Concentrations and emissions of gasoline and other vapors from residential vehicle garages

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Atmospheric Environment 40 (2006) 1828–1844 www.elsevier.com/locate/atmosenv

Concentrations and emissions of gasoline and other vapors from residential vehicle garages Stuart Batterman, Gina Hatzivasilis, Chunrong Jia Environmental Health Sciences, University of Michigan, Ann Arbor, MI 48109-2029, USA Received 16 September 2005; received in revised form 1 November 2005; accepted 1 November 2005

Abstract High concentrations of airborne volatile organic compounds (VOCs) may be present in residential garages due to emissions from vehicles, lawnmowers, storage containers, and many other items stored in the garage. VOC emissions will ultimately be transported into ambient air and, if the garage is attached to a residence or other building, into living spaces. This study reports on VOC concentrations and emissions at 15 residential garages in Michigan that varied in type, size, use and other characteristics. VOCs were measured in garages and in outside air using 4-day passive sampling, thermal desorption, and GC-MS analysis. Effective air exchange rates (AERs) were determined using a perfluorocarbon tracer gas and the constant injection method. A modeling analysis shows the effect of time-varying ventilation. To estimate temporal and spatial variability, concentrations were measured on 7 subsequent occasions at multiple locations in one garage. This garage was well-mixed, and the temporal variation in AERs and concentrations was modest. Across the 15 garages, 36 different VOCs in garage air, and 20 in ambient air, were quantified. Source groups identified and attributed to garage emissions included evaporated gasoline, solvents, paints, oils, and cleaners. Concentrations of gasoline-related VOCs in most garages were high, e.g., benzene levels reached 159 mg m
3 in one garage. TVOC emissions per garage averaged 3.074.1 g day
1, and AERs averaged 0.7770.51 h
1. VOC concentrations and AERs were not strongly correlated to observed house, garage or meteorological factors, but appeared largely dependent on occupant activities (opening of the garage door) and VOC sources present. This study quantifies the importance of attached garages as VOC sources, and the results are significant for understanding and mitigating indoor exposures, and for estimating emissions for source inventory purposes. r 2005 Elsevier Ltd. All rights reserved. Keywords: Air exchange; Indoor air; Garages; Gasoline; Volatile organic compounds

1. Introduction Volatile organic compounds (VOCs) are emitted by the many items stored in garages, including Corresponding author. Tel.: +1 734 763 2417; fax:1 734 764 9424. E-mail address: [email protected] (S. Batterman).

1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2005.11.017

vehicles, lawnmowers, chainsaws, snowblowers, trimmers, gasoline storage containers, solvents, oils, paints, building materials, and pesticides. Emissions from vehicles, which have received more attention than the other sources, occur during startup (hot and cold starts from tailpipe emissions), shutdown (hotsoak emissions), and parking (diurnal breathing and evaporative losses). Understanding concentrations

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and emissions of VOCs in garages is of interest for several reasons. First, when the garage is attached to a building, pollutants can migrate to living spaces and thus degrade indoor air quality. Second, emissions from garages will contribute to ambient air concentrations of VOCs that are precursors to ozone, the most widespread urban air pollutant. Third, garages represent microenvironments in which people are exposed. While most people will spend only a few minutes each day in a garage, much more time may be spent by certain individuals, e.g., automobile enthusiasts and cigarette smokers, the latter being increasingly relegated to outdoor environments and particularly garages in inclement weather. The significance of garages (and vehicles parked within) as emission sources affecting indoor air quality has been recognized for residences (Traynor and Nitschke, 1984; Wallace, 1987, 1989; Hawthorne et al., 1986; Gammage and Matthews, 1988; Cohen et al., 1989; Thomas et al., 1993; Lansari et al., 1996; Graham et al., 1999, 2004; Noseworthy and Graham, 1999; Colome et al., 1994; Lindstrom et al., 1995; Levsen et al., 1999; Lebowitz et al., 1999; Tsai and Weisel, 2000; Brown, 2002; Fugler et al., 2002; Emmerich et al., 2003) and office buildings (Hodgson et al., 1991; Grot et al., 1991). Several other studies have examined automobiles as a source of VOCs in garages (Murphy et al., 1997; Graham et al., 2004). A few several studies have examined VOC concentrations in residential garages (Thomas et al., 1993; Lansari et al., 1996; Graham et al., 2004), air exchange rates (AERs) in garages (Graham et al., 1999; Fugler et al., 2002; Emmerich et al., 2003), and leakage between the garage–house interface (Thomas et al., 1993; Sherman and Chan, 2004). Despite these studies, a recent review concludes that there is an important need to better characterize impacts of attached garages on residential air quality (Emmerich et al., 2003). Additionally, we found no systematic and representative assessments of VOC levels, sources and AERs in residential garages, and no information regarding their significance of emission sources. There has been significantly more work in commercial parking garages (Limb, 1994), but much of this is not applicable to the residential setting. This study is aimed at characterizing AERs, VOC concentrations and emissions in residential garages, including an assessment of major sources and influences on AERs and VOCs.

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2. Methods We recruited 15 participants in our local Ann Arbor, Michigan area. Participants were considered to be eligible if they lived in a single family house with a garage and were willing to participate in all aspects of the study protocol. Recruitment followed procedures approved by the University of Michigan’s Institutional Review Board, and included informed and written consent. An extensive walkthrough checklist was completed to characterize building characteristics and potential VOC sources. Participants also completed a diary to identify activities that may have affected VOC concentrations and AERs. Initially, to investigate temporal and spatial variability of measurements, a single garage (#1) was studied on 7 mostly consecutive 4-day periods with samplers placed at several sites (usually back and sides) in the garage. Then, to examine variability across sites, 14 additional garages were monitored, 1–3 per week, on consecutive weeks. The last measurement at garage #1 (1G) was incorporated into the pooled analysis (achieving n ¼ 15) as its sampling dates were closest to those of the other garages. In each garage, VOC and AER monitoring was conducted for 4 consecutive weekdays. The sampling schedule and prevailing weather information conditions are shown in Table 1. 2.1. AER determinations AERs were determined using the constant injection technique in which a perfluorocarbon tracer (PFT) was injected into the garage at a constant rate, the tracer concentration was measured using passive sampling over a 4-day period, and the garage was considered as a single zone where concentrations were assumed to approach steadystate levels. This method allows for direct measurement of air infiltration without disturbing living conditions (Dietz and Cote, 1982; Stymne and Eliasson, 1991; Sherman, 1990). PFTs make good tracers since they are considered to be non-toxic, stable, non-adsorptive on common materials, and are normally present at exceedingly low concentrations. AERs obtained using constant injections of PFTs have been shown to be equivalent to those determined using sulfur hexafluoride tracer decay tests (Dietz and Cote, 1982; Cheong and Riffat, 1995). In each garage, a liquid hexafluorobenzene (HFB) reservoir provided a constant release of the

4/14 4/18 4/21 4/25 4/28 5/2 5/13 5/23 5/23 5/23 5/31 5/31 6/13 6/13 6/13 6/20 6/20 6/21 6/27 6/27 6/28

4/18 4/21 4/25 4/28 5/2 5/6 5/18 5/26 5/26 5/26 6/3 6/3 6/16 6/16 6/16 6/23 6/23 6/24 6/30 6/30 7/1

Stop

18 4

12 15 3 7 7 5 11 13 13 13 17 17 20 20 20 17 17 19 23 23 22

Ambient data from Weather Underground (2005). a Data not available. b Statistics exclude measurements 1A–F.

Averageb Standard.deviationb

1A 1B 1C 1D 1E 1F 1G 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Start

8 5


2 1
1 1
2
6 2 4 4 4 6 6 10 10 10 4 4 4 18 18 16

Mean Min. temp. (1C) temp. (1C)

Identificatoin site

Sampling dates

Outside/ambient

Sites/dates

27 4

25 27 16 16 14 19 20 22 22 22 25 25 29 29 29 28 28 33 32 32 31

Max. temp. (1C)

78 5

45 62 82 72 55 65 74 82 82 82 71 71 81 81 81 74 74 70 81 81 82

Rel. humidity (%)

Table 1 Sampling schedule, weather data and garage environment during the sampling period

1013 5

1026 1015 1002 1004 1014 1023 1015 1010 1010 1010 1017 1017 1005 1005 1005 1020 1020 1019 1015 1015 1012

Baro. pressure (kPa)

14 3

13 17 23 18 15 13 15 14 14 14 11 11 18 18 18 11 11 13 12 12 14

Wind speed (km h
1)

0.02 0.04

0.00 0.00 0.00 0.00 0.00 0.00 0.14 0.05 0.05 0.05 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 23 4

—a 19 13 13 14 14 — 18 19 18 23 23 23 23 22 — 24 23 29 28 29 18 4

— 16 10 11 11 10 — 15 15 13 19 14 21 20 17 — 16 18 24 22 24

Precipitation Mean Min. temp. (cm) temp. (1C) (1C)

Garage

31 6

— 24 18 16 17 18 — 23 26 25 27 34 25 25 30 — 34 30 44 36 37

Max. temp. (1C)

53 10

— 41 50 55 40 38 — 54 48 52 43 47 58 82 59 — 44 48 51 48 51

Rel. humidity (%)

1830 S. Batterman et al. / Atmospheric Environment 40 (2006) 1828–1844

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tracer through a small diffusion tube similar to that first used by Palmes et al. (1976). Sources were temperature stabilized at 40 1C using dry bath heaters. Emission rates determined in the laboratory by a linear regression of weight change over time for 7 different sources averaged 3.770.3 mg h
1 with excellent linearity (R2 ¼ 0:998). Each source was weighed in the laboratory immediately before and after field deployment, and emission rates were calculated by subtracting final and initial weights and dividing by the time elapsed. 2.2. VOC and PFT measurements Concentrations of VOCs and the HFB tracer were measured in garages and in ambient air using passive samplers that contained Tenax GR adsorbent in a tube type configuration that has been previously shown to provide excellent performance (Peng and Batterman, 2000; Batterman et al., 2005). In garages, duplicate or triplicate samplers were placed on a stands located away from walls and other surfaces, generally on the opposite sides from the PFT source and at a central location away from doors and windows. Final locations depended on the participant’s wishes and the location of an electrical outlet needed for the PFT source. Outside, VOC concentrations were determined at a centrally located site each week, again using duplicate samplers mounted in a stainless steel rain shelter at 1.5 m height placed near one of the study garages. Previous sampling indicated that VOC concentrations in ambient air in residential areas in the study area were low, generally below 1–3 mg m
3 for individual compounds, and spatially quite uniform. Additionally, the garages studied on any individual week were usually close together, e.g., on the same block. Thus, the use of a single outdoor air sampling site is unlikely to cause significant errors, especially since concentrations in garages far exceeded outdoor levels (as shown later). Temperatures at all garage and outdoor samplers were continuously monitored and recorded over the sampling period and used in the calculation of sampling volume. Samplers were deployed for 4 days, after which they were retrieved, capped, and returned to the laboratory. Tube contents were thermally desorbed and analyzed by gas chromatography and mass spectrometry as detailed in Peng and Batterman (2000). Method detection limits (MDLs) were determined by thermally desorbing adsorbent tubes loaded with

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low (close to instrumental DL) concentrations of target VOCs. Following US EPA (1999) guidance for adsorbent tube sampling, 99% confidence MDLs were calculated as 3.14 times the standard deviation of 7 replicate determinations and assuming 2 l sample volumes, representing the minimum concentration above which we have confidence that the analyte is present. As mentioned, all samples were collected using duplicates or triplicates, which were averaged. Non-detects were assigned the MDL. Laboratory and field blanks were collected and analyzed with each batch of samples. The target compounds (with complete calibration curves) included over 90 VOC compounds. A calibration and method evaluation completed for HFB showed excellent recovery (97%) and linearity (R2 ¼ 0:998), and a low MDL (0.024 mg m
3), comparable to that determined for the other VOCs (Peng and Batterman, 2000). 2.3. Calculation of AERs Assuming fully mixed and steady-state conditions, the AER (h
1) is AER ¼ F =ðCV Þ,

(1)
1

where F is the PFT emission rate (mg h ), C the average PFT concentration (mg m
3) in the garage, and V is the volume of the garage (m3). Because the flow rate is not constant in time over the sampling period, this method is said to derive the ‘‘effective’’ AER and ventilation rate (Sherman and Wilson, 1986). The AERs for the garages represent exchange of outside air, both when the garage door is opened and from more or less continuous infiltration though seals and other leaks when the garage door is closed. Also, since most (14 of 15) garages were attached to occupied houses, the AERs also include air exchange with the house, both when the door leading to the house is opened and from more or less continuous leakage through the garage–house interface. While these four pathways cannot be separated with the measurements available in this study, the AERs derived here provide the total air exchange for each garage, a measurement needed to estimated the dilution available in the garage and to estimate emissions. 2.4. Data analysis To identify factors affecting AERs and VOC concentrations, the relationships between house,

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garage, source activity, and meteorological factors were explored using correlation coefficients, scatter plots, regression models and t-tests. Given the skew observed in the concentration data, correlations and other analyses were performed using both parametric and non-parametric measures. To apportion the gasoline fraction, we used a chemical mass balance model and VOC compositions (source profiles) provided in Watson et al. (2001). These analyses used Excel (Microsoft, Redmond, WA) and Systat 10 (SPSS, Chicago, IL).

concentrations above 1 mg m
3. Replicate precision averaged 10% for these compounds, and slightly better (9%) for HFB. Precisions were sometimes, but not always, lower for VOCs detected at lower concentrations. Throughout the whole study period, nearly all laboratory and field blanks were clear of interferences. In a few tubes, trace levels of common VOCs such as toluene and benzene were detected, however, levels remained below MDLs.

3. Results

A summary of the 7 repeated measurements made in garage #1, a 2-car garage attached to a 2-story house, is shown in Table 3. This garage contained a model year 2000 low emitting vehicles (LEV) car, which was driven infrequently (1 or times per week) and a 2004 ultra-low emitting vehicles (ULEV) car, driven into the garage 3 times per day. The electrically powered garage door was opened approximately 8–10 times daily by the 3 occupants of this house. This insulated garage had two passage doors: one led inside to the hallway/ kitchen area; a second door led outside. The latter door was never opened although it was inset with a small cat door which had a magnetic latch that was used perhaps half-dozen times daily. AERs over the 5-week study period at garage #1 averaged 0.6070.07 h
1, varying by only 12% (COV). During this period, outdoor temperatures were cool to mild (3–15 1C as 4-day averages), winds were mild to moderate (12–22 km h
1), and temperatures in the garage at the sampling locations were 13–19 1C, 5–10 1C warmer than outdoors (Table 1). The estimated AERs had no clear relationship to meteorological parameters, suggesting that effects may be averaged out over the 4-day measurement, and/or that occupant activities might compensate for weather changes, e.g., door openings may be shortened in cold weather. Concentrations of individual VOCs were below 100 mg m
3 except for toluene on two events. Gasoline-related compounds (benzene, toluene, m,pxylene, o-xylene, ethylbenzene, n-heptane, methylcylohexane, n-octane, isopropylbenene, 4-ethyl toluene, trimethylbenzene) were dominant and concentrations were highly correlated (r40:9). The total VOC concentration (TVOC), defined here as the sum of the target compounds, averaged 2517 61 mg m
3. As shown later, when ranked by TVOC levels, this garage fell into the lowest tertile of the 15 garages studied. Concentrations of fuel-related

3.1. Garage characteristics Table 2 summarizes characteristics of the 15 houses and garages. House areas were provided by the home owner. Garage dimensions were measured using an acoustic measuring tape. All of the garages were attached to houses via a connecting (passage) doorway except garage #6 which was detached. Of the 15 garages, 7 were located on one side of the house, and 7 had two adjoining walls of which 3 garages had occupied spaces above. The garages had no provisions for ventilation except for #14 which had peak vents. Participants parked cars in all garages except in #14, however, several motorcycles were stored in this garage. Paints, solvents and other VOC-containing materials were stored on shelves, floors, and other open areas in all of the garages except in #5; these materials were also stored in cabinets or other enclosures in garages #5, 6, and 7. Gasoline-powered lawn mowers and fuel (gasoline or kerosene) containers were stored in 10 of the garages. Four garages had evidence of oil or other spills. Temperatures in the garages averaged 2374 1C, warmer than outside air by about 5 1C (Table 1). Outdoor temperature swings over the 4-day sampling periods were moderated in most garages, except in garages 6, 13, 14 and 15 that reached temperatures of 34–44 1C, well above outside temperatures. Relative humidities were lower and more moderated in garages compared to outside air. 3.2. Precision and blanks Measurement precisions for VOC and PFT concentrations were determined by an analysis of the replicate samples collected in each of the 15 garages for the 25 compounds generally detected at

3.3. Temporal and spatial variability

2

15

Y ¼ yes.

2 1 1

12 13 14

1996

1987 1960 1965

1971 1939

F

F F F

F F

202

177 98 148

112 242

121 223 223 158

1 2

F F F P

40

46 23 44

32 46

45 15 41 40

2.7

3.0 2.6 6.1

2.4 5.2

2.4 2.4 2.4 2.4

2

2 1 0

1 2

1 1 2 1

2 2 2 1 2

10 11

1964 1938 1926 2003

3.1 3.1 2.4 2.4 2.7

1 2 2 1

35 35 36 52 35

6 7 8 9

2 car attached garage; 3 surfaces adjacent to home; entrance to hall 2 car attached; 2 surfaces adjacent to home; entrance to hall 2 car attached; 2 surfaces adjacent to home; entrance to hall 1 car attached; 1 surface adjacent to home; entrance to FR 2 car attached garage; 3 surfaces adjacent to home; entrance to kitchen and laundry Detached garage 1 car attached garage; 3 surfaces adjacent to home; entrance to FR 2 car attached; 1 surface adjacent to home; entrance to basement 1 car attached garage; 1 surface adjacent to home; entrance to kitchen 2 car attached garage; 2 surfaces adjacent to home; entrance to hall 2 car attached garage; 1 surface adjacent to home; entrance to LR 2 car attached garage; 1 surface adjacent to home; entrance to hall 1 car attached garage; 1 surface adjacent to home; entrance to hall 2 car attached, spans 2 floors; 1 surface adjacent to home; entrance to LR 2 car attached garage; 2 surfaces adjacent to home; entrance to laundr

214 181 183 177 214

P P F F F

2 2 2 2 2

1 2 3 4 5

1962 1960 1967 1968 1970

House Garage Garage Number area area height cars (m2) (m2) (m) parked in garage (m)

Partial/ Garage type Garage No. Year full basID floors constructed ement in house

2

1 2 2

3 2

3 1 1 1

2 1 2 1 2

Clutter in garage (1–3)

—

— — —

— —

Y Y — —

— — — — Y

Y

Y — Y

Y Y

Y Y Y Y

Y Y Y Y —

Y

Y — Y

Y —

Y Y Y Y

Y — Y — Y

Y

Y — Y

— Y

Y Y Y —

Y — — Y —

—

Y — —

— Y

— — — —

— — — — —

Paints WoodEnclosed Open storage Fuel in con- and working storage of volatiles tainers solvents supplies of volatiles

Potential VOC sources

Dimensions

House and garage description

Table 2 Descriptions of the garages

Y

Y — Y

Y —

Y Y — —

Y Y Y Y Y

Y

Y — Y

Y —

Y — Y —

— Y Y — Y

Y

— — —

Y Y

Y — — —

— — — — —

Evidence Lawn- Lawn mowers, care pro- of spills other IC ducts engines

S. Batterman et al. / Atmospheric Environment 40 (2006) 1828–1844

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Table 3 VOC concentrations and AERs in the temporal study, conducted at garage #1. Concentrations are average of 2 or 3 replicates VOC/AER

Trial 1A (mg m
3)

Concentration (mg m
3) 1,1,1-Trichloroethane Benzene Carbontetrachloride n-Heptane Methyl cyclohexane Methyl isobutyl ketone Toluene n-Octane Tetrachloroethene Ethylbenzene p-Xylene, m-xylene Styrene o-Xylene n-Nonane Isopropylbenzene (cumene) a-Pinene n-Propylbenzene 4-ethyl toluene 1,3,5 -Trimethylbenzene 2-ethyl toluene 1,2,4-Trimethylbenzene n-Decane 1,2,3-trimethyl benzene p-Isopropyltoluene Limonene n-Butylbenzene n-Undecane Naphthalene n-Dodecane Total
1

Air exchange rate (h ) AER

Overall IB (mg m
3)

1C (mg m
3)

ID (mg m
3)

IE (mg m
3)

IF (mg m
3)

1G (mg m
3)

Mean (mg m
3)

COV (%)

5.23 16.17 1.15 11.85 2.95 0.85 103.05 2.50 0.44 12.71 54.46 0.37 18.07 1.57 0.67 1.06 2.93 16.22 4.56 3.72 16.75 1.75 3.91 0.17 7.30 0.73 0.97 3.24 0.39

4.61 14.06 1.19 10.39 2.62 1.47 93.14 2.20 0.33 11.17 48.10 0.21 16.44 1.48 0.66 1.02 2.89 16.09 4.57 3.59 16.65 1.72 4.00 0.21 6.59 0.60 1.01 3.62 0.47

3.12 8.93 1.40 6.33 1.53 0.49 61.30 1.48 0.29 7.53 32.52 0.10 10.94 0.85 0.41 0.58 1.85 10.41 2.92 2.43 10.87 0.93 2.61 0.17 2.32 0.53 0.56 2.66 o0.01

3.29 9.60 0.96 7.24 1.62 o0.06 62.41 1.38 0.23 7.25 30.91 0.08 10.22 0.76 0.36 0.52 1.59 8.94 2.52 1.99 8.99 0.73 2.13 0.09 1.91 0.36 0.43 1.96 o0.01

4.04 13.40 1.50 9.62 2.28 0.52 81.93 2.64 o0.01 12.11 51.38 0.11 17.30 1.34 0.66 0.76 2.92 16.24 4.52 3.71 16.59 1.21 3.83 0.25 3.51 0.66 0.79 3.86 o0.01

4.43 13.91 2.37 9.66 2.51 o0.06 92.21 2.00 0.25 10.37 44.82 0.20 15.20 1.17 0.60 0.68 2.53 13.91 3.94 3.22 13.90 0.96 3.19 0.21 2.97 0.53 0.49 2.78 o0.01

4.34 19.01 1.51 11.89 3.02 o0.06 114.77 2.95 0.30 15.31 65.78 0.13 22.04 1.60 0.84 0.91 3.64 19.71 5.54 4.53 19.69 1.30 4.44 0.28 7.89 0.65 0.66 3.42 0.28

4.15 13.58 1.44 9.57 2.36 0.50 86.97 2.17 0.26 10.92 46.85 0.17 15.74 1.25 0.60 0.79 2.62 14.50 4.08 3.31 14.78 1.23 3.45 0.20 4.64 0.58 0.70 3.08 0.17

18 26 32 22 25 104 23 27 50 26 26 58 26 27 27 27 27 26 26 26 25 32 24 31 54 21 33 21 122

295.75

271.10

176.10

168.50

257.73

249.06

336.46

250.67

24

0.55

0.52

0.53

0.64

0.59

0.66

0.70

0.60

12

compounds and most other VOCs varied by an average of 27% (COV). Several VOCs had greater variability, mostly those measured at low concentrations that were infrequently detected (e.g., methyl isobutyl ketone, styrene, n-dodecane). Limonene showed more variation (54%), and concentrations were highly correlated with a-pinene and n-decane (r ¼ 0:93, 0.98), suggesting a common source or activity, most likely the cleaning and lubrication of bicycles stored in the garage. Chlorinated compounds and naphthalene had lower and inconsistent correlations with other VOCs, also suggesting separate sources. The time variation in measured concentrations (27%) will depend on changes in source emissions

(unknown), AERs (12% variation), and measurement errors (10% variation). Using Gaussian quadrature and assuming independence and normality, the variation in source emissions is estimated to be 22%. Thus, time-varying source emissions appear to be the major factor explaining fluctuations in VOC concentrations, though the amount of variation was small. The observed variation was likely attenuated by the long (4-day) sampling period, and much larger fluctuations are expected for short-term concentrations (shown later). Of course, these results cannot necessarily be generalized, and the variance apportionment is simplified. Concentrations measured at the various sites in this garage were similar, with a COV of 1173%

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As mentioned, the ‘‘effective’’ AER was obtained by assuming that the garage is a single fully mixed zone, and that concentrations and flows are at steady state. When the garage doors are opened, ventilation rates may increase greatly, thus concentrations and flows will be time-varying, depending on the frequency, duration and timing of garage door openings and the difference in flows between open and closed periods. In contrast, the timeweighted average (TWA) AER, constructed as a TWA of short-term or instantaneous measurements, likely will exceed the effective AER (Dietz and Cote, 1982; Sherman and Wilson, 1986; Stymne and Eliasson, 1991). Because the variability in ventilation rates due to garage door openings may be very large, we explored differences between TWA and effective ventilation rates by simulating tracer gas concentrations in garages using a one-compartment, fully mixed mass balance model and a range of door openings durations and AERs. As a base case for a weekday period, we assumed 10 garage door openings per day, reflecting, for example, occupants leaving in the morning (openings at 7:00, 7:15, 7:30, and 8:00 and returning in the afternoon (openings at 15:00, 17:00, 18:00), and three evening openings (21:00, 22:00, and 21:30). To represent the multiday sampling periods, the initial concentration was set to the concentration achieved after a 24 h simulation. Opening the door is assumed to significantly increase the AER. A sensitivity analysis explored effects of different durations of openings and different AERs during open and shut periods. Fig. 1 shows tracer gas concentrations in two simulations. Concentrations in the garage fall below the steady-state level, especially with repeated and long duration door openings, but levels approach steady-state values in the early morning and late afternoon. (Normalized results are not dependent on the tracer gas emission rate or garage volume.) The bias between effective and TWA AERs for door openings from 1 to 30 min and various AERs is shown in Fig. 2. For example, given AERs of 1 and 10 h
1 for closed and opened garage doors, respectively, and 10 openings of 5 min duration

80 Concentration (% of steady-state)

3.4. Time-varying ventilation

100

60 40 Door opening Openings=2 min, AER=1.0/10.0 hr-1 Openings=5 min, AER=0.5/5.0 hr-1

20 0 0

4

8

12 16 Hour of Day

20

24

Fig. 1. Simulated concentration trend of tracer released at a constant rate in a fully mixed space with 10 door openings per day of 2 and 5 min duration and indicated AER rates (door closed/door open). Triangles show time of door openings. 50 40 Relative Bias (%)

(based on 6 repeated sampling events using 3 samples each, and considering VOCs41 mg m
3). This variation is just slightly greater than the experimental precision (discussed previously). These results indicate that this garage was well mixed.

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30 20

AER=1.0/25.0 hr-1 AER=2.0/20.0 hr-1 AER=1.0/10.0 hr-1 AER=0.5/5.0 hr-1

10 0 0

5

10 15 20 Duration of Door Opening (min)

25

30

Fig. 2. Estimated bias between effective AER relative to time weighted average AER. Based on 10 door openings per day of stated duration and AERs, and door opening sequence shown in Fig. 1.

each, the TWA AER (1.31 h
1) is 7.5% larger than the effective AER value (1.21 h
1). The bias increases with longer door openings, lower AERs, larger differences between AERs when doors are open and closed, and more evenly spaced door openings. Given the many variables, estimated biases are approximate. However, if the door is opened only a few minutes each time, TWA and effective AERs will be similar.

3.5. AERs across garages Across the 15 garages, AERs averaged 0.7770.51 h
1 and ranged from 0.16 to 1.80 h
1 (Table 4). The value for garage #2 was obtained with a possibly overheated PFT source and thus may not be reliable, however, this value is not an outlier. The median and interquartile range, 0.58

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S. Batterman et al. / Atmospheric Environment 40 (2006) 1828–1844

and 0.57 h
1, respectively, may be more robust statistics. The AERs showed several interesting associations with the structure, occupant and meteorological variables, however, their statistical significance is doubtful due to the small sample size (n ¼ 15). AERs tended to decrease with increasing garage height, area and volume (correlation coefficients r ¼
0:26,
0.41, and
0.36, respectively). This trend might be explained by several factors. First, the area and perimeter of the garage envelope increases more slowly than the garage volume, thus the same envelope leakage rate (e.g., crack density per area) will result in a lower AERs in larger garages. Second, larger garages like larger homes may be built better with fewer cracks and less infiltration, also leading to lower AERs in larger garages. AERs tended to increase in houses with full basements (r ¼ 0:38), possibly due to additional leakage pathways (Sherman and Chan, 2004). AERs also increased in more cluttered garages (r ¼ 0:46), possibly indicating high utilization of the garage and additional door openings. This explanation is especially speculative since clutter was a qualitative variable and because the correlation was in part driven by garage 10, one of only two garages that were considered to be very cluttered. AERs tended to decline when the passage to the house had both screen and solid doors, compared to a solid door alone (r ¼
0:29), possibly reflecting a tighter garage–house interface. On the other hand, several trends were inexplicable. AERs tended to increase if the space above the garage was occupied (r ¼ 0:30), if the barometric pressure rose (r ¼ 0:43), if the relative humidity dropped (r ¼
0:32), and if the wind speed fell (r ¼
0:45). Multivariate regression models aimed at explaining AERs were not significant. We acknowledge that the explanations provided above are speculations that require additional data and analyses for confirmation. 3.6. VOC levels in garages and outside air A total of 36 VOC compounds on the target list were detected in the garages, and 20 in ambient air (Tables 4 and 5). Nearly all compounds had detection frequencies exceeding 50%. Concentrations in garages exceeded outdoor levels, usually by a large amount. The TVOC concentration in garages, defined as the sum of the target compounds, averaged 6337554 mg m
3 across the 15

garages. Gasoline vapors were by far the dominant VOC source. The adsorbent-based sampling method used does not capture the very volatile components of these vapors, e.g., short n-alkanes (npentane), branched alkanes (2-methyl propane, 2-methylbutane) and alkenes (trans-2-butene, trans-2-pentene), which constitute a large fraction (22–71%) of gasoline vapor (Halder et al., 1986). Moreover, not all VOCs collected are quantified due to co-elution with solvents and peak overlap. Earlier, we compared concentrations determined using adsorbent-based sampling and GC/MS analysis to direct photoionization detection (PID), a general hydrocarbon measurement method. For evaporated gasoline, our target compounds collectively represent about 43% (benzene represented 2.5%) of the PID measurement (Batterman et al., 2005). Other analyses examining the composition of evaporated gasoline give somewhat different fractions, e.g., Watson et al. (2001) report on measurements that show benzene represents 1.2% and 1.3%, respectively, of evaporated and liquid gasoline in California. Scaling our measurements using the PID data, we estimate an adjusted TVOC concentration of 1.571.3 mg m
3. VOC concentrations among the garages varied tremendously (Table 4). A poorly ventilated (AER0.49 h
1) detached 2-car garage (#6) that contained only 1 car but many VOC sources (numerous paint cans and fuel containers) had the highest levels of many compounds, including benzene (159 mg m
3), toluene (72 mg m
3), 1,3,5-, 1,2,4- and 1,2,3-trimethylbenzene (38, 139, 33 mg m
3, respectively), 4ethyl toluene (131 mg m
3), naphthalene (34 mg m
3), and most other aromatics. Concentrations were lower in the other garages, but often still high, e.g., garage #11 had benzene and toluene levels of 83 and 410 mg m
3, respectively. Garages #6 and 11 also had the highest concentrations of n-alkanes. The lowest levels of aromatics were found in garage #13, e.g., benzene and toluene levels were only 1.4 and 17 mg m
3, respectively. While this moderately well ventilated (AER ¼ 1.56 h
1) 1-car garage was rather cluttered and a car was parked inside, it was unique among the tested garages that it was free of other gasoline-powered equipment (e.g., lawnmowers) and containers (for fuel, paint, solvents). This garage is significant as it demonstrates that low VOC concentrations can be achieved in residential garages. Analogous to indoor/outdoor concentration ratios, we calculated garage/outdoor (G/O) ratios.

ARTICLE IN PRESS S. Batterman et al. / Atmospheric Environment 40 (2006) 1828–1844

1837

Table 4 VOC concentrations and AERs for each garage in the study. Concentrations are average of 2 or 3 replicates Measurement and VOC

Concentration (mg m
3) 1,1,1-Trichloroethane Benzene Carbontetrachloride Trichloroethylene n-Heptane Methyl cyclohexane Toluene n-Octane Tetrachloroethene Ethylbenzene p-Xylene, m-xylene Styrene o-Xylene n-Nonane Isopropylbenzene (cumene) a-Pinene n-Propylbenzene 4-ethyl toluene 1,3,5-Trimethylbenzene 2-ethyl toluene 1,2,4-Trimethylbenzene n-Decane 1,2,3-trimethyl benzene p-Isopropyltoluene Limonene n-Butylbenzene n-Undecane Naphthalene n-Dodecane n-Tridecane n-Tetradecane n-Pentadecane n-Hexadecane n-Heptadecane AER (h
1)

Garage number 1G

2

3

4

5

6

4.34 19.01 1.51 o0.01 11.89 3.02 114.77 2.95 0.30 15.31 65.78 0.13 22.04 1.60 0.84 0.91 3.64 19.71 5.54 4.53 19.69 1.30 4.44 0.28 7.89 0.65 0.66 3.42 0.28 o0.01 o0.01 o0.01 o0.01 o0.01

0.45 22.38 1.44 o0.01 14.10 4.36 121.18 3.74 o0.01 18.50 77.65 2.83 24.90 6.69 0.91 0.86 4.82 26.11 7.24 5.50 26.98 11.29 5.95 1.04 14.35 1.11 8.19 9.48 0.79 0.62 0.41 o0.01 o0.01 o0.01

o0.02 20.54 0.96 o0.01 6.62 4.42 70.38 0.85 o0.01 5.26 19.18 0.05 6.62 0.84 0.37 0.64 2.02 10.47 2.67 2.32 9.67 0.82 2.18 0.11 0.20 0.25 0.31 2.02 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01

o0.02 20.03 0.59 o0.01 15.59 4.66 117.51 4.06 1.03 17.91 70.94 0.59 24.04 1.71 0.96 0.83 4.32 24.33 6.43 5.37 23.01 0.89 5.13 0.34 1.46 0.76 0.45 3.49 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01

o0.02 27.22 1.08 o0.01 12.34 4.07 351.88 5.15 0.11 40.16 159.50 0.26 57.54 4.47 3.29 0.85 14.61 76.10 21.85 18.40 75.10 4.70 19.66 1.00 16.88 2.47 1.90 12.25 0.31 o0.01 o0.01 o0.01 o0.01 o0.01

o0.02 0.96 4.96 159.35 23.96 29.38 1.31 2.07 2.56 o0.01 0.27 0.21 57.17 33.07 8.47 18.02 14.90 7.19 729.12 169.99 120.62 20.83 19.89 2.63 o0.01 1.57 o0.01 91.12 46.12 11.41 371.12 169.64 43.73 0.36 2.02 0.55 123.86 63.98 13.44 8.88 6.18 1.47 4.50 3.06 1.19 8.20 41.04 2.11 24.25 13.06 5.75 131.34 69.47 30.29 38.26 20.51 8.42 28.59 16.23 6.79 138.85 72.86 28.13 5.57 3.99 2.13 32.45 17.57 6.46 1.65 1.46 0.47 1.80 12.58 11.05 6.64 2.97 0.76 3.65 2.31 1.12 34.41 10.93 2.74 2.20 6.84 o0.01 1.78 17.20 o0.01 1.19 7.61 o0.01 0.41 1.31 o0.01 o0.01 0.45 o0.01 o0.01 0.14 o0.01

0.80

0.30

1.11

0.58

1.80

Based on average levels, G/O ratios were on the order of 100 (range 91–713) for most fuel-related aromatics, somewhat lower for limonene (92), alkanes (43–58), and pinene (24). G/O ratios will be high if sources are primarily indoors, if outdoor removal rates are rapid, indoor removal rates are slow, or if outdoor measurements fall near MDLs. With the exception of carbon tetrachloride (G/O ratio of 1.4), emission sources in the garage (and possibly in houses for tetrachloroethene) cause the high concentrations in garages. 3.7. VOC sources A classification of VOC sources can be made on the basis of correlations, source origins, and known source compositions. The dominant source included

7

0.49

8

0.73

0.38

9

10

11

12

13

14

15

o0.02 8.12 1.12 o0.01 3.36 1.02 121.58 1.16 o0.01 8.83 37.28 0.20 12.34 0.59 0.49 17.02 2.32 12.54 3.58 2.80 12.61 1.15 2.89 1.23 4.89 0.36 0.76 3.22 0.49 0.61 1.26 0.45 0.36 0.17

o0.02 28.45 1.07 o0.01 17.11 9.13 115.70 2.95 o0.01 5.97 23.06 0.08 7.68 1.54 0.56 1.71 2.82 14.49 4.54 3.39 14.74 1.92 3.56 0.40 4.96 0.39 0.97 6.93 0.60 0.32 3.00 0.63 0.22 0.13

o0.02 82.48 0.88 o0.01 44.37 11.34 409.52 12.91 o0.01 51.55 222.59 0.35 71.39 4.53 3.17 8.4 17.67 92.07 25.11 19.04 93.27 2.83 19.95 0.87 7.24 3.13 1.71 15.82 0.90 0.58 0.43 0.31 0.25 0.15

o0.02 39.31 1.00 o0.01 23.35 7.69 165.14 5.99 o0.01 24.50 105.96 0.11 33.48 2.70 1.13 10.69 6.42 36.39 10.31 7.32 35.80 1.54 7.86 0.32 1.22 1.00 0.71 6.34 0.35 o0.01 o0.01 o0.01 o0.01 o0.01

4.79 1.35 1.27 o0.01 1.62 0.33 16.81 0.90 0.81 3.97 14.96 0.19 4.31 6.27 0.32 1.79 0.79 4.15 1.90 1.61 5.22 14.98 2.42 0.81 2.77 0.27 13.63 0.53 2.30 o0.01 o0.01 o0.01 o0.01 o0.01

3.96 41.02 1.20 o0.01 19.89 6.03 274.97 5.98 o0.01 40.71 169.11 0.63 57.00 2.68 2.20 5.94 11.48 60.05 17.31 13.08 64.04 1.87 14.47 1.17 2.68 2.33 1.01 17.68 1.06 0.80 1.01 0.43 0.60 0.26

o0.02 26.36 0.87 o0.01 13.78 3.72 315.30 5.10 o0.01 39.08 159.16 0.39 46.96 4.39 1.61 10.94 7.02 40.08 12.06 8.61 40.29 3.75 8.83 1.02 7.11 0.95 2.64 4.85 1.61 0.73 0.60 0.39 0.60 0.11

0.33

1.54

0.83

0.42

1.56

0.16

0.58

fuel-related aromatics and C7–C9 n-alkanes, specifically, benzene, toluene, xylene, trimethylbenzene, ethylbenzene, isopropylbenzene, n-propylbenzene, n-butylbenzene, ethyltoluene, isopropyltoluene, methylcyclohexane, n-heptane, n-octane, n-nonane, and naphthalene. All of these compounds were highly correlated, and all are major components of gasoline. We attempted to separate gasoline sources using a chemical mass balance model apportionment for each garage with vehicle exhaust, evaporative emissions, and evaporated gasoline source profiles from a California study reported in Watson et al. (2001). Fig. 3 shows these profiles, along with the average concentration found across the 15 garages, scaled to the adjusted TVOC concentration. Evaporative emissions were a statistically significant source in 13 of 15 garages (all but #7 and #8). In

40 100 100 13 100 100 100 100 33 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 80 53 53 47 40 40

—

Total

635.1

1.31 36.60 1.26 0.04 18.85 6.66 214.30 6.34 0.26 28.03 113.98 0.58 37.97 3.64 1.64 6.80 8.07 43.17 12.38 9.57 44.02 3.92 10.25 0.81 6.47 1.60 2.67 8.94 1.19 1.51 1.04 0.27 0.17 0.07

Det.Fr (%) Mean (mg m
3)

Garage

1,1,1-Trichloroethane Benzene Carbontetrachloride Trichloroethylene n-Heptane Methyl cyclohexane Toluene n-Octane Tetrachloroethene Ethylbenzene p-Xylene, m-xylene Styrene o-Xylene n-Nonane Isopropylbenzene (cumene) a-Pinene n-Propylbenzene 4-Ethyl toluene 1,3,5-Trimethylbenzene 2-Ethyl toluene 1,2,4-Trimethylbenzene n-Decane 1,2,3-trimethyl benzene p-Isopropyltoluene Limonene n-Butylbenzene n-Undecane Naphthalene n-Dodecane n-Tridecane n-Tetradecane n-Pentadecane n-Hexadecane n-Heptadecane

VOC

507.8

2.03 38.51 0.50 0.08 15.33 4.94 180.30 6.41 0.48 23.74 96.96 0.79 32.65 2.51 1.30 10.65 6.75 35.85 10.29 7.84 37.61 4.07 8.75 0.47 5.20 1.71 3.62 8.72 1.73 4.37 1.99 0.36 0.23 0.08

StDev. (mg m
3)

111.1

o0.02 1.35 0.59 o0.01 1.62 0.33 16.81 0.85 o0.01 3.97 14.96 0.05 4.31 0.59 0.32 0.64 0.79 4.15 1.90 1.61 5.22 0.82 2.18 0.11 0.20 0.25 0.31 0.53 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01

o0.02 26.36 1.12 o0.01 14.10 4.66 121.58 4.06 o0.01 18.50 77.65 0.35 24.90 2.70 1.13 1.79 5.75 30.29 8.42 6.79 28.13 2.13 6.46 0.87 4.96 0.95 1.12 6.34 0.60 0.32 0.41 o0.01 o0.01 o0.01 423.9

Min. (mg m
3)

Median (mg m
3)

Table 5 Statistics of VOC concentrations in garages (n ¼ 15) and outdoors (n ¼ 8)

2046.9

4.96 159.35 2.56 0.27 57.17 18.02 729.12 20.83 1.57 91.12 371.12 2.83 123.86 8.88 4.50 41.04 24.25 131.34 38.26 28.59 138.85 14.98 32.45 1.65 16.88 6.64 13.63 34.41 6.84 17.20 7.61 1.31 0.60 0.26

Max. (mg m
3)

—

0 100 100 0 0 13 100 0 0 100 100 0 100 13 25 88 75 88 88 88 88 13 75 25 25 0 13 0 0 0 0 0 0 0 5.18

o0.02 0.40 0.92 o0.01 o0.15 0.04 1.24 o0.06 o0.01 0.21 0.69 o0.01 0.22 0.09 0.00 0.28 0.04 0.28 0.11 0.09 0.30 0.10 0.07 0.02 0.07 o0.01 0.05 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01

Det.Fr (%) Mean (mg m
3)

Outdoor

1.66

— 0.12 0.23 — — 0.02 0.45 — — 0.11 0.27 — 0.09 0.21 0.00 0.17 0.03 0.14 0.05 0.05 0.15 0.25 0.06 0.01 0.13 — 0.13 — — — — — — —

StDev. (mg m
3)

5.31

o0.02 0.36 0.91 o0.01 o0.15 o0.03 1.31 o0.06 o0.01 0.21 0.70 o0.01 0.23 o0.02 o0.00 0.30 0.05 0.31 0.12 0.09 0.31 o0.01 0.07 o0.01 o0.00 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01

Median (mg m
3)

2.44

o0.02 0.26 0.53 o0.01 o0.15 o0.03 0.53 o0.06 o0.01 0.02 0.22 o0.01 0.04 o0.02 o0.00 o0.00 o0.00 o0.00 o0.02 o0.00 o0.00 o0.01 o0.00 o0.01 o0.00 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01

Min. (mg m
3)

7.12

o0.02 0.59 1.37 o0.01 o0.15 0.08 1.80 o0.06 o0.01 0.37 1.05 o0.01 0.34 0.61 0.01 0.50 0.08 0.42 0.17 0.16 0.44 0.72 0.16 0.05 0.31 o0.01 0.37 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01 o0.01

Max. (mg m
3)

1838 S. Batterman et al. / Atmospheric Environment 40 (2006) 1828–1844

ARTICLE IN PRESS

ARTICLE IN PRESS S. Batterman et al. / Atmospheric Environment 40 (2006) 1828–1844

Evaporated

Liquid

2-Ethyl toluene

Observed

10

n-Undecane

n-Decane

n-Nonane

n-Octane

4-Ethyl toluene

1,2,3-TMB

1,3,5-TMB

o-Xylene

m,p-Xylene

Toluene

0.1

Ethylbenzene

1

Benzene

Fraction of Total (%)

Exhaust

1,2,4-TMB

100

Fig. 3. Source profiles for gasoline exhaust, evaporated gasoline, and liquid gasoline (from Watson et al., 2001), and observed average fractional composition of VOCs in garages.

contrast, exhaust emissions were statistically meaningful in only one garage (#12), as was liquid gasoline (#7). No source was statistically significant in garage #8, though evaporative emissions approached marginal significance (p ¼ 0:12). This analysis is preliminary since source profiles for Michigan fuels and other VOC sources are needed. Our previous analyses of Michigan gasoline showed fractional compositions that generally matched the California gasoline within 16% for the most of the major components (toluene, ethylbenzene, m,pxylene, o-xylene, 1,2,4- and 1,3,5-trimethylbenzene), however, benzene was found at about twice the California level. This mismatch is also indicated in Fig. 3. Despite the lack of site-specific source profiles, these results strongly suggest that evaporated gasoline contributes the bulk of VOC concentrations. The other n-alkanes found in the garages are constituents of many materials, e.g., gasoline, paints, adhesives, gasoline lubricating oils, and varnishes, thus identifications are tentative. The C10 and C11 compounds may be constituents of fast-drying paints, primers and sealers, along with a portion of toluene and other aromatics (Censullo et al., 1996 as reported in Watson et al., 2001). The C12–C15 compounds are ingredients in other paints, varnishes and caulks, while the C16 and C17 alkanes may be oils and lubricants, along with a variety of other hydrocarbons (e.g., other paraffins, olefins, aromatics). A subset of garages showed chlorinated compounds, including 1,1,1-trichloroethane, trichloroethylene, and tetrachloroethene, at low levels (up

1839

to 5 but generally p1 mg m
3). These VOCs were rarely detected outdoors. Tetrachloroethene and 1,1,1-trichloroethane are likely dry cleaning residues. The trichloroethylene source is unclear. Carbon tetrachloride was detected in all garages and also outdoors at similar concentrations (G/O ratio of 1.4). In the past, this gas was used in aerosol cans, pesticides, fire extinguishers and degreasers, and global atmospheric background levels are quite stable at 0.8 mg m
3 (ATSDR, 2004). a-Pinene and limonene were found in all garages. Pinene averaged 7 mg m
3 but reached 41 mg m
3 in garage #7. Limonene also averaged 7 mg m
3 and reached 18 mg m
3 in garage #5. Terpenes are widely used in scented deodorizers, cleaning products, tobacco, food, and flavor applications. Pinene is also emitted by wood-containing materials. These VOCs are also in deodorizers, including those often hung from the rear view mirrors in cars. 3.8. Factors affecting VOC concentrations VOC concentrations and AERs were inversely related, as expected, although the association was not strong, e.g., individual VOCs had (Spearman rank) correlation coefficients from
0.1 to
0.4. To explain a larger fraction of variability, multiple regression models were constructed using step-wise regression. A typical model for toluene is logðC tol Þ ¼ 2:23ð0:12Þ
0:35ð0:13ÞAER þ 0:50ð0:16ÞI ENCL þ 0:45ð0:15ÞI SPILL , R2 ¼ 0:67,

ð2Þ

where log(Ctol) is the logarithm of the toluene concentration (used since the distribution was highly skewed), AER the estimated AER in the garage (h
1), and IENCL and ISPILL are the indicator variables noting the presence in the garage of enclosed VOC sources and spills, respectively. Values in parentheses are the standard errors of the coefficients, all of which were significant (po0:05). Regression models fit to other compounds were often similar, although the selection, number and magnitude of the coefficients varied. The AER was included in all models, and its coefficient had a larger magnitude for other VOCs than toluene, e.g.,
0.7270.28 for n-undecane and
0.6870.19 for n-tridecane, again indicating the expected inverse relationship as well as the importance of AER.

ARTICLE IN PRESS 1840

S. Batterman et al. / Atmospheric Environment 40 (2006) 1828–1844

These models suggest the importance of VOCemitting materials in the garage along with the AER in determining VOC concentrations. Contrary to expectations, the number of vehicles stored in the garage, garage temperature, and meteorological variables were never significant predictors. The regression analysis is limited by the only modest predictive ability of the models (R2 from 0.5 to 0.8), and the need for better indicators of emissions and larger sample sizes.

4. Discussion 4.1. AERs in garages AER measurements are key parameters in understanding indoor air quality and emissions, and longterm estimates are necessary for many applications. Previous studies of AERs in garages are limited, especially compared to the extensive testing conducted in residences. Previously, Graham et al. (1999) conducted SF6 measurements in 4 attached garages at Canadian houses in the winter season, finding AERs ranged from 1.8 to 2.7 h
1. Fugler et al. (2002) used fan pressurization tests in 25 Canadian homes, and noted that AERs in garages were 10 times higher than that found in the houses. Also using fan pressurization tests at 5 houses in the Washington, DC area, Emmerich et al. (2003) found that garages were at least twice as leaky as houses, and suggested that findings of Fugler et al. (2002) were due to the much tighter houses in Canada (compared to US houses). Sherman and Chan (2004) review two studies in new and low-energy Belgian houses that show significant leakage through the garage–house interface, and also provide a comprehensive discussion of the many factors affecting airtightness. Most of the 15 garages tested were relatively tight. Given Michigan’s climate, most houses are well insulated and weatherized, e.g., house windows and passage doors usually have effective seals. Seals on garage doors are often much less effective, though most doors do have flexible seals on the bottom and sides. Temperatures during most of the study period were moderate, minimizing temperature differences that might increase ventilation. As suggested by Emmerich et al. (2003), the average AER found across the 15 garages, 0.7470.53, represents approximately twice that expected in area houses, based on earlier measurements of

seasonal average AERs in single and multifamily homes in the region just north of the study area, specifically, 0.3670.28, 0.4470.31, 0.8270.69, and 0.2570.15 h
1 in winter, spring, summer and fall, respectively (Murray and Burmaster, 1995). We did not directly measure the amount of time garage doors were open, which is probably the most significant influence on the AER. In addition, the sample size was likely too small and the garages too varied for secondary influences on AERs, such as temperature and wind speed, to achieve statistical significance in correlations or regression models. Residential studies have shown that opening windows can tremendously increase short-term AERs, e.g., Wallace et al. (2002) found a large (2–3 h
1) increase in a 3-story townhouse, that indoor–outdoor temperatures differences were important, and commensurate with our results, that wind speed and direction had minimal effects. 4.2. VOC levels and gasoline vapors Levels of gasoline-related VOCs found in the 15 garages greatly exceeded levels found in residences, based on a literature review of 50 IAQ studies (Brown, 2002), more recent tests in 10 houses by van Winkle and Scheff (2001), and many other studies. The garages showed the highest concentrations of VOCs we have detected in non-occupational or ambient environments, including short-term measurements at busy traffic intersections and on hightraffic roads (Batterman et al., 2002a, b). Outdoor concentrations were negligible for most compounds in the suburban and non-industrial neighborhoods tested. Concentrations were inversely related to AERs, as expected, however, concentrations appeared to depend mainly on the presence and strength of VOC sources. Benzene levels exceeded risk-based advisory or screening levels used in Michigan and elsewhere. A somewhat older study examining four garages (Thomas et al., 1993) found in one garage a benzene concentration of 196 mg m
3, surpassing the highest level found here, 160 mg m
3 (garage #6). However, concentrations in most garages fell in the range (13–45 mg m
3) given by US EPA (2000) for a 1 in 10,000 lifetime cancer risk, while the highest concentration measured fell into the 1 in 1000 risk category. These are high-risk levels that would normally require mitigation though, as stated at the onset, exposure durations in garages would be short for most individuals.

ARTICLE IN PRESS S. Batterman et al. / Atmospheric Environment 40 (2006) 1828–1844

4.3. Garages as VOC emission sources We estimated the source strength of garages by multiplying the air exchange flow rate by the TVOC concentrations. Using the benzene fraction of 2.5% (discussed previously) and averaging across the 15 garages, the TVOC emission rate averages 3.074.1 g day
1 (1.171.5 kg yr
1) per garage. If the garages measured were a representative sample, then TVOC emissions from the roughly 1 million homes with attached garages in the SE Michigan area would be 1,100,00071,500,000 kg yr
1. We can only speculate as to whether or not these garages are a representative sample. VOC emissions from residential garages may be compared to emission estimates from other residential sources. van Winkle and Scheff (2001) used the same approach to calculate emissions from 10 Chicago homes without attached garages. Emission rates of 0.3671.3 and 1.372.1 mg h
1 were estimated for benzene and toluene, respectively. In comparison, rates across the 15 garages for these two compounds were an order of magnitude higher, 3.174.3 and 19724 mg h
1. VOC emissions also may be compared to emissions from vehicles, the dominant urban source of VOCs. The estimated emissions from garages could easily result from vehicles undergoing hot soaks, evaporative losses, and diurnal breathing losses, especially if the car’s emission control system is defective. The VOC compositions, discussed earlier (see Fig. 3), indicate that it is primarily evaporative emissions that accumulate in garages, which is logical given that exhaust emissions that may enter the garage occur only briefly as vehicles enter or exit the garage, while evaporative emissions (and hot soaks) will occur continuously when the car is parked within the garage. US EPA’s diurnal breathing test allows 2 g of total hydrocarbon (THC) per car, which includes hot soaks and other losses. Hot soak emissions measured from 20 vehicles using the Sealed Housing for Evaporative Determination (SHED) method averaged 0.53 g for ‘‘normal’’ conditions, 1.58 g with the gas cap removed; and 3.29 g with the vehicle’s carbon canister hose removed (Murphy et al., 1997). Graham et al. (2004) measured higher emissions from an older (1993) vehicle, especially in the coldstart condition. We previously estimated that emissions for a modern car meeting recent US emission standards and driven 12,000 miles annually are about 18 g day
1 (Batterman et al., 2005),

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thus, for a 1 car family, garage emissions are equivalent to about 16% of a vehicle’s NMOC emissions. Because vehicles were parked in nearly all of the garages, it was impossible to separate automotive emissions from the evaporation of gasoline from other sources, e.g., lawnmowers and fuel containers. The average garage’s 3 g day
1 emission rate might be a result of 6 ‘‘normal’’ hot-soaks, though it seems unlikely that the frequency of hot-soaks was that high (the average number of cars was 1.47 car per garage, implying a perhaps unrealistically high 4 hot soaks per car), and that all of the emissions would be captured in the garages. Evidence showing emissions from non-vehicle sources was apparent, e.g., garages without cars had high concentrations, and VOC compositions indicated a number of species not associated with gasoline, from which non-gasoline source factors were identified. The role of vehicle contributions might best be identified by examining VOC levels with and without vehicles present in the same garages. 4.4. Method evaluation The use of miniature PFT sources and passive sampling for AER measurements, which was first described over two decades ago by Dietz and Cote (1982), has been proven to be an effective method in both laboratory and field studies (e.g., Leaderer et al., 1985). We have updated this method using a reliable PFT emission source and combined PFT/ VOC measurements. The new method is attractive for several reasons. First, it provides measurements over a long and potentially representative time periods. Second, PFT concentrations are conveniently obtained along with other VOCs, including many compounds that are toxic, ozone precursors, or themselves tracers of emission sources. Third, the passive sampling approach is far more convenient than active sampling. 4.5. Study limitations This study has several limitations. While perhaps the largest study of VOC levels in garages in the literature, the sample size is modest, limiting the analysis possibilities and the generalizability of results. We tested garages in a single season and did not examine seasonal and temperature differences that will affect vapor pressures, vehicle emissions, gasoline formulations, activity patterns,

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and house/garage stack effects. We tested garages in a single geographic area. We had no direct records regarding the opening and closing of exterior windows and doors, and had only visual evidence of emission sources in garages. As mentioned, the AERs incorporate flows into the garage from outside air, as usually considered for AERs, as well as flows from the attached house into the garage. While these flows are not separated, the house air entering the garage is likely to be a small fraction of the total exchange since both temperature and pressure gradients favor the opposite flow. Further work is necessary to quantify these flows. Finally, spatial and temporal variability was examined in a single garage. 4.6. Recommendations Residential garages have high concentrations of gasoline vapors due to the storage of vehicles, other gasoline-powered equipment, and gasoline containers. While there are often numerous sources of VOCs in indoor air, it is important to recognize that the migration of air from the garage into living spaces will transport potentially significant levels of pollutants. Mitigating measures include tightening the garage–house interface, ventilating the garage, maintaining a negative pressure gradient in the garage, and reducing emissions in the garage (eliminating or sealing sources, warming up and shutting down cars outside of the garage). Emissions to indoor and outdoor air from gasolinepowered equipment, fuel cans and other sources might be reduced using enclosures and simple activated carbon traps or separate exhausts. Steps to address problems of contaminant levels in residential garages and pollutant migration to living spaces for new construction have been formulated in ventilation standards for new construction (ASHRAE, 2003), and tests to verify their effectiveness are recommended. 5. Conclusions Using an enhanced perfluorocarbon tracer gas method that features miniature PFT sources and passive VOC/PFT sampling, long-term (4-day) AERs and VOC concentrations were measured in 15 garages that varied in type, size, use and other characteristics. AERs averaged 0.7770.51 h
1, and showed a large range. Differences between the effective AER, measured here, and the TWA AER

can be sizable if garage doors remain open for extended periods, based on a modeling analysis. Nearly all of the garages showed high VOC levels, largely due to the evaporation of gasoline. Additional emission sources identified included paints, solvents, and oils. By combining AER and concentration information, we estimated an average TVOC emission rate of 1.171.5 kg yr
1 per garage. The significance of garages as VOC emission sources that affect indoor and ambient air quality has not been adequately recognized. Acknowledgments The authors thank the participants for their cooperation in the study, and laboratory and field team. Chris Godwin assisted in the recruitment of participants and helped develop protocols. Sergei Chernyak assisted with VOC analyses. Scott Roberts helped with data entry and data analysis. Jaehwan Lee helped with the PFT calibrations. Portions of the study were financially supported by the American Chemistry Council (Grant 2401) and the Michigan Education and Research Center (funded by the National Institute of Occupational Safety and Health (Grant T42 CC5410428)). References Agency for Toxic Substances and Disease Registry (ATSDR), 2004. ToxFAQs for chloroform. http://www.atsdr.cdc.gov/ tfacts6.html. Accessed July 2005. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE), 2003. Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings. ASHRAE Standard 62, 2-2003. Batterman, S., Metts, T., Kalliokoski, P., Barnett, E., 2002a. Low-flow active and passive sampling of VOCs using thermal desorption tubes: theory and application at an offset printing facility. Journal of Environmental Monitoring 4, 361–370. Batterman, S.A., Peng, C.-Y., Braun, J., 2002b. Levels and composition of volatile organic compounds on commuting routes in Detroit, Michigan. Atmospheric Environment 36, 6015–6030. Batterman, S.A., Yungdae, Y., Jia, C., Godwin, C.C., 2005. Nonmethane hydrocarbon emissions from vehicle fuel caps. Atmospheric Environment 39, 1855–1867. Brown, S.K., 2002. Volatile organic pollutants in new and established buildings in Melbourne, Australia. Indoor Air 12 (1), 55–63. Cheong, K.W., Riffat, S.B., 1995. New approach for measuring airflows in buildings using a perfluorocarbon tracer. Applied Energy 51, 223–232. Cohen, M.A., Ryan, P.B., Yanagisawa, Y., Spengler, J.D., Ozkaynak, H., Epstein, P.S., 1989. Indoor/outdoor measurements of volatile organic compounds in the Kanawha Valley

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Further reading Graham, L., 1999. Characterizing the cold start exhaust and hot soak evaporative emissions of the test vehicle for the attached garage study. ERMD Report # 99-26768-1, Environment Canada.