- Email: [email protected]
Supermicron particle deposition from turbulent chamber flow onto smooth and rough vertical surfaces
ARTICLE IN PRESS
Atmospheric Environment 39 (2005) 4893–4900 www.elsevier.com/locate/atmosenv
Supermicron particle deposition from turbulent chamber flow onto smooth and rough vertical surfaces A.C.K. Laia,, W.W. Nazaroffb a School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 637987 Department of Civil and Environmental Engineering, University of California, Berkeley, California, CA 94720-1710 USA
b
Received 28 February 2005; received in revised form 18 April 2005; accepted 30 April 2005
Abstract Deposition to indoor surfaces influences human exposures and material damage from airborne particulate matter. Experiments were conducted to study the deposition of monodisperse particles in the diameter range 0.9–9 mm from turbulent flow onto smooth and rough vertical chamber surfaces. Fluorescent particles were injected continuously into a stirred 1.8-m3 aluminum chamber for a period of several hours. Deposition was measured on smooth glass plates and sandpaper with four different roughness scales that had been mounted on two opposing vertical sidewalls. Deposition velocities were determined as the ratio of deposited particle flux density to airborne particle concentrations. Contrary to expectations, particle deposition onto smooth and rough vertical surfaces was observed to increase with diameter for most conditions, especially for the larger particle sizes. Deposition velocity increased only moderately with increasing surface roughness. r 2005 Elsevier Ltd. All rights reserved. Keywords: Aerosol; PM10; Pollutant dynamics; Indoor air quality; Soiling
1. Introduction Particle deposition onto indoor surfaces is significant in areas broadly categorized as material damage and human health risk. Indoor particle deposition may pose a hazard to artwork, which may become soiled or chemically damaged (Nazaroff and Cass, 1991), and to electronic devices, whose performance may degrade because of contamination (Litvak et al., 2000). Beneficial effects of particle deposition onto indoor surfaces may accrue from the reduced inhalation exposure and dose owing to lower airborne particle concentrations. On the other hand, deposition from a short-term release Corresponding author. Tel.: +65 6790 4458; fax: +65 6794 2035. E-mail address: [email protected] (A.C.K. Lai).
followed by subsequent resuspension can cause exposures over much a longer period than the original event, altering the total inhaled dose and the population exposed. Although much of the concern for adverse human health effects from exposure to particulate matter focuses on submicron particles, certain supermicron particles can also pose important risks. Examples include allergens, such as pollen, fungal spores, and dust-mite detritus (Institute of Medicine, 2000); and airborne infectious agents (Weis et al, 2002; Nicas et al., 2005). Understanding the factors that control the concentrations and fates of supermicron particles can improve the basis for evaluating human health risks and for designing more effective engineering interventions. In general, deposition processes are complex, varying markedly with particle characteristics (such as size),
1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2005.04.036
ARTICLE IN PRESS 4894
A.C.K. Lai, W.W. Nazaroff / Atmospheric Environment 39 (2005) 4893–4900
surface characteristics (such as orientation), and airflow characteristics (such as degree of turbulence). As reviewed by Lai (2002), many studies have been conducted to quantify particle deposition rates to indoor surfaces and to understand the factors that influence them. However, the present state of knowledge is far from complete. Among deposition experiments conducted in rooms or in chambers, some researchers have inferred deposition rates by measuring the particle concentration decay rate (Xu et al., 1994; Fogh et al., 1997; Abadie et al., 2001; Thatcher et al., 2002). Others have investigated deposition by measuring the indoor–outdoor concentration relationship and interpreting it using a massbalance model (O¨zkaynak et al., 1996; Long et al., 2001; Vette et al., 2001; Thatcher et al., 2003). These concentration-decay and material-balance approaches only permit determination of an overall loss rate, equal to the area-weighted-average deposition velocity times the surface-to-volume ratio of the space. For supermicron particles, gravitational settling to upward surfaces dominates the overall loss rate. Therefore, experiments that measure the overall loss rate cannot be used to determine the rate of supermicron particle deposition to vertical surfaces. A few studies have been reported that measure particle deposition directly by quantifying particle accumulation onto surfaces in relation to the corresponding airborne particle concentration. By this method, orientation-dependent deposition velocities can be evaluated. Studies that resolve particle deposition by size have been conducted in museums (Ligocki et al., 1990) and in laboratory chambers (Byrne et al., 1995; Thatcher et al., 1996; Lai et al., 2002). Very few experiments have been performed to study particle deposition onto vertically oriented rough surfaces in chambers or rooms. No firm conclusions can yet be drawn on the influence of roughness on deposition for typical indoor conditions. Studies by Harrison (1979), Shimada et al. (1987), and Abadie et al. (2001) only measured the airborne particle concentrations and so their experiments do not provide direct evidence regarding deposition to vertical surfaces. Shimada et al. (1987) showed increases of one to two orders of magnitude in overall loss rate for sandpapercovered surfaces relative to a smooth tank. However, compared to conditions of interest for indoor air, their experiments employed high turbulence intensity in a very small chamber. Abadie et al. (2001) found a much smaller effect of rough wallpaper on increasing deposition relative to a smooth surface. We know of only two studies that have reported size-resolved, coarse-particle deposition to rough vertical chamber surfaces from mechanically induced turbulent flow (Byrne et al., 1995; Lai et al., 2002). With one exception (wallpaper used by Byrne et al.), the roughness elements in these studies
were much larger than would be commonly found on indoor walls. The literature contains extensive studies of particle deposition from turbulent pipe flow, as reviewed by Sippola (2002). This work informs studies of particle deposition from turbulent flow in chambers or rooms. However, because of the considerable differences in near-surface airflow conditions and in surface topography, deposition from pipe flow is not a substitute for research that directly addresses particle deposition to indoor surfaces. In light of the incomplete understanding and in view of the complexity and importance of particle deposition onto indoor surfaces, we conducted the study described here. Our primary goal was to measure the deposition of supermicron (1–10 mm diameter) particles from turbulent flow onto vertical surfaces that varied from aerodynamically smooth (glass) to roughness similar to conditions found in ordinary indoor environments.
2. Methods Experiments were conducted in a 1.8-m3 cubic aluminum chamber. Except for the front wall, the surfaces were all thermally insulated to minimize the influence of natural convection and thermophoresis. The chamber walls were electrically grounded and the Perspex front door was covered with aluminum foil to minimize electric field effects on particle deposition. In separate experimental runs, eight distinct sizes of fluorescent particles were generated using a vibrating orifice aerosol generator, VOAG (TSI, Saint Paul, MN, Model 3450). The VOAG generates highly monodisperse particles. It was oriented in an upside-down position to maximize the transmission efficiency of coarse particles into the chamber. A positive displacement, high-pressure pump (Rainin, Model HP) supplied a stable fluorescent liquid stream to the VOAG. The output aerosol was neutralized to a Boltzmann charge distribution using a Kr-85 source (TSI, Saint Paul, MN, Model 3054). Particles were injected through copper tubing at low velocity into the center of the chamber adjacent to the fan. Mixing and turbulent flow near surfaces was induced by means of a small instrumentcooling fan, which was mounted near the center of the ceiling, 13.5 cm below the top, with the induced flow oriented vertically downward. Constant voltage at the rated input of 12 V was provided to the fan using a regulated power supply throughout each experiment. The main goal of the present work was to investigate particle deposition onto vertically oriented surfaces with various roughness scales similar to those found indoors. We selected sandpaper to permit systematic variation of surface roughness with regular and easily characterized features. In addition, there is some knowledge about
ARTICLE IN PRESS A.C.K. Lai, W.W. Nazaroff / Atmospheric Environment 39 (2005) 4893–4900
airflow characteristics pertinent to particle deposition in the vicinity of sand type roughness elements (Grass, 1971; Browne, 1974; Wood, 1981; El-Shobokshy, 1983; Guha, 1997). 2.1. Selection of deposition surfaces Many materials found in commercially available sandpaper possess detectable fluorescence levels. The sandpapers used in this study were carefully selected and tested to ensure negligible background level. Four different grades of aluminum oxide sandpapers were selected to represent, approximately, the range of roughness that might be expected on indoor walls: P60, P100, P150 and P220, with corresponding average grit sizes of 250, 150, 100 and 70 mm, respectively (CAFA, 2005). The dimensions of each sandpaper sheet were 23 cm 28 cm. In addition to the sandpapers, ordinary window glass was also tested. The plate dimensions were 28 cm 8 cm and the mean roughness height was approximately 10 mm. 2.2. Preliminary testing: Mixing conditions For proper interpretation of the results, it was important to ensure that the airborne particle concentration was uniform throughout the chamber. Nonuniform particle concentration could cause non-uniform deposits, and hence inaccurate determination of deposition velocity. Experiments were conducted to test the well-mixed assumption. Eight copper foils (each approximately 2 cm 2 cm) were positioned at welldistributed locations across the chamber floor. Monodisperse fluorescent particles of 3 mm diameter, generated by the vibrating orifice aerosol generator, were introduced continuously for one hour with the mixing fan in operation. Fluorescence readings were measured for each foil. The maximum relative discrepancy among all the samples was less than 15%. Hence the approximation that concentrations in the chamber were spatially uniform is reasonable. 2.3. Experimental procedure The chamber facility and methods are similar to those reported by Thatcher et al. (1996). Key aspects are reported here. In summary, the procedure for each experiment involved cleaning the chamber and sample surfaces, affixing sandpaper and glass plates to paper mounted on the walls, injecting monodisperse fluorescent particles into the chamber, and then, after an exposure period during which particles were also collected from air on filters, analyzing surface and airborne samples for fluorescence. The particles were extracted from the deposition surfaces and the filter samples using a buffer solution and the amount of
4895
fluorescent material was quantified by measuring the extracts with a fluorometer. To elaborate, before each run, the chamber surfaces were cleaned thoroughly with mild detergent and water. After drying, each sampling wall was covered with clean paper (45 cm 80 cm) to which the test surfaces were affixed. (The purpose of the paper, which was discarded after each experiment, was to protect the surfaces from inadvertent contamination.) The test surfaces were fixed to the paper with adhesive tape. Glass plates and sandpaper samples with four different roughness scales (typically two pieces for each grit size) were mounted onto two vertical walls. At the start of a run, a portable air cleaner was placed inside the chamber, which was then sealed. The air cleaner was operated remotely for approximately 10–20 min, after which the total particle count was effectively zero. Monodisperse fluorescent particles of known size were then generated, introduced continuously into the chamber, and allowed to deposit over a period of 8–16 h. Typical particle concentrations were 5–50 mg m3. Ammonium fluorescein particles were used for all experiments. Solutions were prepared from mixtures of oleic oil, isopropyl alcohol, deionized water, ammonium hydroxide and fluorescein (sodium salt). Upon evaporation, the droplet residue primarily comprises a mixture of oleic oil and sodium fluorescein. Particle size and concentration were monitored in real time by means of a particle sizer (Aerosol Particle Sizer, TSI, Saint Paul, MN, Model 3320). To determine deposition velocity, the average airborne particle concentration was measured from timeintegrated filter samples. In each experiment, two filter samples were collected by drawing air through the filter at a constant rate throughout the sampling period. The filters were membrane filters with 0.8 mm pore size (AAWP25, Millipore). Sample pumps were calibrated using a Gillibrator bubble flow meter (Gillian Instrument Corporation) and were found to be stable at a typical sampling rate of 1.6 L min1. The chamber particle concentration was determined as the average of the two filter samples. For all experiments, the concentration difference between the two filters was less than 10%. (This observation also supports the wellmixed representation of the chamber air, which was assumed to hold.) For each particle size, replicate experiments were conducted at least 3 times (typically 5 times) to determine experimental precision. At the end of the each experimental run, the aerosol generator was turned off, the delivery tube was removed, and the air cleaner was turned on for about 10 min or until the particle count was again effectively zero. The chamber door was then opened. The amount of material deposited or filtered was determined by measuring the fluorescence of the extract with a fluorometer (Turner Designs Corporation,
ARTICLE IN PRESS A.C.K. Lai, W.W. Nazaroff / Atmospheric Environment 39 (2005) 4893–4900
4896
Sunnyvale, CA, Model TD-700). The mass of particles deposited for each sample was determined using 10 mL of buffer solution. Glass plates were similarly analyzed, following buffer extraction of deposited particles from the immersed sample plates. Each glass plate was carefully removed from the clean paper and then immersed into a cleaned galvanized steel tray filled with 20 mL of buffer. Unlike sandpaper, which was discarded after each experiment, the glass plates were reused. Deposition was parameterized in terms of a deposition velocity, Vd (m s1), evaluated as follows: Vd ¼
J , C1
(1)
where J is the average particle flux density to the surface (mg m2 s1) and CN (mg m3) is the particle concentration in the chamber air. The particle flux density and particle concentration were evaluated by the following expressions: J¼
M sample , Asample ts
(2)
M filter , Qs t s
(3)
C1 ¼
where Msample and Mfilter are the fluorescein masses measured on a deposition surface and on the air filters, respectively, Asample is the area of the deposition surface, ts is the duration of particle exposure (and air sampling) and Qs is the volumetric flow rate through the air filter samples. Preliminary evaluations were undertaken to test the accuracy and repeatability of the final measurement protocols for all materials. We are confident that the errors associated with extracting and measuring fluorescein were small (less than 10%) as compared with observed variability either within experiments or between experiments. The extraction efficiency for the airsampling filters has been demonstrated to be 98% or better (Thatcher et al., 1996). To test the extraction efficiency from different types of surfaces, a few pieces of sandpaper and smooth copper foil were placed on the chamber floor adjacent to each other, so that deposition would be the same, as controlled by the mechanism of settling. The measured fluxes on these two types of surfaces were very close to each other. Extraction efficiency for sandpaper surfaces was also tested by means of carrying out a second extraction on selected samples and demonstrating that the fluorescence of the second buffer was very much smaller than that from the first extraction. It was also confirmed that the backing paper used in the experiments exhibited negligible fluorescence when subjected to extraction and analysis. The mixing fan near the chamber ceiling induced turbulent airflow in the chamber. The mean airspeed at
3 cm away from the wall surfaces was measured by means of an omnidirectional air velocity transducer (TSI, Saint Paul, MN, Model 8470) at ten locations on each of the two sampling walls. The measurements confirmed that the speeds at the different sampling locations were fairly consistent with mean values of 0.17 and 0.14 m s1 on the left and right side walls, respectively. The friction velocity, u*, was estimated by measuring the airspeed in the logarithmic flow region near the chamber wall (Lai and Nazaroff, 2000), using a laser doppler velocimeter (TSI, Saint Paul, MN, Model Laservec Diode Velocimeter). The measurements, taken at a single position on a vertical wall, yielded u ¼ 2:6 cm s1 : According to Nikuradse’s roughness classification (Nikuradse, 1933), the dimensionless roughness height of all of the sandpapers tested is less than the thickness of the fluid mechanical viscous sublayer, so that the surfaces can be considered aerodynamically smooth. However, in light of their very small diffusion coefficients, particle concentration boundary layers are much thinner than the fluid momentum boundary layer. So, although the sandpaper surfaces are aerodynamically smooth, it is expected that their roughness elements protrude across the particle concentration boundary layer and could therefore influence the particle deposition rate.
3. Results Table 1 presents the detailed experimental results, reporting average and standard deviation of the particle deposition velocity as a function of particle diameter for all five surface conditions. Considering all particle sizes and surfaces, the deposition velocity lies within the range (0.4–56) 106 m s1. Fig. 1 depicts the measured particle deposition velocity onto smooth glass surfaces for particle size ranging from 0.9 to 9 mm. The dominant experimental trend is counterintuitive. Starting from the smallest particles, as size increases, deposition velocity decreases initially (as expected) but then increases for particle sizes greater than about 3 mm. For particle sizes greater than 7 mm, the deposition velocity appears to level off. Measured particle deposition velocities for four roughness grades of sandpaper are shown in Fig. 2. A general trend that is similar to that for the smooth glass can be observed. Beginning with 0.9 mm particles, with increasing particle diameter, deposition velocity remains approximately constant or decreases slightly initially and then increases for particles larger than 3 mm. For all results presented, it is noted that the deposition velocity attains a fairly steady value for particle size larger than 7 mm. Another significant finding is that the deposition rate increases with increasing roughness scale. However,
ARTICLE IN PRESS A.C.K. Lai, W.W. Nazaroff / Atmospheric Environment 39 (2005) 4893–4900
4897
Table 1 Particle deposition velocity as a function of particle diameter for vertical deposition surfaces with different degrees of surface roughnessa Glass
Sand 220
Sand 150
Sand 100
Sand 60
dp
Vd
dp
Vd
dp
Vd
dp
Vd
dp
Vd
0.9 1.6 2.2 3.5 5.0 7.0 7.8 9.1
1.770.7 1.070.7 0.470.2 1.170.8 4.572.3 1277 5.570.8 1173
1.1 1.8 2.4 3.7 5.2 7.2 8.0 9.3
3.471.5 1.670.9 1.070.5 3.272.0 3.771.9 22718 22713 30713
1.1 1.8 2.4 3.7 5.2 7.2 8.0 9.3
1.671.1 1.670.6 1.370.2 6.872.7 5.473.4 25716 19716 20714
1.0 1.7 2.3 3.6 5.1 7.1 7.9 9.2
1.870.3 1.970.9 2.370.5 5.872.2 1675 29727 1177 43719
1.0 1.7 2.3 3.6 5.1 7.1 7.9 9.2
1.870.4 3.870.6 2.671.3 6.572.5 1077 34723 26711 5677
a
Deposition velocity (Vd) reported as mean7standard deviation with units of 106 m s1. Particle diameter (dp) reported in units of mm. Surface roughness of the glass plates was nominally 10 mm. For the sandpaper, grain sizes were as follows: sand 220 ¼ 70 mm; sand 150 ¼ 100 mm; sand 100 ¼ 150 mm; sand 60 ¼ 250 mm.
deposition velocity (µm/s)
30 10 3 1
u* = 9.8 cm/s
0.3 0.1 u* = 2.6 cm/s
0.03 1
3 5 particle diameter (µm)
10
Fig. 1. Particle deposition velocity onto smooth vertical surfaces as a function of particle diameter. The points indicate the mean values and the bars represent one standard deviation of the measurements. The curves represent model predictions for two different friction velocities, u* (Lai and Nazaroff, 2000).
the differences with changing roughness are relatively small compared with the influence of particle size on deposition.
4. Discussion The experimental data reveal two dominant trends. First, the deposition velocity tends to increase with particle size for most of the conditions studied. For the glass plates, and for the finest grain sandpaper, the deposition velocity decreases as particle size increases from 0.9 to 2.5 mm, and then begins to rise. For the roughest two sandpapers, the deposition velocity in-
creases monotonically with increasing particle diameter throughout the experimental range. Second, the effect of particle size is much larger than the effect of surface roughness. For each surface considered separately, the ratio of the largest to smallest deposition velocity as particle size varies lies in the range 20–30. With particle size fixed, changing roughness only influences particle deposition by a factor of 6 or less. We note that the relaxation times for particles in this study are low. With a friction velocity (u*) of 2.6 cms1 and particle densities in the range 1–1.4 g cm3, the dimensionless relaxation times for 0.9–9 mm particles would be in the range 1.3 104 to 0.016. Based on studies of particle deposition from vertically oriented turbulent pipe flow, this range places particle conditions for our study in what is known as the diffusion regime (Sippola, 2002). Here, the influence of inertial impaction is expected to be small, and deposition would be expected to decrease with increasing particle size. This expectation is contrasted with that for the diffusion– impaction regime, for which the dimensionless relaxation time is in the range 0.1–10, and particle deposition increases with increasing particle size. The model of Lai and Nazaroff (2000) was developed to be applicable to the conditions of this experiment for the glass surface. However, the model predictions do not correspond well with the experimental observations reported here. Fig. 1 illustrates this point, comparing the experimental data for glass with model predictions. The lower model prediction curve corresponds to the experimentally determined friction velocity of 2.6 cm s1. The upper curve, for a friction velocity of 9.8 cm s1, represents a best fit of the model predictions to the three smallest-diameter data points. Clearly, the model predicts neither the trend nor the magnitude for the larger particle sizes studied. The divergence for the largest particle sizes is striking: for 9 mm particles, the
ARTICLE IN PRESS A.C.K. Lai, W.W. Nazaroff / Atmospheric Environment 39 (2005) 4893–4900
particle diameter (µm) 3 5
9
1
particle diameter (µm) 3 5
deposition velocity (µm/s)
deposition velocity (µm/s)
80 sandpaper 220 (70 µm)
30
10 80
sandpaper 150 (100 µm)
30
10
10
3
3
1
1
30
sandpaper 100 (150 µm)
sandpaper 60 (250 µm)
30
10
10
3
3
1
1
0.2
deposition velocity (µm/s)
1
deposition velocity (µm/s)
4898
0.2 1
3 5 particle diameter (µm)
9
1
3 5 particle diameter (µm)
10
Fig. 2. Particle deposition velocity onto vertically oriented sandpaper surfaces. Points indicate mean values and bars represent one standard deviation of the measurements. The gray bars indicate deposition onto smooth glass plates (from Fig. 1) and are shown for reference.
measured deposition velocity was 11 106 m s1, a factor of 30–150 larger than the modeled deposition velocities for friction velocities of 2.6–9.8 cm s1. Models for particle deposition from turbulent flow to rough vertical surfaces have also been developed, but they do not yield good agreement with these new measurements, either. Fundamentally, we believe that the discrepancy between model predictions and measurements is a consequence of inadequacies in the models, because some key physical processes affecting transport and deposition of coarse particles to vertical surfaces are not appropriately represented. We have investigated possible explanations. One that appears unlikely to contribute significantly is electrostatic drift. Because of the grounded chamber surfaces, electric fields within the chamber would be small. Image force effects are also expected to be small for particles charged with a Boltzmann distribution (Chen and Lai, 2004).
On the other hand, even though the dimensionless relaxation times are low, we believe that potentially significant contributions might be attributable to inertial transport of particles through boundary layers, possibly enhanced by turbulent burst phenomena near surfaces. A possible contributor that might help reconcile the observations in these experiments with the expectations from studies of turbulent pipe flow is the potential for more highly irregular turbulent flow near surfaces in a chamber. Modeling efforts may be fruitful if they focus on understanding the effect on particle deposition of the high-energy tail of the distribution of near-wall turbulent fluctuations. Another factor we considered is gravitational settling onto the upward surfaces of roughness elements. Turbulent transport need only transport particles to the interstices between grains if settling to the upward side of the grain surface is sufficiently rapid. However, while this factor could be important for sandpaper, it is
ARTICLE IN PRESS A.C.K. Lai, W.W. Nazaroff / Atmospheric Environment 39 (2005) 4893–4900
deposition loss rate coefficient (per h)
unlikely to be important for glass. That the dominant deposition features are similar for all surfaces in these experiments suggests that settling onto roughness elements is not a key factor. One further contributing factor to the divergence between model and measurement might be the lift force (Saffman, 1965). Coarse particles suspended in a shear flow, as in the viscous sublayer adjacent to the vertical surfaces in these experiments, will experience a force toward the wall (enhancing deposition) when the overall airflow is downward, and the particle is traveling more rapidly than the air speed owing to gravitational settling. Holmberg and Li (1998) have carried out numerical simulations of the airflow field in a chamber configured similarly to ours, albeit larger (8 m3 vs. 2 m3). Fig. 3 from their paper indicates mean downward flow along the upper half of the wall centerlines. Even for positions along walls where the mean flow is upward, as appears to be the case in the lower portion of the walls in their figure, the weak overall flow in such a chamber leaves open the possibility that for some fraction of time largescale eddies induce net downward flow locally in the viscous sublayer. In simulating particle deposition from turbulent flow in ducts, Zhang and Ahmadi (2000; Fig. 7) have shown strong effects of the lift force influencing deposition in a vertical duct for friction velocities comparable with those in our experiments (0.03 and 0.1 m s1) and for dimensionless particle relaxation times not inordinately greater (102 to 1 in Zhang and Ahmadi as compared with 104 to 102 in our experiments).
10
room (Thatcher et al., 2002) walls (sandpaper 60)
1
0.1
0.01 1
3 5 particle diameter (µm)
10
Fig. 3. Particle loss-rate coefficients by means of deposition onto indoor surfaces as a function of particle diameter. The ‘‘room’’ traces reflect deposition to all surfaces for ‘‘bare’’ conditions in a 2.2 m 2.7 m 2.4 m (high) enclosure under four different levels of airflow intensity (Thatcher et al., 2002). The data points reflect deposition only to walls, and were determined by applying Eq. (4) to the experimental data reported in the present paper for sandpaper 60 (grit size ¼ 250 mm), with Awalls =V ¼ 1:65 m2 m3 .
4899
The model-measurement discrepancy reported here does not significantly alter the understanding of the overall rate of depositional loss of coarse particles from indoor air. Deposition onto upward surfaces by means of gravitational settling, possibly augmented by inertial impaction still dominates. This point is illustrated in Fig. 3, which shows that the loss-rate coefficients for walls based on results in the current study are more than an order of magnitude smaller than the measured overall loss-rate coefficients for rooms (Thatcher et al., 2002). In making this comparison, we converted deposition velocity to a wall loss-rate coefficient, bwall, using the relationship V d Awall , (4) V where Awall is the wall area of a room and V is the room volume. The comparison in Fig. 3 also reinforces a point made in the introduction. Because coarse-particle deposition to upward surfaces dominates, one cannot investigate coarse-particle deposition to walls by measurement methods that only consider the overall rate of particle loss from indoor air. Because of the weak dependence of total depositional loss on deposition to walls, such experiments are effectively ‘‘blind’’ to the rate of coarse particle deposition on walls.
bwall ¼
5. Conclusion Particle deposition onto indoor surfaces is a complex, multifaceted phenomenon that has relevance to issues as disparate as predicting human exposure to airborne particles and protecting works of art from soiling. As was already well known, and is further substantiated by the evidence presented in this paper, particle size plays a major role influencing particle deposition. Ample evidence indicates that airflow conditions and surface characteristics are also important. Particle deposition experiments are challenging to conduct, and the potentially important influence of many otherwise subtle processes means that modeling is also difficult. The contributions of the present paper lie in the experimental data and the observations about model-measurement discrepancies. Although progress on understanding particle deposition is hard won, we believe that continued efforts to advance our understanding of such processes are warranted.
Acknowledgements The experimental work was carried out at the laboratory of Department of Civil and Environmental Engineering in University of California, Berkeley. Shannon Coulter-Burke assisted in conducting the
ARTICLE IN PRESS 4900
A.C.K. Lai, W.W. Nazaroff / Atmospheric Environment 39 (2005) 4893–4900
experiments. This research was partly supported by the Office of Research and Development, Office of Nonproliferation and National Security, US Department of Energy under Contact No. DE-AC03-76SF00098.
References Abadie, M., Limam, K., Allard, F., 2001. Indoor particle pollution: effect of wall textures on particle deposition. Building and Environment 36, 821–827. Browne, L.W.B., 1974. Deposition of particles on rough surfaces during turbulent gas-flow in a pipe. Atmospheric Environment 8, 801–816. Byrne, M.A., Goddard, A.J.H., Lange, C., Roed, J., 1995. Stable tracer aerosol deposition measurements in a test chamber. Journal of Aerosol Science 26, 645–653. CAFA, 2005. Abrasives Info/Grit Comparison Chart. Coated Abrasives Fabricators Association. Available from http:// www.cafa-info.org/main.html. Chen, F., Lai, A.C.K., 2004. An Eulerian model for particle deposition under electrostatic and turbulent conditions. Journal of Aerosol Science 35, 47–62. El-Shobokshy, M.S., 1983. Experimental measurements of aerosol deposition to smooth and rough surfaces. Atmospheric Environment 17, 639–644. Fogh, C.L., Byrne, M.A., Roed, J., Goddard, A.J.H., 1997. Size specific indoor aerosol deposition measurements and derived I/O concentrations ratios. Atmospheric Environment 31, 2193–2203. Grass, A.J., 1971. Structural features of turbulent flow over smooth and rough boundaries. Journal of Fluid Mechanics 50, 233–255. Guha, A., 1997. A unified Eulerian theory of turbulent deposition to smooth and rough surfaces. Journal of Aerosol Science 28, 1517–1537. Harrison, A.W., 1979. Quiescent boundary-layer thickness in aerosol enclosures under convective stirring conditions. Journal of Colloid and Interface Science 69, 563–570. Holmberg, S., Li, Y., 1998. Modelling of the indoor environment—Particle dispersion and deposition. Indoor Air 8, 113–122. Institute of Medicine, 2000. Clearing the Air: Asthma and Indoor Air Exposures. National Academy Press, Washington, DC (Chapter 10). Lai, A.C.K., 2002. Particle deposition indoors: a review. Indoor Air 12, 211–214. Lai, A.C.K., Nazaroff, W.W., 2000. Modeling indoor particle deposition from turbulent flow onto smooth surfaces. Journal of Aerosol Science 31, 463–476. Lai, A.C.K., Byrne, M.A., Goddard, A.J.H., 2002. Experimental studies of the effect of rough surfaces and air speed on aerosol deposition in a test chamber. Aerosol Science and Technology 36, 973–982; 37, 401. Ligocki, M.P., Liu, H.I.H., Cass, G.R., John, W., 1990. Measurements of particle deposition rates inside southern California museums. Aerosol Science and Technology 13, 85–101. Litvak, A., Gadgil, A.J., Fisk, W.J., 2000. Hygroscopic fine mode particle deposition on electronic circuits and resulting
degradation of circuit performance: an experimental study. Indoor Air 10, 47–56. Long, C.M., Suh, H.H., Catalano, P.J., Koutrakis, P., 2001. Using time-and size-resolved particulate data to quantify indoor penetration and deposition behavior. Environmental Science & Technology 35, 2089–2099. Nazaroff, W.W., Cass, G.R., 1991. Protecting museum collections from soiling due to the deposition of airborne particles. Atmospheric Environment 25A, 841–852. Nicas, M., Nazaroff, W.W., Hubbard, A., 2005. Toward understanding the risk of secondary airborne infection: emission of respirable pathogens. Journal of Occupational and Environmental Hygiene 2, 143–154. Nikuradse, J., 1933. Laws for flow in rough pipes. VDI Forschungsheft (English translation, NACA TM 1292, 1950). O¨zkaynak, H., Xue, J., Spengler, J., Wallace, L., Pellizzari, E., Jenkins, P., 1996. Personal exposure to airborne particles and metals: results from the Particle TEAM study in Riverside, California. Journal of Exposure Analysis and Environmental Epidemiology 6, 57–78. Saffman, P.G., 1965. The lift on a small sphere in a slow shear flow. Journal of Fluid Mechanics 22, 385–400. Shimada, M., Okuyama, K., Kousaka, Y., Ohshima, K., 1987. Turbulent and Brownian diffusive deposition of aerosol particles onto a rough wall. Journal of Chemical Engineering of Japan 20, 57–64. Sippola, M.R., 2002. Particle deposition in ventilation ducts. Ph.D. Dissertation. University of California, Berkeley, CA, USA. Thatcher, T.L., Fairchild, W.A., Nazaroff, W.W., 1996. Particle deposition from natural convection enclosure flow onto smooth surfaces. Aerosol Science and Technology 25, 359–374. Thatcher, T.L., Lai, A.C.K., Moreno-Jackson, R., Sextro, R.G., Nazaroff, W.W., 2002. Effects of room furnishings and air speed on particle deposition rates indoors. Atmospheric Environment 36, 1811–1819. Thatcher, T.L., Lunden, M.M., Revzan, K.L., Sextro, R.G., Brown, N.J., 2003. A concentration rebound method for measuring particle penetration and deposition in the indoor environment. Aerosol Science and Technology 37, 847–864. Vette, A.F., Rea, A.W., Lawless, P.A., Rodes, C.E., Evans, G., Highsmith, V.R., Sheldon, L., 2001. Characterization of indoor–outdoor aerosol concentration relationships during the Fresno PM exposure studies. Aerosol Science and Technology 34, 118–126. Weis, C.P., Intrepido, A.J., Miller, A.K., Cowin, P.G., Durno, M.A., Gebhardt, J.S., Bull, R., 2002. Secondary aerosolization of viable Bacillus anthracis spores in a contaminated US Senate office. Journal of the American Medical Association 288, 2853–2858. Wood, N.B., 1981. A simple method for the calculation of turbulent deposition to smooth and rough surfaces. Journal of Aerosol Science 12, 275–290. Xu, M.D., Nematollahi, M., Sextro, R.G., Gadgil, A.J., Nazaroff, W.W., 1994. Deposition of tobacco-smoke particles in a low ventilation room. Aerosol Science and Technology 20, 194–206. Zhang, H., Ahmadi, G., 2000. Aerosol particle transport and deposition in vertical and horizontal turbulent duct flows. Journal of Fluid Mechanics 406, 55–80.
Atmospheric Environment 39 (2005) 4893–4900 www.elsevier.com/locate/atmosenv
Supermicron particle deposition from turbulent chamber flow onto smooth and rough vertical surfaces A.C.K. Laia,, W.W. Nazaroffb a School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 637987 Department of Civil and Environmental Engineering, University of California, Berkeley, California, CA 94720-1710 USA
b
Received 28 February 2005; received in revised form 18 April 2005; accepted 30 April 2005
Abstract Deposition to indoor surfaces influences human exposures and material damage from airborne particulate matter. Experiments were conducted to study the deposition of monodisperse particles in the diameter range 0.9–9 mm from turbulent flow onto smooth and rough vertical chamber surfaces. Fluorescent particles were injected continuously into a stirred 1.8-m3 aluminum chamber for a period of several hours. Deposition was measured on smooth glass plates and sandpaper with four different roughness scales that had been mounted on two opposing vertical sidewalls. Deposition velocities were determined as the ratio of deposited particle flux density to airborne particle concentrations. Contrary to expectations, particle deposition onto smooth and rough vertical surfaces was observed to increase with diameter for most conditions, especially for the larger particle sizes. Deposition velocity increased only moderately with increasing surface roughness. r 2005 Elsevier Ltd. All rights reserved. Keywords: Aerosol; PM10; Pollutant dynamics; Indoor air quality; Soiling
1. Introduction Particle deposition onto indoor surfaces is significant in areas broadly categorized as material damage and human health risk. Indoor particle deposition may pose a hazard to artwork, which may become soiled or chemically damaged (Nazaroff and Cass, 1991), and to electronic devices, whose performance may degrade because of contamination (Litvak et al., 2000). Beneficial effects of particle deposition onto indoor surfaces may accrue from the reduced inhalation exposure and dose owing to lower airborne particle concentrations. On the other hand, deposition from a short-term release Corresponding author. Tel.: +65 6790 4458; fax: +65 6794 2035. E-mail address: [email protected] (A.C.K. Lai).
followed by subsequent resuspension can cause exposures over much a longer period than the original event, altering the total inhaled dose and the population exposed. Although much of the concern for adverse human health effects from exposure to particulate matter focuses on submicron particles, certain supermicron particles can also pose important risks. Examples include allergens, such as pollen, fungal spores, and dust-mite detritus (Institute of Medicine, 2000); and airborne infectious agents (Weis et al, 2002; Nicas et al., 2005). Understanding the factors that control the concentrations and fates of supermicron particles can improve the basis for evaluating human health risks and for designing more effective engineering interventions. In general, deposition processes are complex, varying markedly with particle characteristics (such as size),
1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2005.04.036
ARTICLE IN PRESS 4894
A.C.K. Lai, W.W. Nazaroff / Atmospheric Environment 39 (2005) 4893–4900
surface characteristics (such as orientation), and airflow characteristics (such as degree of turbulence). As reviewed by Lai (2002), many studies have been conducted to quantify particle deposition rates to indoor surfaces and to understand the factors that influence them. However, the present state of knowledge is far from complete. Among deposition experiments conducted in rooms or in chambers, some researchers have inferred deposition rates by measuring the particle concentration decay rate (Xu et al., 1994; Fogh et al., 1997; Abadie et al., 2001; Thatcher et al., 2002). Others have investigated deposition by measuring the indoor–outdoor concentration relationship and interpreting it using a massbalance model (O¨zkaynak et al., 1996; Long et al., 2001; Vette et al., 2001; Thatcher et al., 2003). These concentration-decay and material-balance approaches only permit determination of an overall loss rate, equal to the area-weighted-average deposition velocity times the surface-to-volume ratio of the space. For supermicron particles, gravitational settling to upward surfaces dominates the overall loss rate. Therefore, experiments that measure the overall loss rate cannot be used to determine the rate of supermicron particle deposition to vertical surfaces. A few studies have been reported that measure particle deposition directly by quantifying particle accumulation onto surfaces in relation to the corresponding airborne particle concentration. By this method, orientation-dependent deposition velocities can be evaluated. Studies that resolve particle deposition by size have been conducted in museums (Ligocki et al., 1990) and in laboratory chambers (Byrne et al., 1995; Thatcher et al., 1996; Lai et al., 2002). Very few experiments have been performed to study particle deposition onto vertically oriented rough surfaces in chambers or rooms. No firm conclusions can yet be drawn on the influence of roughness on deposition for typical indoor conditions. Studies by Harrison (1979), Shimada et al. (1987), and Abadie et al. (2001) only measured the airborne particle concentrations and so their experiments do not provide direct evidence regarding deposition to vertical surfaces. Shimada et al. (1987) showed increases of one to two orders of magnitude in overall loss rate for sandpapercovered surfaces relative to a smooth tank. However, compared to conditions of interest for indoor air, their experiments employed high turbulence intensity in a very small chamber. Abadie et al. (2001) found a much smaller effect of rough wallpaper on increasing deposition relative to a smooth surface. We know of only two studies that have reported size-resolved, coarse-particle deposition to rough vertical chamber surfaces from mechanically induced turbulent flow (Byrne et al., 1995; Lai et al., 2002). With one exception (wallpaper used by Byrne et al.), the roughness elements in these studies
were much larger than would be commonly found on indoor walls. The literature contains extensive studies of particle deposition from turbulent pipe flow, as reviewed by Sippola (2002). This work informs studies of particle deposition from turbulent flow in chambers or rooms. However, because of the considerable differences in near-surface airflow conditions and in surface topography, deposition from pipe flow is not a substitute for research that directly addresses particle deposition to indoor surfaces. In light of the incomplete understanding and in view of the complexity and importance of particle deposition onto indoor surfaces, we conducted the study described here. Our primary goal was to measure the deposition of supermicron (1–10 mm diameter) particles from turbulent flow onto vertical surfaces that varied from aerodynamically smooth (glass) to roughness similar to conditions found in ordinary indoor environments.
2. Methods Experiments were conducted in a 1.8-m3 cubic aluminum chamber. Except for the front wall, the surfaces were all thermally insulated to minimize the influence of natural convection and thermophoresis. The chamber walls were electrically grounded and the Perspex front door was covered with aluminum foil to minimize electric field effects on particle deposition. In separate experimental runs, eight distinct sizes of fluorescent particles were generated using a vibrating orifice aerosol generator, VOAG (TSI, Saint Paul, MN, Model 3450). The VOAG generates highly monodisperse particles. It was oriented in an upside-down position to maximize the transmission efficiency of coarse particles into the chamber. A positive displacement, high-pressure pump (Rainin, Model HP) supplied a stable fluorescent liquid stream to the VOAG. The output aerosol was neutralized to a Boltzmann charge distribution using a Kr-85 source (TSI, Saint Paul, MN, Model 3054). Particles were injected through copper tubing at low velocity into the center of the chamber adjacent to the fan. Mixing and turbulent flow near surfaces was induced by means of a small instrumentcooling fan, which was mounted near the center of the ceiling, 13.5 cm below the top, with the induced flow oriented vertically downward. Constant voltage at the rated input of 12 V was provided to the fan using a regulated power supply throughout each experiment. The main goal of the present work was to investigate particle deposition onto vertically oriented surfaces with various roughness scales similar to those found indoors. We selected sandpaper to permit systematic variation of surface roughness with regular and easily characterized features. In addition, there is some knowledge about
ARTICLE IN PRESS A.C.K. Lai, W.W. Nazaroff / Atmospheric Environment 39 (2005) 4893–4900
airflow characteristics pertinent to particle deposition in the vicinity of sand type roughness elements (Grass, 1971; Browne, 1974; Wood, 1981; El-Shobokshy, 1983; Guha, 1997). 2.1. Selection of deposition surfaces Many materials found in commercially available sandpaper possess detectable fluorescence levels. The sandpapers used in this study were carefully selected and tested to ensure negligible background level. Four different grades of aluminum oxide sandpapers were selected to represent, approximately, the range of roughness that might be expected on indoor walls: P60, P100, P150 and P220, with corresponding average grit sizes of 250, 150, 100 and 70 mm, respectively (CAFA, 2005). The dimensions of each sandpaper sheet were 23 cm 28 cm. In addition to the sandpapers, ordinary window glass was also tested. The plate dimensions were 28 cm 8 cm and the mean roughness height was approximately 10 mm. 2.2. Preliminary testing: Mixing conditions For proper interpretation of the results, it was important to ensure that the airborne particle concentration was uniform throughout the chamber. Nonuniform particle concentration could cause non-uniform deposits, and hence inaccurate determination of deposition velocity. Experiments were conducted to test the well-mixed assumption. Eight copper foils (each approximately 2 cm 2 cm) were positioned at welldistributed locations across the chamber floor. Monodisperse fluorescent particles of 3 mm diameter, generated by the vibrating orifice aerosol generator, were introduced continuously for one hour with the mixing fan in operation. Fluorescence readings were measured for each foil. The maximum relative discrepancy among all the samples was less than 15%. Hence the approximation that concentrations in the chamber were spatially uniform is reasonable. 2.3. Experimental procedure The chamber facility and methods are similar to those reported by Thatcher et al. (1996). Key aspects are reported here. In summary, the procedure for each experiment involved cleaning the chamber and sample surfaces, affixing sandpaper and glass plates to paper mounted on the walls, injecting monodisperse fluorescent particles into the chamber, and then, after an exposure period during which particles were also collected from air on filters, analyzing surface and airborne samples for fluorescence. The particles were extracted from the deposition surfaces and the filter samples using a buffer solution and the amount of
4895
fluorescent material was quantified by measuring the extracts with a fluorometer. To elaborate, before each run, the chamber surfaces were cleaned thoroughly with mild detergent and water. After drying, each sampling wall was covered with clean paper (45 cm 80 cm) to which the test surfaces were affixed. (The purpose of the paper, which was discarded after each experiment, was to protect the surfaces from inadvertent contamination.) The test surfaces were fixed to the paper with adhesive tape. Glass plates and sandpaper samples with four different roughness scales (typically two pieces for each grit size) were mounted onto two vertical walls. At the start of a run, a portable air cleaner was placed inside the chamber, which was then sealed. The air cleaner was operated remotely for approximately 10–20 min, after which the total particle count was effectively zero. Monodisperse fluorescent particles of known size were then generated, introduced continuously into the chamber, and allowed to deposit over a period of 8–16 h. Typical particle concentrations were 5–50 mg m3. Ammonium fluorescein particles were used for all experiments. Solutions were prepared from mixtures of oleic oil, isopropyl alcohol, deionized water, ammonium hydroxide and fluorescein (sodium salt). Upon evaporation, the droplet residue primarily comprises a mixture of oleic oil and sodium fluorescein. Particle size and concentration were monitored in real time by means of a particle sizer (Aerosol Particle Sizer, TSI, Saint Paul, MN, Model 3320). To determine deposition velocity, the average airborne particle concentration was measured from timeintegrated filter samples. In each experiment, two filter samples were collected by drawing air through the filter at a constant rate throughout the sampling period. The filters were membrane filters with 0.8 mm pore size (AAWP25, Millipore). Sample pumps were calibrated using a Gillibrator bubble flow meter (Gillian Instrument Corporation) and were found to be stable at a typical sampling rate of 1.6 L min1. The chamber particle concentration was determined as the average of the two filter samples. For all experiments, the concentration difference between the two filters was less than 10%. (This observation also supports the wellmixed representation of the chamber air, which was assumed to hold.) For each particle size, replicate experiments were conducted at least 3 times (typically 5 times) to determine experimental precision. At the end of the each experimental run, the aerosol generator was turned off, the delivery tube was removed, and the air cleaner was turned on for about 10 min or until the particle count was again effectively zero. The chamber door was then opened. The amount of material deposited or filtered was determined by measuring the fluorescence of the extract with a fluorometer (Turner Designs Corporation,
ARTICLE IN PRESS A.C.K. Lai, W.W. Nazaroff / Atmospheric Environment 39 (2005) 4893–4900
4896
Sunnyvale, CA, Model TD-700). The mass of particles deposited for each sample was determined using 10 mL of buffer solution. Glass plates were similarly analyzed, following buffer extraction of deposited particles from the immersed sample plates. Each glass plate was carefully removed from the clean paper and then immersed into a cleaned galvanized steel tray filled with 20 mL of buffer. Unlike sandpaper, which was discarded after each experiment, the glass plates were reused. Deposition was parameterized in terms of a deposition velocity, Vd (m s1), evaluated as follows: Vd ¼
J , C1
(1)
where J is the average particle flux density to the surface (mg m2 s1) and CN (mg m3) is the particle concentration in the chamber air. The particle flux density and particle concentration were evaluated by the following expressions: J¼
M sample , Asample ts
(2)
M filter , Qs t s
(3)
C1 ¼
where Msample and Mfilter are the fluorescein masses measured on a deposition surface and on the air filters, respectively, Asample is the area of the deposition surface, ts is the duration of particle exposure (and air sampling) and Qs is the volumetric flow rate through the air filter samples. Preliminary evaluations were undertaken to test the accuracy and repeatability of the final measurement protocols for all materials. We are confident that the errors associated with extracting and measuring fluorescein were small (less than 10%) as compared with observed variability either within experiments or between experiments. The extraction efficiency for the airsampling filters has been demonstrated to be 98% or better (Thatcher et al., 1996). To test the extraction efficiency from different types of surfaces, a few pieces of sandpaper and smooth copper foil were placed on the chamber floor adjacent to each other, so that deposition would be the same, as controlled by the mechanism of settling. The measured fluxes on these two types of surfaces were very close to each other. Extraction efficiency for sandpaper surfaces was also tested by means of carrying out a second extraction on selected samples and demonstrating that the fluorescence of the second buffer was very much smaller than that from the first extraction. It was also confirmed that the backing paper used in the experiments exhibited negligible fluorescence when subjected to extraction and analysis. The mixing fan near the chamber ceiling induced turbulent airflow in the chamber. The mean airspeed at
3 cm away from the wall surfaces was measured by means of an omnidirectional air velocity transducer (TSI, Saint Paul, MN, Model 8470) at ten locations on each of the two sampling walls. The measurements confirmed that the speeds at the different sampling locations were fairly consistent with mean values of 0.17 and 0.14 m s1 on the left and right side walls, respectively. The friction velocity, u*, was estimated by measuring the airspeed in the logarithmic flow region near the chamber wall (Lai and Nazaroff, 2000), using a laser doppler velocimeter (TSI, Saint Paul, MN, Model Laservec Diode Velocimeter). The measurements, taken at a single position on a vertical wall, yielded u ¼ 2:6 cm s1 : According to Nikuradse’s roughness classification (Nikuradse, 1933), the dimensionless roughness height of all of the sandpapers tested is less than the thickness of the fluid mechanical viscous sublayer, so that the surfaces can be considered aerodynamically smooth. However, in light of their very small diffusion coefficients, particle concentration boundary layers are much thinner than the fluid momentum boundary layer. So, although the sandpaper surfaces are aerodynamically smooth, it is expected that their roughness elements protrude across the particle concentration boundary layer and could therefore influence the particle deposition rate.
3. Results Table 1 presents the detailed experimental results, reporting average and standard deviation of the particle deposition velocity as a function of particle diameter for all five surface conditions. Considering all particle sizes and surfaces, the deposition velocity lies within the range (0.4–56) 106 m s1. Fig. 1 depicts the measured particle deposition velocity onto smooth glass surfaces for particle size ranging from 0.9 to 9 mm. The dominant experimental trend is counterintuitive. Starting from the smallest particles, as size increases, deposition velocity decreases initially (as expected) but then increases for particle sizes greater than about 3 mm. For particle sizes greater than 7 mm, the deposition velocity appears to level off. Measured particle deposition velocities for four roughness grades of sandpaper are shown in Fig. 2. A general trend that is similar to that for the smooth glass can be observed. Beginning with 0.9 mm particles, with increasing particle diameter, deposition velocity remains approximately constant or decreases slightly initially and then increases for particles larger than 3 mm. For all results presented, it is noted that the deposition velocity attains a fairly steady value for particle size larger than 7 mm. Another significant finding is that the deposition rate increases with increasing roughness scale. However,
ARTICLE IN PRESS A.C.K. Lai, W.W. Nazaroff / Atmospheric Environment 39 (2005) 4893–4900
4897
Table 1 Particle deposition velocity as a function of particle diameter for vertical deposition surfaces with different degrees of surface roughnessa Glass
Sand 220
Sand 150
Sand 100
Sand 60
dp
Vd
dp
Vd
dp
Vd
dp
Vd
dp
Vd
0.9 1.6 2.2 3.5 5.0 7.0 7.8 9.1
1.770.7 1.070.7 0.470.2 1.170.8 4.572.3 1277 5.570.8 1173
1.1 1.8 2.4 3.7 5.2 7.2 8.0 9.3
3.471.5 1.670.9 1.070.5 3.272.0 3.771.9 22718 22713 30713
1.1 1.8 2.4 3.7 5.2 7.2 8.0 9.3
1.671.1 1.670.6 1.370.2 6.872.7 5.473.4 25716 19716 20714
1.0 1.7 2.3 3.6 5.1 7.1 7.9 9.2
1.870.3 1.970.9 2.370.5 5.872.2 1675 29727 1177 43719
1.0 1.7 2.3 3.6 5.1 7.1 7.9 9.2
1.870.4 3.870.6 2.671.3 6.572.5 1077 34723 26711 5677
a
Deposition velocity (Vd) reported as mean7standard deviation with units of 106 m s1. Particle diameter (dp) reported in units of mm. Surface roughness of the glass plates was nominally 10 mm. For the sandpaper, grain sizes were as follows: sand 220 ¼ 70 mm; sand 150 ¼ 100 mm; sand 100 ¼ 150 mm; sand 60 ¼ 250 mm.
deposition velocity (µm/s)
30 10 3 1
u* = 9.8 cm/s
0.3 0.1 u* = 2.6 cm/s
0.03 1
3 5 particle diameter (µm)
10
Fig. 1. Particle deposition velocity onto smooth vertical surfaces as a function of particle diameter. The points indicate the mean values and the bars represent one standard deviation of the measurements. The curves represent model predictions for two different friction velocities, u* (Lai and Nazaroff, 2000).
the differences with changing roughness are relatively small compared with the influence of particle size on deposition.
4. Discussion The experimental data reveal two dominant trends. First, the deposition velocity tends to increase with particle size for most of the conditions studied. For the glass plates, and for the finest grain sandpaper, the deposition velocity decreases as particle size increases from 0.9 to 2.5 mm, and then begins to rise. For the roughest two sandpapers, the deposition velocity in-
creases monotonically with increasing particle diameter throughout the experimental range. Second, the effect of particle size is much larger than the effect of surface roughness. For each surface considered separately, the ratio of the largest to smallest deposition velocity as particle size varies lies in the range 20–30. With particle size fixed, changing roughness only influences particle deposition by a factor of 6 or less. We note that the relaxation times for particles in this study are low. With a friction velocity (u*) of 2.6 cms1 and particle densities in the range 1–1.4 g cm3, the dimensionless relaxation times for 0.9–9 mm particles would be in the range 1.3 104 to 0.016. Based on studies of particle deposition from vertically oriented turbulent pipe flow, this range places particle conditions for our study in what is known as the diffusion regime (Sippola, 2002). Here, the influence of inertial impaction is expected to be small, and deposition would be expected to decrease with increasing particle size. This expectation is contrasted with that for the diffusion– impaction regime, for which the dimensionless relaxation time is in the range 0.1–10, and particle deposition increases with increasing particle size. The model of Lai and Nazaroff (2000) was developed to be applicable to the conditions of this experiment for the glass surface. However, the model predictions do not correspond well with the experimental observations reported here. Fig. 1 illustrates this point, comparing the experimental data for glass with model predictions. The lower model prediction curve corresponds to the experimentally determined friction velocity of 2.6 cm s1. The upper curve, for a friction velocity of 9.8 cm s1, represents a best fit of the model predictions to the three smallest-diameter data points. Clearly, the model predicts neither the trend nor the magnitude for the larger particle sizes studied. The divergence for the largest particle sizes is striking: for 9 mm particles, the
ARTICLE IN PRESS A.C.K. Lai, W.W. Nazaroff / Atmospheric Environment 39 (2005) 4893–4900
particle diameter (µm) 3 5
9
1
particle diameter (µm) 3 5
deposition velocity (µm/s)
deposition velocity (µm/s)
80 sandpaper 220 (70 µm)
30
10 80
sandpaper 150 (100 µm)
30
10
10
3
3
1
1
30
sandpaper 100 (150 µm)
sandpaper 60 (250 µm)
30
10
10
3
3
1
1
0.2
deposition velocity (µm/s)
1
deposition velocity (µm/s)
4898
0.2 1
3 5 particle diameter (µm)
9
1
3 5 particle diameter (µm)
10
Fig. 2. Particle deposition velocity onto vertically oriented sandpaper surfaces. Points indicate mean values and bars represent one standard deviation of the measurements. The gray bars indicate deposition onto smooth glass plates (from Fig. 1) and are shown for reference.
measured deposition velocity was 11 106 m s1, a factor of 30–150 larger than the modeled deposition velocities for friction velocities of 2.6–9.8 cm s1. Models for particle deposition from turbulent flow to rough vertical surfaces have also been developed, but they do not yield good agreement with these new measurements, either. Fundamentally, we believe that the discrepancy between model predictions and measurements is a consequence of inadequacies in the models, because some key physical processes affecting transport and deposition of coarse particles to vertical surfaces are not appropriately represented. We have investigated possible explanations. One that appears unlikely to contribute significantly is electrostatic drift. Because of the grounded chamber surfaces, electric fields within the chamber would be small. Image force effects are also expected to be small for particles charged with a Boltzmann distribution (Chen and Lai, 2004).
On the other hand, even though the dimensionless relaxation times are low, we believe that potentially significant contributions might be attributable to inertial transport of particles through boundary layers, possibly enhanced by turbulent burst phenomena near surfaces. A possible contributor that might help reconcile the observations in these experiments with the expectations from studies of turbulent pipe flow is the potential for more highly irregular turbulent flow near surfaces in a chamber. Modeling efforts may be fruitful if they focus on understanding the effect on particle deposition of the high-energy tail of the distribution of near-wall turbulent fluctuations. Another factor we considered is gravitational settling onto the upward surfaces of roughness elements. Turbulent transport need only transport particles to the interstices between grains if settling to the upward side of the grain surface is sufficiently rapid. However, while this factor could be important for sandpaper, it is
ARTICLE IN PRESS A.C.K. Lai, W.W. Nazaroff / Atmospheric Environment 39 (2005) 4893–4900
deposition loss rate coefficient (per h)
unlikely to be important for glass. That the dominant deposition features are similar for all surfaces in these experiments suggests that settling onto roughness elements is not a key factor. One further contributing factor to the divergence between model and measurement might be the lift force (Saffman, 1965). Coarse particles suspended in a shear flow, as in the viscous sublayer adjacent to the vertical surfaces in these experiments, will experience a force toward the wall (enhancing deposition) when the overall airflow is downward, and the particle is traveling more rapidly than the air speed owing to gravitational settling. Holmberg and Li (1998) have carried out numerical simulations of the airflow field in a chamber configured similarly to ours, albeit larger (8 m3 vs. 2 m3). Fig. 3 from their paper indicates mean downward flow along the upper half of the wall centerlines. Even for positions along walls where the mean flow is upward, as appears to be the case in the lower portion of the walls in their figure, the weak overall flow in such a chamber leaves open the possibility that for some fraction of time largescale eddies induce net downward flow locally in the viscous sublayer. In simulating particle deposition from turbulent flow in ducts, Zhang and Ahmadi (2000; Fig. 7) have shown strong effects of the lift force influencing deposition in a vertical duct for friction velocities comparable with those in our experiments (0.03 and 0.1 m s1) and for dimensionless particle relaxation times not inordinately greater (102 to 1 in Zhang and Ahmadi as compared with 104 to 102 in our experiments).
10
room (Thatcher et al., 2002) walls (sandpaper 60)
1
0.1
0.01 1
3 5 particle diameter (µm)
10
Fig. 3. Particle loss-rate coefficients by means of deposition onto indoor surfaces as a function of particle diameter. The ‘‘room’’ traces reflect deposition to all surfaces for ‘‘bare’’ conditions in a 2.2 m 2.7 m 2.4 m (high) enclosure under four different levels of airflow intensity (Thatcher et al., 2002). The data points reflect deposition only to walls, and were determined by applying Eq. (4) to the experimental data reported in the present paper for sandpaper 60 (grit size ¼ 250 mm), with Awalls =V ¼ 1:65 m2 m3 .
4899
The model-measurement discrepancy reported here does not significantly alter the understanding of the overall rate of depositional loss of coarse particles from indoor air. Deposition onto upward surfaces by means of gravitational settling, possibly augmented by inertial impaction still dominates. This point is illustrated in Fig. 3, which shows that the loss-rate coefficients for walls based on results in the current study are more than an order of magnitude smaller than the measured overall loss-rate coefficients for rooms (Thatcher et al., 2002). In making this comparison, we converted deposition velocity to a wall loss-rate coefficient, bwall, using the relationship V d Awall , (4) V where Awall is the wall area of a room and V is the room volume. The comparison in Fig. 3 also reinforces a point made in the introduction. Because coarse-particle deposition to upward surfaces dominates, one cannot investigate coarse-particle deposition to walls by measurement methods that only consider the overall rate of particle loss from indoor air. Because of the weak dependence of total depositional loss on deposition to walls, such experiments are effectively ‘‘blind’’ to the rate of coarse particle deposition on walls.
bwall ¼
5. Conclusion Particle deposition onto indoor surfaces is a complex, multifaceted phenomenon that has relevance to issues as disparate as predicting human exposure to airborne particles and protecting works of art from soiling. As was already well known, and is further substantiated by the evidence presented in this paper, particle size plays a major role influencing particle deposition. Ample evidence indicates that airflow conditions and surface characteristics are also important. Particle deposition experiments are challenging to conduct, and the potentially important influence of many otherwise subtle processes means that modeling is also difficult. The contributions of the present paper lie in the experimental data and the observations about model-measurement discrepancies. Although progress on understanding particle deposition is hard won, we believe that continued efforts to advance our understanding of such processes are warranted.
Acknowledgements The experimental work was carried out at the laboratory of Department of Civil and Environmental Engineering in University of California, Berkeley. Shannon Coulter-Burke assisted in conducting the
ARTICLE IN PRESS 4900
A.C.K. Lai, W.W. Nazaroff / Atmospheric Environment 39 (2005) 4893–4900
experiments. This research was partly supported by the Office of Research and Development, Office of Nonproliferation and National Security, US Department of Energy under Contact No. DE-AC03-76SF00098.
References Abadie, M., Limam, K., Allard, F., 2001. Indoor particle pollution: effect of wall textures on particle deposition. Building and Environment 36, 821–827. Browne, L.W.B., 1974. Deposition of particles on rough surfaces during turbulent gas-flow in a pipe. Atmospheric Environment 8, 801–816. Byrne, M.A., Goddard, A.J.H., Lange, C., Roed, J., 1995. Stable tracer aerosol deposition measurements in a test chamber. Journal of Aerosol Science 26, 645–653. CAFA, 2005. Abrasives Info/Grit Comparison Chart. Coated Abrasives Fabricators Association. Available from http:// www.cafa-info.org/main.html. Chen, F., Lai, A.C.K., 2004. An Eulerian model for particle deposition under electrostatic and turbulent conditions. Journal of Aerosol Science 35, 47–62. El-Shobokshy, M.S., 1983. Experimental measurements of aerosol deposition to smooth and rough surfaces. Atmospheric Environment 17, 639–644. Fogh, C.L., Byrne, M.A., Roed, J., Goddard, A.J.H., 1997. Size specific indoor aerosol deposition measurements and derived I/O concentrations ratios. Atmospheric Environment 31, 2193–2203. Grass, A.J., 1971. Structural features of turbulent flow over smooth and rough boundaries. Journal of Fluid Mechanics 50, 233–255. Guha, A., 1997. A unified Eulerian theory of turbulent deposition to smooth and rough surfaces. Journal of Aerosol Science 28, 1517–1537. Harrison, A.W., 1979. Quiescent boundary-layer thickness in aerosol enclosures under convective stirring conditions. Journal of Colloid and Interface Science 69, 563–570. Holmberg, S., Li, Y., 1998. Modelling of the indoor environment—Particle dispersion and deposition. Indoor Air 8, 113–122. Institute of Medicine, 2000. Clearing the Air: Asthma and Indoor Air Exposures. National Academy Press, Washington, DC (Chapter 10). Lai, A.C.K., 2002. Particle deposition indoors: a review. Indoor Air 12, 211–214. Lai, A.C.K., Nazaroff, W.W., 2000. Modeling indoor particle deposition from turbulent flow onto smooth surfaces. Journal of Aerosol Science 31, 463–476. Lai, A.C.K., Byrne, M.A., Goddard, A.J.H., 2002. Experimental studies of the effect of rough surfaces and air speed on aerosol deposition in a test chamber. Aerosol Science and Technology 36, 973–982; 37, 401. Ligocki, M.P., Liu, H.I.H., Cass, G.R., John, W., 1990. Measurements of particle deposition rates inside southern California museums. Aerosol Science and Technology 13, 85–101. Litvak, A., Gadgil, A.J., Fisk, W.J., 2000. Hygroscopic fine mode particle deposition on electronic circuits and resulting
degradation of circuit performance: an experimental study. Indoor Air 10, 47–56. Long, C.M., Suh, H.H., Catalano, P.J., Koutrakis, P., 2001. Using time-and size-resolved particulate data to quantify indoor penetration and deposition behavior. Environmental Science & Technology 35, 2089–2099. Nazaroff, W.W., Cass, G.R., 1991. Protecting museum collections from soiling due to the deposition of airborne particles. Atmospheric Environment 25A, 841–852. Nicas, M., Nazaroff, W.W., Hubbard, A., 2005. Toward understanding the risk of secondary airborne infection: emission of respirable pathogens. Journal of Occupational and Environmental Hygiene 2, 143–154. Nikuradse, J., 1933. Laws for flow in rough pipes. VDI Forschungsheft (English translation, NACA TM 1292, 1950). O¨zkaynak, H., Xue, J., Spengler, J., Wallace, L., Pellizzari, E., Jenkins, P., 1996. Personal exposure to airborne particles and metals: results from the Particle TEAM study in Riverside, California. Journal of Exposure Analysis and Environmental Epidemiology 6, 57–78. Saffman, P.G., 1965. The lift on a small sphere in a slow shear flow. Journal of Fluid Mechanics 22, 385–400. Shimada, M., Okuyama, K., Kousaka, Y., Ohshima, K., 1987. Turbulent and Brownian diffusive deposition of aerosol particles onto a rough wall. Journal of Chemical Engineering of Japan 20, 57–64. Sippola, M.R., 2002. Particle deposition in ventilation ducts. Ph.D. Dissertation. University of California, Berkeley, CA, USA. Thatcher, T.L., Fairchild, W.A., Nazaroff, W.W., 1996. Particle deposition from natural convection enclosure flow onto smooth surfaces. Aerosol Science and Technology 25, 359–374. Thatcher, T.L., Lai, A.C.K., Moreno-Jackson, R., Sextro, R.G., Nazaroff, W.W., 2002. Effects of room furnishings and air speed on particle deposition rates indoors. Atmospheric Environment 36, 1811–1819. Thatcher, T.L., Lunden, M.M., Revzan, K.L., Sextro, R.G., Brown, N.J., 2003. A concentration rebound method for measuring particle penetration and deposition in the indoor environment. Aerosol Science and Technology 37, 847–864. Vette, A.F., Rea, A.W., Lawless, P.A., Rodes, C.E., Evans, G., Highsmith, V.R., Sheldon, L., 2001. Characterization of indoor–outdoor aerosol concentration relationships during the Fresno PM exposure studies. Aerosol Science and Technology 34, 118–126. Weis, C.P., Intrepido, A.J., Miller, A.K., Cowin, P.G., Durno, M.A., Gebhardt, J.S., Bull, R., 2002. Secondary aerosolization of viable Bacillus anthracis spores in a contaminated US Senate office. Journal of the American Medical Association 288, 2853–2858. Wood, N.B., 1981. A simple method for the calculation of turbulent deposition to smooth and rough surfaces. Journal of Aerosol Science 12, 275–290. Xu, M.D., Nematollahi, M., Sextro, R.G., Gadgil, A.J., Nazaroff, W.W., 1994. Deposition of tobacco-smoke particles in a low ventilation room. Aerosol Science and Technology 20, 194–206. Zhang, H., Ahmadi, G., 2000. Aerosol particle transport and deposition in vertical and horizontal turbulent duct flows. Journal of Fluid Mechanics 406, 55–80.