Indoor Aerosol Particle Deposition in an Empty Office

INDOOR AEROSOL PARTICLE DEPOSITION IN AN EMPTY OFFICE

JIRI SMOLIK1,∗, MIHALIS LAZARIDIS2, PAVEL MORAVEC1,JAROSLAV SCHWARZ1, SHAMIL K. ZARIPOV3 and VLADIMIR ZDIMAL1

1Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic,16502 Prague, Czech Republic; 2Technical University of Crete, Department of EnvironmentalEngineering, 73100 Chania, Greece; 3Kazan State University, Institute of Mathematics and

Mechanics, 420008 Kazan, Republic of Tatarstan, Russia(∗author for correspondence, e-mail: [email protected]; Tel: 420-296780247,

Fax: 420-220920661)

(Received 25 August 2004; accepted 19 April 2005)

Abstract. The size- and time-resolved indoor/outdoor aerosol concentration relationships were stud-ied experimentally in an empty office without internal particle sources. Two Scanning Mobility ParticleSizers (SMPS) and an Aerodynamic Particle Sizer (APS) sampled alternately from indoor and outdoor,together covering the size range 3–10,000 nm. The results showed that the indoor aerosol concentra-tion depends mainly on the air exchange and deposition rates, and the outdoor concentration. At higherair exchange rates the indoor aerosol concentration approaches the outdoor one. This was observed forthe accumulation mode particles. The size-dependent deposition rates were estimated using a simpleaerosol dynamics experiment. It was based on equilibration of the indoor/outdoor concentrations byopening windows and analyzing the concentration decay curves after the windows were closed. Forthis purpose a simple mass balance model was used. The overall loss rate and the asymptotic valueof indoor concentration were found by applying a non-linear least squares method on the time de-pendence of the indoor concentrations. The air exchange rate was estimated from the overall loss rateand the steady state values of the indoor/outdoor concentration of the accumulation mode particles.

Keywords: indoor/outdoor aerosol concentrations, on-line particle size distribution measurements,indoor aerosol dynamics, air exchange rate, deposition rate, penetration factor

Introduction

In recent years, extensive research efforts have been invested in examining therelationship between exposure to particulate matter and resulting health effects.Data from epidemiological studies, conducted to date, demonstrate associationsbetween ambient particulate concentrations and increased morbidity and mortality(e.g. EPA, 1996a,b; Pope et al., 1995; Schlesinger, 1995; Schwartz, 1994). Al-though these studies have focused on outdoor particles, most people’s exposure toparticulate matter occurs indoors, where people spend about 85–90% of their lives(EPA, 1996a,b). Therefore, understanding the fate of indoor particles is importantfor the assessment of human health.

Results from studies of indoor particle deposition characteristics have been pub-lished, e.g. by Alzona et al. (1979), Dockery et al. (1981), Offermann et al. (1985),

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302 J. SMOLIK ET AL.

Raunemaa et al. (1989), Koutrakis et al. (1992), Byrne et al. (1995), Thatcher andLayton (1995), Fogh et al. (1997), Jones et al. (2000), Abt et al. (2000), Vetteet al. (2001), Mosley et al. (2001), Long et al. (2001), Koponen et al. (2001), Rileyet al. (2002) and Thatcher et al. (2002). The results show that the most importantfactors influencing the indoor/outdoor particle concentration ratios are the strengthof indoor sources, deposition rates and penetration factors together with the airexchange rate by ventilation. A model of a homogeneous indoor particle concen-tration within a well-mixed room has been utilised in many works describing theindoor aerosol dynamics. Here particle concentration is assumed to be uniformwithin the enclosured volume except in thin boundary layers near the walls, thefloor and the ceiling or other surfaces and particles penetrate the boundary layerby turbulent, Brownian diffusion and gravitational settling. Corner and Pendlebury(1951) developed the first analytical formula for a particle deposition rate in arectangular enclosure under a homogeneously turbulent flow. Crump and Seinfeld(1981) extended the Corner-Pendlebury formula to calculate the particle depositionrate for an enclosure of arbitrary shape based on the same assumptions. McMurryand Rader (1985) applied the Crump-Seinfeld theory to account for an electrostaticdrift. Shimada et al. (1989) studied the influence of particle inertia on depositionvelocities. Nazaroff and Cass (1989) extended the model of Corner-Pendleburyregarding non-isothermal room surfaces where thermophoresis noticeably influ-ences the deposition of particles. Lai and Nazaroff (2000) proposed a new modelthat accounts for the functional dependence of deposition velocity on particle size.Based on the results of recent studies of near-surface turbulent diffusion they madeassumptions about particle eddy diffusion in the boundary layer and developed newformulae for particle deposition velocity on vertical and horizontal surfaces.

Due to the importance of indoor human exposure to particles, an extensive re-search project, URBAN-EXPOSURE, has been conducted in six European citiesduring the summer and winter 2001–2002. The measurements were carried out inAthens (Greece), Prague (Czech Republic), Oslo (Norway), Milan (Italy), London(U.K.), and Hanover (Germany). In this paper, we report on some results of theexperimental campaign carried out in Prague over the period December 2001–January 2002. The aim of the campaign was to test the measuring system designedto continuously monitor indoor and outdoor particle concentrations (along withindoor/outdoor temperature, pressure and relative humidity), and, if possible, todetermine the air exchange rate and size-dependent particle deposition rates. Thesystem is based on parallel aerosol sampling by two Scanning Mobility ParticleSizers (SMPS) and an Aerodynamic Particle Sizer (APS). The system was laterused in experimental campaigns carried out in Oslo, Milan, and Prague. A keypart of the campaign was measurements in an empty room, i.e. without carpetsand furniture, and also without internal particle sources. Since the instrumentationfor an independent determination of the air exchange rate was not available, anattempt was made to estimate this parameter using a simple aerosol dynamics ex-periment combined with a simple mass balance model. It is shown that under certain

INDOOR PARTICLE DEPOSITION 303

assumptions this approach may be sufficient to estimate both the air exchange rateand the size-dependent particle deposition rates. The estimated deposition rateswere compared with predictions of the model by Lai and Nazaroff (2000).

Experimental Methods

EXPERIMENTAL ROOM

The measurements were carried out in one of the buildings belonging to the Instituteof Chemical Process Fundamentals (ICPF AS CR) in Prague. It is a four storeyrectangular building, constructed in 1960s from bricks covered with plaster, about40 m long and 12 m wide. A straight corridor goes centrally through each floor,doors to offices (one to each) are located SW and doors to laboratories NE. Theoffices are of one size and there are no doors interconnecting them. The experimentalroom was a freshly remodeled rectangular office 3.9 m long, 2.85 m wide and 3.6m high. It is situated on the third floor approximately in the middle of the flat sidewall of the building so that offices surrounded it from both sides, and also fromthe bottom and from the top. There was no HVAC system in the room, i.e. that theonly ventilation of the room was through windows and the door. Six independentwindows in two columns of three covered the outside wall from about one third ofits height up to the ceiling. The entrance door located in the wall opposite to thewindows was sealed during the experiments. The room was empty, i.e. it containedno furniture and no carpets. Local heating (radiators located under the windows) wasswitched off, heating due to sunlight radiation was suppressed by sun shades. Boththe walls and the ceiling of the room were covered by plaster freshly painted withlatex; floor material was PVC. In order to prevent possible generation of aerosol bythe measuring system and to suppress its influence on the flow pattern inside theoffice, all the instruments were located in the office adjacent to the experimentalroom. Lines for indoor sampling connected the rooms through the lowest adjacentwindows, where glass was replaced with dense styrofoam carefully sealed in theframe. Sampling lines were sealed inside the foam and thermally insulated outsidethe windows. Outdoor samples were taken directly through the styrofoam window,the inlet was located about 1 m from the window.

EXPERIMENTAL SET-UP AND DATA PROCESSING

Indoor and outdoor particle size concentrations were measured by two ScanningMobility Particle Sizers (SMPS, models 3936N25 and 3934C, TSI Incorporated,U.S.A.) and an Aerodynamic Particle Sizer (APS, model 3320, TSI Incorporated,U.S.A.). Each instrument was used to sample from both inside and outside theroom using its own sampling train formed by copper and conductive rubber tubing,provided with an electrically actuated three-way ball valve (Whitey). Valves were


connected to common programmable controller that used a CPC voltage (control-ling the high voltage on the central rod of the DMA) as a signal for switching. Thecontroller allowed selection of the switching cycle concerning the number of sam-ples taken alternately from inside and outside. The SMPS 3936N25 was operatedat 1.5 l/min polydisperse aerosol flow and measured over a size range of (nomi-nally) 3–65 nm. The SMPS 3934C sampled the aerosol with a flow rate 0.3 l/min,resulting in size range of 15–730 nm. The APS 3320 was operated with 5 l/minaerosol flow rate and measured particles in the effective size range 0.7–10 µm. BothSMPS used 3 min upward scan, followed by one minute downward scan with aone minute delay necessary to separate samples and wash the sampling trains aftervalve switching. Eventually, three five-minute sampling cycles for indoor samplingfollowed by three five-minute cycles for outdoor sampling were used. Data fromeach of the particle instruments were collected using Aerosol Instrument Managersoftware (AIM v.1.0, TSI), where particle losses inside the sampling trains were in-corporated using so called “efficiency files”. In addition to the particle instruments,basic meteorological data (temperature, atmospheric pressure, and relative humid-ity) were collected both indoors and outdoors. The temperature was measured usingplatinum resistance thermometers, relative humidity with capacity meters and at-mospheric pressure with pressure transducers. Signals from sensors were collectedusing a DAQ board and Virtual Bench Software (LabView, National Instruments).

SIMPLIFIED AEROSOL DYNAMICS MODEL

Using a simple mass balance approach (e.g. Wallace et al., 2004) the rate of changein indoor aerosol concentrations for any particle size with respect to time is generallyexpressed as:

VdCi

dt= V λ(PC0 − Ci ) + Q − S, (1)

where V is the room volume, Ci and C0 are the indoor and outdoor aerosol concen-trations, respectively, t is the time, λ is the air exchange rate, P is the penetrationfactor, Q is the source strength, and S is the sink strength. Assuming that there areno indoor particle sources, that particles are not resuspended, are physically andchemically stable, and there is no coagulation, the source term may be neglectedand hence Q = 0. Furthermore, the sink strength can be expressed as S = Ci V β,where β is the deposition rate in the room comprising all deposition mechanismsand all surfaces. Equation (1) then can be written as:

dCi

dt= −λ∗Ci + λPC0, (2)


with the overall loss rate:

λ∗ = λ + β. (3)

The deposition rate β of the entire room is defined as:

β = vdv Av + vdu Au + vdd Ad

V(4)

where Av is the area of the vertical surfaces, Au is the area of the upward-facingsurfaces, Ad is the area of the downward-facing surfaces, vdv, vdu , vdd are the depo-sition velocities on vertical, upward horizontal and downward horizontal surfaces,respectively.

Assuming time-independent parameters and the initial concentration equal toCi , we obtain for the initial condition Ci (0) = C0 the solution:

Ci (t) = C0

[(1 − λP

λ∗

)e−λ∗t + λP

λ∗

](5)

Asymptotic solution of Equation (2) at t → ∞ can be written as:

Ci (∞) = C∞ = λP

λ∗ C0 (6)

and therefore we can rewrite the Equation (5) as:

Ci (t) = C∞ + (C0 − C∞)e−λ∗t . (7)

Results and Discussion

INDOOR AND OUTDOOR AEROSOL CONCENTRATIONS

Preliminary experiments were carried out in December 2001 and the first halfof January 2002. They included the instrument inter-comparison and tests of thevalve switching system, which found an optimum sample length and switchingstrategy. The two week intensive measuring campaign between January 17–31, 2002included measurements of the dynamics of indoor/outdoor aerosol concentrations,and observation of their diurnal variation.

Generally, the indoor concentrations were lower than those outdoors, especiallyfor ultrafine (<20 nm) and coarse (>2.5 µm) particles. This was caused by thewell known fact that the particle deposition rate depends strongly on particle size,has a pronounced minimum in the size range 0.1–1 µm and increases rapidly both


towards smaller sizes (due to diffusion) and towards larger sizes (due to gravita-tional settling). The differences between indoor and outdoor concentrations weregreater during calm days. However, during windy days the indoor concentrationsapproached those of outdoors. This was especially true for particles with sizesbetween 0.1–1 µm. In order to study the dynamics of the temporal variation ofindoor aerosol concentrations an aerosol dynamics experiment was carried out.Every morning the windows of the office were opened for about two hours to fa-cilitate infiltration of the outdoor aerosol. During this period the indoor aerosolconcentrations increased to outdoor values. After the windows were closed the in-door concentrations started to decrease exponentially to levels existing prior to theopening of the windows. An example of behavior of various aerosol size fractionsis shown in Figure 1. The separation of indoor and outdoor concentrations canbe observed only in those size ranges where the deposition prevailed over the airexchange rate.

Figure 1. Dynamics of the indoor/outdoor concentrations for several wide size fractions. The windowwas closed at about 11 o’clock. The individual size ranges were measured by the following instruments:<20 nm (SMPS, 3936N25), 20–700 nm (SMPS, 3934C), >700 nm (APS, 3320).


ESTIMATION OF THE INDOOR PARTICLE DEPOSITION RATES

If the air exchange rate and the outdoor aerosol concentration are known and con-stant during the decay period, Equations (3) and (7) can be used to determine thesize-dependent indoor particle deposition rate β directly from the experiment. Sincethe air exchange rate was not directly measured in our experiments, an attempt wasmade to estimate both the air exchange and the deposition rates using a procedurewhich is outlined below. Due to the fact that the deposition rate depends stronglyon particle size, the highly size resolved data from both SMPS and APS had tobe used for the evaluation. Each particle size had been processed independently asfollows. First, the overall loss rate λ∗ and the asymptotic value C∞ were found byapplying a non-linear least squares method (the Levenberg-Marquardt algorithm)on the time dependence of indoor concentrations. The time when windows wereclosed was taken as the initial time, the initial indoor concentration was set equalto the outdoor concentration at that initial time, and the outdoor concentration C0

was represented by its mean value during the decay experiment. This is illustratedin Figure 2 where the measured temporal evolution of indoor and outdoor concen-trations is shown for several particle size bins. The prediction of indoor aerosolconcentration by Equation (7) is presented as well using the optimal estimates ofthe model parameters.

Figure 2 allows us to check some of the assumptions on which the model isbased. It was assumed that the outdoor concentration during the decay experimentwas constant. It is difficult to fulfill this condition in real experimental campaigns,but in the case under study the outdoor concentrations in any size bin changed byless than factor of two over the whole measurement. It was also assumed that theair exchange rate was constant during the decay period. Since it depends on windvelocity and difference between indoor and outdoor temperatures (e.g. Etheridgeand Sandberg, 1996) this assumption is supported by the fact that the weatherconditions were very stable during the 5 h long experiment (SSW wind 4–6 m s−1,outdoor temperature 9–11 ◦C).

The result of this analysis is given in Figure 3 where the size dependent valuesof λ∗ are presented together with the information about the instrument used andwith the typical error bars corresponding to the uncertainty of the nonlinear fit. Theresulting curve has the expected U shape since the overall loss rate is a linear com-bination of the air exchange rate (supposedly constant) and strongly size-dependentdeposition rate.

Encouraged by these findings the air exchange rate λ was estimated using theEquation (6) and assuming that in the accumulation region the penetration factor Pequals to unity with a good accuracy (e.g. Mosley et al., 2001). Using the particlediameters only from the lower part of the APS size range (Dp < 1 µm) and puttingthe results of the nonlinear fit (the overall loss rate λ∗ and the asymptotic value ofthe indoor concentration C∞) into the Equation (6), the mean value of air exchangerate has been obtained as λ = 0.39 ± 0.06 h−1. Even though one may argue that


Figure 2. Temporal evolution of indoor/outdoor concentrations of several narrow size fractions duringthe decay period of the experiment. Time 0 corresponds to the closing of windows. The experimentalvalues of indoor concentrations are compared with the model prediction according to Equation (7).

the assumption concerning the penetration factor is rather stringent, Figure 2 showsit may be used with confidence. The deposition rate of particles with sizes in theaccumulation mode is almost negligible compared to the average value of the airexchange rate. Since the indoor and outdoor concentrations of particles in this sizerange are pretty much the same, it may be concluded that the assumption P ∼ 1 israther strongly supported by the experimental data.

Finally, the size-dependent deposition rate β was obtained from Equation (3)using pre-determined values of the individual overall loss rate λ∗ and a mean airexchange rate λ. In order to perform a qualitative comparison of the overall deposi-tion rates (estimated by the procedure outlined bellow) with data from the literature,the results were compared with a prediction based on the Lai and Nazaroff (2000)model, where all the relevant deposition mechanisms as well as all wall orientationswere summed together. For small particle sizes a numerical calculation of the inte-gral (Lai and Nazaroff, 2000) was used. The value of friction velocity (parameter


Figure 3. The overall loss rate λ∗ as a function of particle size. The error bars correspond to theuncertainties of the nonlinear fit.

of the model) was chosen to be u∗ = 3 cm/s. The result of the comparison is shownin Figure 4. As can be seen, the values estimated from the aerosol dynamics experi-ment are systematically higher than the model predictions. This might be caused bythe uncertainties on both the theoretical calculations and the experimental data. Theuncertainties related with the current evaluation procedure were already discussed.Uncertainties on the side of theoretical predictions may be caused by the deviationsof real experimental conditions from the model assumptions (e.g. homogeneouslyturbulent air flow, wall roughness, friction velocity). Furthermore, Equation (6) wasused to estimate the size-dependent penetration factor for particle sizes outside theaccumulation mode. However, the uncertainty of this estimate was too large dueto the fact that each of the terms in Equation (6) was determined with significantuncertainty. The qualitative finding from the theoretical predictions was that thepenetration factor has a value close to one for particle size around 2 microns and isa decreasing function of particle size both towards larger and smaller (<0.1 µm)particle sizes. In order to determine the penetration factor with better precision itwould be necessary to measure the air exchange rate independently.

Conclusions

An empty office without any internal aerosol sources was used to study size-and time-resolved indoor/outdoor aerosol concentration relationships in relation


Figure 4. Comparison of experimentally estimated and theoretical indoor particle deposition rate.Theoretical deposition rates are based on the model of Lai and Nazaroff (2000) at the friction velocity3 cm/s.

to ambient conditions. The sampling system was located in a neighboring officeand consisted of two SMPS and one APS operating in parallel. The whole systemenabled the sampling of aerosol particles over a size range of 3–10000 nm in 5 mintime intervals. A series of samples were taken alternately from indoor and outdoor.It was found that the indoor aerosol concentration depends both on air exchangeand deposition rates. If the air exchange rate dominates the deposition losses, thenthe indoor aerosol concentration approaches the outdoor one. This was observedespecially for the accumulation mode particles.

In addition, a simple mass balance model was used, to estimate both the airexchange rate and the particle size-dependent deposition rates. Assuming that thepenetration factor reaches unity in the case of the accumulation mode particles, themean air exchange rate was determined as λ = 0.39 ± 0.06 h−1. Experimentallydetermined overall deposition rates were compared with the prediction of the Laiand Nazaroff (2000) model, where the deposition rate was summed over all relevantmechanisms and all wall surfaces. The values of the deposition rate estimated byevaluating the aerosol dynamics experiment are systematically higher than thoseobtained from the theoretical model. This might be caused both by uncertainties inthe estimation of air exchange rate and by the fact that the model does not includeother possible mechanisms of particle deposition (e.g. thermophoretic depositionon windows). In order to suppress the experimental uncertainties it would be nec-essary to measure the air exchange rate independently using some non-depositing


substance, e.g. SF6. This would allow to determine the particle deposition velocitieswith higher accuracy and also to evaluate the size-dependent penetration factor.

Acknowledgements

This work was supported by the European Commission under grant EVK4-CT-2002-00090 and by the GA CR grant No. 101/04/1190. The authors would like tothank their colleague David Brus, MSc. for technical assistance during the experi-ments.

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Indoor Aerosol Particle Deposition in an Empty Office

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