactivity of each material with ozone decreased with increasing cumulative exposure. An example of the time profile of the inlet and outlet ozone levels for a whole-carpet test is shown in Figure 1a. The feedback control program adjusted the inlet concentration to maintain the outlet ozone level constant at 100 ppb. As exposure progresses and the carpet becomes less reactive to ozone, a lesser upstream level is needed to maintain 100 ppb within the reactor.This surface quenching is interpreted in terms of a decrease in the apparent reaction probability.

FIGURE 1. Time profile of inlet and outlet ozone mole fraction: (a) in a CMFR experiment measuring ozone uptake on a whole carpet sample, CP2; and (b) in a PFR experiment measuring ozone uptake on carpet fibers from sample CP2. In some CMFR experiments, the ozone concentration drifted slightly above the set point of 100 ppb because the lower limit of ozone generation had been reached. In the first hour of the CMFR experiment, the carpet is not exposed to ozone, and the ozone control system is calibrated. This is represented by a stair-step profile in the inlet ozone plot. Electronic noise from the ozone analyzer triggered brief deviations from the set point ozone level, as seen in the small, sharp peaks in (a). Figure 1b shows a time profile for a fixed bed experiment on CP2 fibers. No control routine is used to maintain a fixed outlet ozone concentration in the fiber experiments. The inlet mole fraction is set (but not controlled) at the target level, and the outlet mole fraction is observed to increase as the fiber surfaces become quenched. The deposition velocity, vdc, varied significantly over the course of the CMFR experiments due to ozone aging of carpet surfaces. Representative values of vdc are 0.17 cm s-1 at the beginning of an experiment with a very reactive carpet (e.g. CP3) and 0.04 cm s-1at the end of an experiment with a relatively nonreactive carpet (e.g. CP4). The value of vt was measured to be about 0.17 cm s-1 for all carpets. The ozone reaction probabilities as measured on the whole carpet, γo, carpet fibers, γ j f, and carpet backing, γb, are presented in Tables 2-4. The experimental uncertainty is estimated to be about (55%, (15%, and (20%, respectively, for whole carpet and carpet backing reaction probabilities of 10-4, 10-5, and 10-6. The uncertainty is estimated to be about (25%, (15%, and (35%, respectively, for fiber reaction probabilities of 10-5, 10-6, and 10-7. Uncertainties in the measurement of the CMFR reaction probability, γ, were derived by an error propagation analysis of eqs 2 and 3 that incorporated typical measured values and uncertainties of j f was Q, Sc, vt, C, and Cinlet. Similarly, the uncertainty in γ determined by error propagation analysis of eq 6. Uncertainties in C, Cinlet, and CL were 1% for values at or above 100 ppbv or 1 ppbv for values below 100 ppbv. The uncertainties in Sc and V were estimated to be less than 5%. Uncertainty in df was (5 µm. The uncertainty in vt was obtained by replicate measurements and was approximately 0.01 cm s-1. Uncertainty in the CMFR reaction probability increases as γ approaches 1 because ozone flux to the surface approaches the mass transport limited flux. As vdc approaches vt, the uncertainty of the difference shown in eq 3 becomes relatively large. As γ approaches zero, the value of C approaches Cinlet, making the uncertainty in the ratio of C and Cinlet relatively large in eq 2. A similar analysis can be applied to the PFR. The reaction probabilities are reported as initial and final values. For whole carpet and carpet backing experiments, the initial value of apparent reaction probability is taken 15 min after exposure begins. This delay from t ) 0 is necessary because the rapid changes that occur in the ozone level in the reactor during the initial period of exposure create very large uncertainties in the reaction probability. For carpet VOL. 34, NO. 23, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Initial and Final Reaction Probabilities, Cumulative Uptake, and Fitting Parameters for Whole Carpet Samplesa,b carpet

exptc

initial γo

final γo

Φo (µg cm-2)

r (µg cm-2)-β

β

r2

timed (h)

CP1 CP2 CP3

90405 90323 71028 81130 70812 71111 90412

3 × 10-5 6 × 10-5 >10-4 10-4 6 × 10-5 >10-4 >10-4

6.6 × 10-6 1.1 × 10-5 3.1 × 10-5 1.1 × 10-5 1.2 × 10-5 9.3 × 10-6 6.3 × 10-6

0.9 1.2 1.7 1.3 1.3 1.1 0.9

7 × 10-6 1 × 10-5 3 × 10-5 2 × 10-5 1 × 10-5 9 × 10-6 6 × 10-6

-0.12 -0.07 -0.16 -0.10 -0.26 -0.18 -0.29

0.93 0.66 0.72 0.86 0.87 0.91 0.91

48 48 48 38 48 48 48

CP4

a The symbols have the following meaning: γ is the apparent reaction probability, Φ is the cumulative ozone uptake on the sample, R and o o β are empirical parameters for the relationship shown as eq 9, and r2 is the square of the correlation coefficient, r, between log(γo) and log(Φo). b Values in this table were obtained in a continuously mixed flow reactor (CMFR). Note that the values of reaction probability and cumulative uptake are not directly comparable to those values obtained for fibers in the tubular reactor (see text). c The start date of the experiment in the form YMMDD. For example, 90405 designates that this experiment started in 1999 (9) on April 5 (04 and 05). d Duration of the experiment.

fiber experiments, the initial value of reaction probability reflects the first acceptable data point, typically obtained within the first minute of exposure. Although the initial reaction probability was generally higher than could be resolved with the experimental setup, an exception was found for ozone reacting with olefin fibers (CP2, CP4). In most of these runs, the initial reaction probably was measurable at 1 × 10-6 to 8 × 10-6. In all cases, the final value reported is that obtained at the end of the experiment. The final apparent reaction probabilities of whole carpet and carpet backing spanned a factor of 5 from 6.6 × 10-6 (CP1, whole carpet) to 3.1 × 10-5 (CP3, whole carpet). The magnitude of these values, ∼10-5, is comparable to the ozone reactivity of latex paint (12). The final reaction probability for fiber surfaces spanned an order of magnitude from 5 × 10-8 to 5 × 10-7. The low reaction probability of aged carpet fibers is similar to those of Lucite, nylon, and plate glass (14). Although the final values are low, the initial values are greater than ∼10-5 for the residential carpets CP1 and CP3. The initial γ j f value for commercial carpet fibers is somewhat lower, ∼10-6 to ∼10-5. However, after aging, there is no clear distinction between commercial and residential carpets with respect to ozone reactivity. The low reaction probability of individual carpet fibers combine with the high specific surface area of carpeting to produce an overall apparent reaction probability for whole carpet that is moderately high. Ignoring the complexities of ozone transfer through the carpet fiber mat, the whole-carpet reaction probability, γo, can be crudely estimated by adding the contribution of each component (fiber and backing), with appropriate area weighting factors. As an example, for carpet CP2, the normalized surface area associated with fibers is 32 (Rf - 1). Multiplying this value by the final average fiber reaction probability, 3.6 × 10-7, and adding the result to the final reaction probability for carpet backing, 1.4 × 10-5, yields an estimate for γo of 2.6 × 10-5. By comparison, the measured value of γo for CP2 was about 40% of this value, 1.1 × 10-5. The approximate agreement between the estimated and measured values of the overall reaction probability suggests that we have accounted for the most important factors. We see from this estimate that the contribution of fibers to the whole carpet reaction probability is similar to that of the backing. We explored the aging phenomenon by relating the reaction probability to the cumulative ozone uptake, Φ, on a surface, defined to be the time integral of the ozone flux to the surface (with units of µg cm-2)

Φ)

∫

texp

0

(vdC)dt

(7)

where texp is the accumulated exposure time. For the wholecarpet and carpet-backing experiments, the cumulative 4966

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uptake is computed by solving eq 2 for vdcC and substituting into eq 7

Φ)

∫

texp(Cinlet

0

- C - Srvdr)Q dt Sc

(8)

where, recall, Sc is the area covered by the carpet. We made the term Srvdr negligibly small by exposing the empty chamber to a high ozone level (∼4000 ppbv) in advance of each carpet experiment. A similar expression holds for the carpet-fiber experiments, with Sc replaced by Sf, the total surface area of fibers in the reactor. Final values of cumulative uptake are shown in Tables 2-4 as Φo, Φf, and Φb for whole carpet, carpet fibers, and carpet backing, respectively. For the 48-h exposure periods, the cumulative uptake on whole carpet and backing is remarkably similar across all samples, with an average of 1.3 µg cm-2 and a range of 0.71.9 µg cm-2. Whole carpet and backing from sample CP3 consumed the most ozone. High ozone uptake on CP3 corresponds to a high emission rate of ozone-induced oxidized volatile organic compounds from the surface of this sample (17). Interestingly, the backing tends to take up slightly more ozone over the same period than whole carpet for each sample, suggesting that the fiber mat serves as a resistance to mass-transport. However, emission rates of oxidized species were not consistently higher for carpet backing than for whole carpet samples (17). This may be explained by the presence of styrene-butadiene rubber (SBR) in the backing. Some double bonds may remain after polymerization of SBR, providing sites for ozone reactions whose products are semivolatile or nonvolatile. A comparison of Φo and Φf in Tables 2 and 3 would seem to suggest that the cumulative uptake on fibers was lower than that for whole carpet. The cumulative uptake of ozone obtained for whole carpet is the result of ozone deposition to all fiber surfaces and backing but is defined over the much smaller area of the planar interface at the carpet fiber tips. Thus the large cumulative uptake values shown in Table 2, if referenced to all available surface area, would be much smaller. A rough estimate of the degree of “accelerated aging” that occurred in the fixed bed reactor can be obtained by multiplying the fiber cumulative uptake by the normalized surface area, Rf, and dividing this by the whole-carpet cumulative uptake. Using 48-h experimental data for carpets CP1, CP2, CP3, and CP4, respectively, these values are 5, 4, 9 and 1. Since some of the ozone is presumed to have reacted with backing materials in the whole-carpet studies, the cumulative ozone uptake of fibers per unit fiber area is higher in all cases in the fiber experiments than in the whole-carpet experiments. All of the materials tested were exposed to clean air in the chambers beginning in August 1997. However, the experiments took place over a subsequent period of more than 1.5

TABLE 3. Initial and Final Reaction Probabilities, Cumulative Uptake, and Fitting Parameters for Carpet Fibersa,b carpet

exptc

initial γf

final γf

Φf (µg cm-2)

r (µg cm-2)-β

β

r2

timed (h)

CP1

80810 90427 80727 80817 81116 80722 80824 80824 81209 80820 81118

>10-5 8 × 10-6 8 × 10-6 6 × 10-6 >10-5 >10-5 >10-5 >10-5 >10-5 2 × 10-6 1 × 10-6

6.2 × 10-8 5.0 × 10-8 3.4 × 10-7 2.8 × 10-7 4.8 × 10-7 4.0 × 10-7 3.9 × 10-7 2.0 × 10-7 1.4 × 10-7 9.2 × 10-8 9 × 10-8

0.07 0.022 0.149 0.153 0.055 0.355 0.335 0.475 0.43 0.042 0.018

2 × 10-11 6 × 10-13 6 × 10-8 4 × 10-8 9 × 10-8 1 × 10-9 3 × 10-9 1 × 10-8 1 × 10-9 6 × 10-9 4 × 10-9

-2.2 -2.9 -0.8 -0.9 -0.5 -5.6 -4.4 -2.9 -5.1 -0.8 -0.8

0.95 0.96 0.95 0.98 0.99 0.99 0.95 0.88 0.97 0.98 0.97

48 31 48 48 24 48 48 168 120 48 24

CP2 CP3

CP4

a The symbols have the following meaning: γ is the reaction probability, Φ is the cumulative ozone uptake on the sample, R and β are empirical f f parameters for the relationship shown as eq 9, and r2 is the square of the correlation coefficient, r, between log(γf) and log(Φf). b Values in this table were obtained in a tubular reactor. Note that the values of reaction probability and cumulative uptake are not directly comparable to those values obtained for carpets in a CMFR (see text). c The start date of the experiment in the form YMMDD. For example, 80810 designates that this experiment started in 1998 (8) on August 10 (08 and 10). d Duration of the experiment.

TABLE 4. Initial and Final Reaction Probabilities, Cumulative Uptake, and Fitting Parameters for Carpet Backinga,b carpet

exptc

initial γb

final γb

Φb (µg cm-2)

r (µg cm-2)-β

β

r2

timed (h)

CP1

80310 80408 90423 80602 90414 80325 81215 80401 90419

>10-4 >10-4 >10-4 >10-4 >10-4 >10-4 >10-4 >10-4 >10-4

1.2 × 10-5 1.4 × 10-5 1.4 × 10-5 1.2 × 10-5 1.5 × 10-5 2.8 × 10-5 2.0 × 10-5 1.0 × 10-5 1.1 × 10-5

1.3 1.3 0.7 1.2 1.3 1.7 1.9 1.3 1.3

1 × 10-5 1 × 10-5 1 × 10-5 1 × 10-5 1 × 10-5 3 × 10-5 4 × 10-5 1 × 10-5 1 × 10-5

-0.37 -0.38 -0.31 -0.28 -0.12 -0.18 -0.47 -0.3 -0.28

0.9 0.88 0.77 0.76 0.86 0.85 0.94 0.92 0.94

48 48 24 48 48 48 48 48 48

CP2 CP3 CP4

a The symbols have the following meaning: γ is the reaction probability, Φ is the cumulative ozone uptake on the sample, R and β are empirical b b parameters for the relationship shown as eq 9, and r2 is the square of the correlation coefficient, r, between log(γb) and log(Φb). b Values in this table were obtained in a continuously mixed flow reactor (CMFR). Note that the values of reaction probability and cumulative uptake are not directly comparable to those values obtained for fibers in the tubular reactor (see text). c The start date of the experiment in the form YMMDD. For example, 80310 designates that this experiment started in 1998 (8) on March 10 (03 and 10). d Duration of the experiment.

FIGURE 2. Reaction probability plotted against cumulative uptake of ozone on whole carpet, carpet backing, and carpet fibers. Data from carpets CP1-CP4 are shown in frames (a)-(d), respectively. A number designating the start date of the respective experiment is shown in parentheses (see note c in Tables 2-4). y. Measurements were made on some materials that had been aired for only a few days, while others had been aired for a much longer period before testing. Nevertheless, there appears to be no systematic difference in the experimental results owing to differences in airing times. The data do suggest small differences in cumulative uptake of ozone for whole carpet sample CP4 in separate experiments performed over the duration of the project. Decreases are observed in

Φo from 8/97 (1.3 µg cm-2), through 11/97 (1.1 µg cm-2), and finally in 4/99 (0.9 µg cm-2). This decrease may be a consequence of the oxidation of surface sites during airing by some species other than ozone. Figure 2 presents plots of the reaction probability, γ, versus cumulative uptake, Φ, for whole carpet, carpet fibers, and carpet backing. Presented in this form, carpet fibers show much more pronounced aging (steeper profiles) than whole carpet or carpet backing samples. In considering this comparison, one should be mindful that the normalizing surface area for determining cumulative uptake is larger by a factor of Rf ) 30-66 for fibers as compared to whole carpet or backing. Normalizing on a consistent basis of floor area covered by carpet would cause the figures for fibers to be shifted to the right by 1.5-1.8 decades; the slope of the curves would not change. In previous work investigating ozone reactions with duct surfaces (16), a power-law (log-linear) relationship was used to describe the dependence of reaction probability on cumulative ozone uptake. We found this relationship to again hold reasonably well for carpet and its constituents

γi ) R(Φi)β

(9)

where R and β are fitted parameters and the subscript i is an index which is designated o for whole carpet, f for carpet fibers, and b for carpet backing. Equation 9 describes a straight-line relationship between γi and Φi when plotted on log-log coordinates. Figure 2 shows that such a relationship holds for portions of all experiments. In Tables 2-4 we have reported values of R, β, and a goodness of fit measure, r2. To obtain these parameters VOL. 34, NO. 23, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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for the whole-carpet and carpet-backing experiments, we used only those experimental data points during which the ozone concentration had stabilized. In a typical CMFR experiment, the amount of data excluded by this criterion was minimal because the time to reach 100 ppb O3 was only about 0.5 h. The carpet-fiber experiments give a length-averaged value of reaction probability, γ j f, rather than the reaction probability of individual fibers, γf. Initially, no fiber has been exposed, and so the fiber reaction probability in the packed bed is uniform (γf ) γ j f at t ) 0). However, in some cases (e.g., for CP3 fibers), the initial outlet concentration, CL, was lower than the limit of detection. The initial reaction probability was so high that all of the ozone was removed within the bed and fibers near the outlet were unexposed. As the fiber surfaces age, the reaction probability decreases, and more ozone is allowed to pass deeper into the bed. During these periods, the spatial variation in γf may be large. After long exposure, CL approaches Cinlet, and all of the fibers are exposed to nearly the same concentration. If the integrated exposure of all fiber surfaces is nearly the same, then we may assume that γf is also nearly the same throughout, or γf = γ j f. Therefore, measurements near the completion of a fixed-bed reactor experiment better reflect the aging phenomena of individual fibers. For consistency in calculating the fitting parameters R and β, we fit eq 9 to data beginning when the ozone level at the reactor outlet reached 70% of the inlet value. Fitted parameters R and β are similar among different samples for whole carpet. Based on mean values for each carpet, the parameter R lies in the range (0.7-2.5) × 10-5 (µg cm-2)-β, and the range of β is -0.07 to -0.24. For carpet backing, similar results were found, with R in the range (13.5) × 10-5 (µg cm-2)-β, and β in the range -0.2 to -0.35. For carpet fibers, there was a distinct difference in the parameter β for fibers of different type. The nylon fibers had large negative values of β, averaging -2.5 (CP1) and -4.5 (CP3). The olefin fibers (CP2 and CP4) exhibited much smaller values of β, approximately -0.8. The physical or chemical properties of the fibers that cause these differences are as yet unknown. Summarizing, the overall apparent reaction probability for whole carpet samples was found to be on the order of 10-5. This corresponds to a deposition velocity for typical indoor airflow conditions on the order of 1 m h-1 (10). For wall-to-wall carpeting in rooms with a 2.5-m ceiling, the carpet-loading factor would be 0.4 m2 per m3 of indoor air. The product of these two numberssdeposition velocity and loading factorsyields an effective indoor air-cleaning rate per room volume associated with ozone scavenging by carpet. Measurements in this study indicate that the typical rate of passive ozone scavenging by carpet yields a benefit in reduced exposure for room occupants that is comparable to processing 0.4 room volumes per hour of air through an efficient ozone-scavenging air filter. However, we have observed (17), and it has been reported by others (8) that volatile aldehydes,

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ketones, and carboxylic acids are produced by ozone reactions with carpeting. Such reaction products degrade indoor air quality to an uncertain extent. It is not clear whether the net effect of ozone-carpet chemistry is beneficial or detrimental with respect to human health.

Acknowledgments This research was funded through a grant from the U.S. Environmental Protection Agency STAR Fellowship Program and by the Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy, under contract No. DE-AC0376SF00098. We are grateful to Al Hodgson and the late Dr. Joan Daisey for their advice and support.

Literature Cited (1) Allen, R. J.; Wadden, R. A.; Ross, E. D. Am. Ind. Hyg. Assoc. J. 1978, 39, 466. (2) Boeniger, M. F. Am. Ind. Hyg. Assoc. J. 1995, 56, 590. (3) Boelter, K. J.; Davidson, J. H. Aerosol Sci. Technol. 1997, 27, 689. (4) Weschler, C. J.; Shields, H. C.; Naik, D. V. JAPCA: J. Air Waste Manage. Assoc. 1989, 39, 1562. (5) Lee, K.; Valarino, J.; Dumyahn, T.; O ¨ zkaynak, H.; Spengler, J. D. J. Air Waste Manage. Assoc. 1999, 49, 1238. (6) Avol, E. L.; Navidi W. C.; Colome S. D. Environ. Sci. Technol. 1998, 32, 463. (7) Sutton, D. J.; Nodolf, K. M.; Makino, K. K. ASHRAE J. 1976, 18, 21. (8) Weschler, C. J.; Hodgson, A. T.; Wooley, J. D. Environ. Sci. Technol. 1992, 26, 2371. (9) Moriske, H. J.; Ebert, G.; Konieczny, L.; Menk, G.; Scho¨ndube, M. Toxicol. Lett. 1998, 96-97, 319. (10) Cano-Ruiz, J. A.; Kong, D.; Balas, R. B.; Nazaroff W. W. Atmos. Environ. 1993, 27A, 2039. (11) Nazaroff, W. W.; Gadgil, A. J.; Weschler, C. J. In Modeling of Indoor Air Quality and Exposure; Nagda, N. L., Ed.; STP 1205, American Society for Testing and Materials: Philadelphia, PA, 1993; pp 81-104. (12) Reiss, R.; Ryan, P. B.; Koutrakis, P. Environ. Sci. Technol. 1994, 28, 504. (13) Mueller, F. X.; Loeb, L.; Mapes, W. H. Environ. Sci. Technol. 1973, 7, 342. (14) Sabersky, R. H.; Sinema, D. A.; Shair, F. H. Environ. Sci. Technol. 1973, 7, 347. (15) Cox, R. A.; Penkett, S. A. Atmos. Environ. 1972, 6, 365. (16) Morrison, G. C.; Nazaroff, W. W.; Cano-Ruiz, J. A.; Hodgson, A. T.; Modera, M. P. J. Air Waste Manage. Assoc. 1998, 48, 941. (17) Morrison, G. C. Ozone-Surface Interactions: Investigations of Mechanisms, Kinetics, Mass Transport, and Implications for Indoor Air Quality, Dissertation, University of California, Berkeley, CA, 1999. (18) Fogler, S. H. Elements of Chemical Reaction Engineering; Prentice-Hall: Englewood, NJ, 1986. (19) Parmar, S. S.; Grosjean, D. Atmos. Environ. 1990, 24A, 2695. (20) Schlatter, J. C. A Practical Guide to Catalyst Testing; Catalytica, Inc.: Mt. View, CA, 1987.

Received for review June 12, 2000. Revised manuscript received September 14, 2000. Accepted September 21, 2000. ES001361H