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Thornburg 2001 - Bibliotheek T D W Uitsluitend VOOI‘...

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Unformatted text preview: Bibliotheek T» D W Uitsluitend VOOI‘ eigen gebruik / for own use only /////A Prometheusplein 1 Datum: 0 2 —OCt — 0 3 Postbus 98 2600 MG DELFT Bonnummer: 7 0 6 2 1 5 Telefoon: 015 - 2784636 Fax: 015 - 2785673 Email: [email protected] Aan; T.N.O. 'I'ECHN. PHYSISCHfi DIfiNSi POSTBUS 155 2600 AD DELFT NfiD *. RTAND Tav: A. van de Runstraat Aantal kopieén: 13 Uw referentie(s): 008 . 05105/01 . 01 Artikelomschrijving bij aanvraagnummer: 7 0 6 2 l 5 Artikel: Penetration of particles into buildings and associate phys Auteur: T nornburg e : al . Tijdschrift; A'QROSOT. SCI *-NC *. AND LLCHNOLOGY J aar: 2 0 0 1 Vol. 3 4 Aflevering: Pagina(s); l - l 3 Plaatsnr.: 5 9 8 0 Met ingang van 1 april 2003 zullen de prijzen v00rf0t0k0pie levering buitenland stijgen met 6‘ 0,05 per pagina From April I 2003, prices for photocopy delivery abroad will increase by 6‘ 0.05 per page Aerosol Science and Technology 34: 284—296 (2001) © 2001 American Association for Aerosol Research Published by Taylor and Francis 0278~6826/01/$12.00 + .00 Penetration of Particles into Buildings and Associated Physical Factors. Part 1: Model Development and Computer Simulations J. Thornburg,1 D. S. Ensor,1 C. E. Rodes,1 P. A. Lawless,1 L. E. Sparks,2 and R. B. Mosley2 I Research Triangle Institute, Research Triangle Park, North Carolina 2Nationol Risk Management Research Laboratory, United States Environmental Protection Agency, Research Triangle Park, North Carolina The PM” standard proposed by the US. Environmental Pro- tection Agency (EPA) has stimulated research on the relationships between particulate matter concentrations and the exposures and subsequent health responses of sensitive subpopulations, such as the elderly. Since individuals in these subpopulations may spend more than 90% of their time indoors, understanding the relation— ship between outdoor particle concentrations and those found in indoor microenvironments is critical. This research resulted in a time~dependent indoor air quality model incorporating all poten- tial particle sources and loss mechanisms. Monte Carlo simula- tions of the model identified the mechanisms, such as particle loss during penetration through the building envelope, that modify the outdoor particle size distribution during transport into the interior of a building, calculated indoor-to-outdoor (I/O) concentration ra- tios, and estimated penetration factors as a function of particle size. Indoor particle generation and transport of outdoor particles through the HVAC system were the most important contributors to the indoor concentration in residential and commercial buildings, respectively. The most significant removal mechanisms included ventilation through and particle removal by the HVAC filter if an HVAC system was present, or particle deposition on indoor surfaces if an HVAC system was not present. The modeled I/O concentration ratios varied between 0.05 and 0.5, depending on particle size and type of ventilation system, and agreed well with published experi- mental results. Pcnetration factors less than unity were calculated for particles with aerodynamic diameters larger than 0.2 pm if the air exchange rate and steady-state [/0 concentration ratio were cor- related during the simulations. An additional correlation between the air exchange rate and particle deposition velocity is required if penetration factors less than unity are to be modeled for particles with aerodynamic diameters smaller than 0.2 pm. These results Received 29 July 1999; accepted 12 May 2000. This work was supported by cooperative agreement CR 822870-01 between the Research Triangle Institute and the National Risk Mana- gement Research Laboratory of the U.S. Environmental Protection Agency. Address correspondence to Jonathan Thomburg, Research Triangle Institute, P.O. Box 12194, 3040 Cornwallis Road, Research Triangle Park, NC 27709-2194. E-mail: [email protected] 284 support the possibility that appropriate experimental studies will yield penetration factors less than unity. The U.S. Environmental Protection Agency’s (EPA) pro- posed PM” standard (U.S. EPA 1997) has stimulated research into the causes and effects of exposure to fine particles. Under— standing how sensitive subpopulations, such as the elderly with cardiopulmonary diseases, are exposed to low concentrations of size-specific ambient aerosols has become a high priority fol— lowing the recommendations by the National Research Council (1998). Some susceptible subpopulations may spend more than 90% of their time inside commercial managed-care facilities or in private residences. Since individuals from these populations may influence substantially morbidity and mortality statistics. understanding the building and the heating, ventilation, and air- conditioning (HVAC) system features that determine the indoor particle concentration and size distribution in their residences has become very important. Characterization of these building and HVAC features, designated informally as physical factors, should help identify the scenarios that lead to the most— and least-exposed individuals. The transport of outdoor particles across the building enve— lope (i.e.. penetration) is an important physical factor that con» tributes to the particle concentration and size distribution inside buildings. Although the penetration factor can be unity when doors and windows are open, the possibility that aerosol pene— tration is unity under other conditions is counterintuitive. If the building envelope acts like a filter when the doors and windows are closed, the penetration factor should be a function of particle size and less than one. While traversing the building envelope, particles with aerodynamic diameters larger than 1 nm should be removed due to gravitational settling or turbulent impaction, and particles with aerodynamic diameters smaller than 0.2 pm should be removed by diffusion. As a result, penetration through the building envelope should be the greatest for particles with )/_______________________._ PENETRATION OF PARTICLES INTO BUILDINGS aerodynamic diameters between 0.3 and 0.5 mu, as might be predicted by general filtration theory (Hinds 1982). The goal of this article is to clarify the role of the penetra— tion factor in determining the indoor particle concentration. To accomplish this goal, published penetration factors calculated with a steady-state, single zone indoor air quality (lAQ) model were reviewed. A secondary goal is to determine the attributes of the indoor environment that may have the greatest effect on personal exposure to particulate matter. Then. a more robust, single zone lAQ model incorporating the physical factors most likely to affect the indoor particle concentration and size distri- bution was developed for various ventilation scenarios. For each scenario. Monte Carlo simulations determined the model pa— rameters that significantly influenced the indoor concentration. The sensitivity analysis performed on the indoor concentration identified frequently overlooked parameters. such as HVAC sys- tem characteristics, that may reduce the unexplained variation in field exposure data. Indoorvto—outdoor (I/O) concentration ratios were modeled with the Monte Carlo method for each scenario to identify the predominant size—dependent mechanisms modi~ fyin g the outdoor particle size distribution during penetration of the building envelope. Finally. size—dependent particle penetra— tion factors fora simplified case were computed using the Monte Carlo method and compared with previously published values. Part II of this study will present experimental measurements of penetration factors for a range of particle sizes and ventilation scenarios. A robust. singIc—compartmcnt. sixc~depcndent lAQ model has a number of attributes that an: useful for both predictive and planning purposes. Identification of the physical aspects of buildings that affect the penetration and fate of ambient particles can improve the design ol‘subsequcnt experimental PM” expo— sure rescarch by defining important measurements. as well as by aiding in the interpretation ofcollccted exposure data. Addition, ally, a more thorough understanding of the factors that influence indoor PM” concentrations can be used to reduce uncertain- ties in cpit‘lemiological investigations relating both ambient and indoor I’M” concentrations to health effects. PREVIOUS PHYSICAL FACTOR RESEARCH Previous investigations ol'the importance ofphysical factors on the transport and fate of particles into buildings addressed two general areas: statisti 'al regression of indttor-outdoor data to fit a simple lAQ model and the use of this model to calculate penetration factors using experimental deposition velocity data. Previous investigations used a steady-state. singlc-compartmcnt lAQ modcl similar to Equation (1 ): lll where C is the indoor concentration. I’ is the penetration factor. I... is the air exchange rate. (7,, is the outdoor concentration. A.) is the particle deposition rate. V is the building volume. and G 285 is the generation rate for all indoor sources. Note that the parti— cle deposition rate divided by the surface—area-to-volume ratio equals the particle deposition velocity, 1),]. Though used under a variety of experimental conditions by previous researchers, Equation ('1) is a special case of the more robust IAQ model developed in the next section. Dockery and Spengler (I981) first used the model to statisti- cally interpret personal exposure data. They calculated an aver- age I/O concentration ratio between 0.31 and 0.7 for respirable particulate matter without considering particle deposition. With a qualitative inclusion of building characteristics, Dockery and Spengler concluded that air conditioning decreased their 1/0 concentration ratio measurements. Koutrakis et al. (1992) as— sumed a particle deposition velocity of 0.18 m/h and performed statisti‘al regressions using Equation (1) to calculate average penetration factors between 0.6 and 1 for total PM” mass and a variety of elements. Koutrakis et al. also noted that the value of the penetration factor probably varies between homes and seasons. Wallace (1996) reviewed IAQ literature, with an em— phasis on the statistical interpretation of personal exposure data. Wallace concluded that the penetration factor, deposition veloc— ity, and air exchange rate were vital parameters affecting the indoor concentration. Wallace reported that average 110 con- centration ratios from the PTEAM study for PMM and PM“) were 0.67 and 0.57, respectively. Nonlinear regression of the re— sults from the PTEAM study yielded penetration factors “very close to unity” and average deposition rates of 0.39 h—l and 0.65 h“l for PM2_5 and PM“), respectively. Assuming a typical surfacc—area-to-volume ratio of 1.7 in", these deposition rates are equivalent to deposition velocities of 0.23 m/h and 0.38 m/h. respectively. Other researchers used Equation ( l) to experimentally de- termine particle deposition velocities. penetration factors, and I/O concentration ratios in buildings without mechanical venti— lation. In a residence with air exchange rates less than 0.1 h“, McMurry et al. (1985) measured I/O concentration ratios be» tween 0.2 and 0.6 for particle aerodynamic diameters between 0.01 and 1.0 run. McMurt‘y etal. concluded their data supported the hypothesis that particle removal during infiltration is the pri- mary loss mechanism causing I/O concentration ratios less than one. In a single residence study, Thatcher and Layton (1995) ‘alculated deposition velocities between 0.64 and 4.15 m/h for I to 6 mn aerodynamic diameter particles, and reported an aver— age penetration factor for all particle aerodynamic diameters of one. In several residences. Fogh ct al. (1997) measured particle deposition velocities between 0.18 and LI m/h for particle aero- dynamic diameters between 0.5 and 5.5 um. Calculated 1/0 con— centration ratios varied between 0.1 and 0.7 for the same sized particles for assumed penetration factors equal to 1. Tang et al. (1999) developed a time-dependent lAQ model. However. the 8-hour integrated inside-outside concentration measurements col lectcd in adjacent rooms effectively reverted the time— dependent solution back to Equation (1). Tang et al. reported an extremely low average deposition velocity of 0.037 m/h and 286 J. THORNBURG ET AL. an average penetration factor of 0.78 for PM“) in an enclosed office. In an enclosed chamber, Lewis (1995) experimentally measured penetration factors between 0 and 0.8 for l to 7 am aerodynamic diameter particles with particle deposition rates between 0.25 and 1.1 h‘l. The equivalent deposition velocities are 0.15 m/h and 0.65 m/h, assuming a surface-area—to—volume ratio of 1.7 m”. Lewis also demonstrated that increasing the pressure differential or aperture width of an opening increased the penetration factor for all particle sizes. Obviously, the range of published penetration factors varies considerably. One possible explanation for these differences is the value of the particle deposition rate measured, calculated, or assumed by various researchers. As shown in Equation (1), the penetration factor is proportional to the particle deposition rate. Accordingly, penetration factor cannot be calculated using Equation ( 1) without first knowing the particle deposition rate. Therefore, it is not surprising to find that Thatcher and Layton (1995) calculated average penetration factors of one consider- ing their deposition rates were the highest, while Koutrakis et al. (1992) and Tung et al. (1999) used lower deposition rates to cal— culate penetration factors less than unity. Interestingly, Lewis (1995) measured deposition velocities 4 to 8 times lower than Thatcher and Layton over the same particle size range, and calculated penetration factors less than unity. Wallace (1996) did not present definite penetration factor values for PM” and PM“), so his work cannot be compared appropriately to the other results. Separating the influence of deposition rate and penetration factor on the indoor concentration is difficult because both phys- ical factors occur simultaneously, and both parameters affect the indoor concentration of specific particle sizes in a similar man- ner. Diffusion, turbulent deposition, gravitational settling, and thermophoresis remove particles from the air inside the room and during transport through the building shell. Diffusion re- duces the penetration and dominates the deposition velocity of 0.1 [rm aerodynamic diameter or smaller particles; whereas, gravitational settling and inertial impaction reduce the penetra- tion and increase the deposition velocity for 1.0 ttm aerodynamic diameter or larger particles. Particles with aerodynamic diame~ ters between 0. land 1.0 um have higher penetration factors and lower deposition velocities. As a result, the most accurate pene- tration factors will be calculated by considering narrow particle size ranges over which the deposition velocity and filtration ef- ficacy of the building envelope will be essentially independent of particle size. The Monte Carlo simulations conducted in this study separated the confounding influence of the penetration factor and deposition rate by using specific particle sizes when modeling the indoor concentration. MODEL DEVELOPMENT Current IAQ models are either simple box models or complex models developed as computer programs. Equation (1) is a box model that does not account for all the physical factors found in residential and commercial buildings with HVAC systems unless natural ventilation is the primary air exchange mecha- nism. Personal computer—based IAQ models (e.g., CONTAM, INDOOR) that account for transport within multiroom build- ings, ventilation system characteristics, pollutant types, and pol- lutant sources and sinks have been developed. These models can predict either the indoor concentration or personal exposure to gases and particles. Austin et al. (1992) discussed the features of the various multiroom IAQ models, but a peer-reviewed as- sessment on the accuracy of these multiroom IAQ models has not been conducted. Also, the focus of this study was to under- stand the overall particle removal and generation mechanisms, not inter~room transport. Therefore, multiroom models were not used to conduct this research. Instead, the IAQ model developed here contained the relevant physical factors, but was based on a box model of a single compartment. This model was based on the IAQ equations developed by Meckler (1995) and Tung et al. (1999), but included additional terms to account for different ventilation scenarios. Physical factors contributing to the indoor concentration are penetration of particles from outdoors through crevices or open windows, recirculation of indoor particles between microenvi- ronments, and indoor sources (e.g., cooking and smoking). Key factors removing particles from the building are filtration losses within the building envelope, deposition to surfaces, and capture by the HVAC system. The conceptual IAQ in Figure 1 assume that the building contains one well—mixed compartment with a constant volume, and that the indoor concentration is not explic— itly dependent on particle size. Particle size can be accounted for by applying the IAQ model to individual particle sizes or particle size ranges. Also, the outdoor concentration and meteorological conditions are assumed to be constant. Models were developed for three building~HVAC combina— tions common in U.S. construction: a commercial building with atypical commercial HVAC system, a residence with a typical residential HVAC system, and a residence without an HVAC system. These systems differ primarily in the presence of de— liberate makeup air for the commercial system and the type of HVAC filter present. The IAQ model for the most complex scenario, a commer- cial building with an HVAC system, will be developed first. Although several HVAC system configurations exist for com— mercial buildings, the most common configuration, where the outdoor makeup air and the return air from the room pass through the same HVAC filter, was modeled as shown in Figure l (ASHRAE 1996). Other possible HVAC-filter configurations in— clude the makeup air or return air not passing through an HVAC filter or passing through different filters. The particle mass that penetrates and exits the building en- velope via natural convection, the generation rate of particles indoors, the particle deposition rate, and the contribution or re— moval of particles by the HVAC system determine the rate of change of the indoor particle concentration. The particle mass balance within the indoor space in Figure l, with input and output PENETRATION OF PARTICLES INTO BUILDINGS 1n filtration, anf Outdoor Air, an (commerical only) HVAC Flow, Q1“, Exhaust Air. Qe (commercial only) 287 Extiltration, Qexi / indoor Concentration Indoor Generation . . Deposttion Figure 1. Sources and sinks of particles within a building. from the I-IVAC system, is given mathematically by: dC V(~—) : PCerini' 'i' G h ‘A‘S'Ud — CQL‘“ (It ‘i‘ Clrr,IQ/tl' — CQra — CQW [2] where V is the building volume. C is the indoor particle con— centration, r is time, P is the fraction of particles that penetrate the building envelope due to natural convection, Cu is the out- door particle concentration, QM is the infiltration air flow due to natural convection, G is the indoor generation rate, it, is the surface—area available for deposition, 1),, is the deposition veloc- ity, chr is the exliltration airflow due to natural convection, Cl”, is the particle concentration coming from the l-IVAC system, Q,,,, is the flow from the HVAC system, Q“l is the return air- flow from the room to the I~IVAC system, and Q(. is the airflow mechanically exhausted to the outdoors. A particle mass balance on the PIVAC system in Figure 1 yields: ‘ (IC‘I t CIer/m ‘i‘ Vim”; : (I! II ‘“ 7))CuQnm ‘i‘ ...
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