Environ. Sci. Technol. 2003, 37, 2340-2349
Characterization of Polar Organic Compounds in the Organic Film on Indoor and Outdoor Glass Windows QIN-TAO LIU,† RACHEL CHEN,‡ BRIAN E. MCCARRY,‡ M I R I A M L . D I A M O N D , * ,† A N D BAGHER BAHAVAR† Environmental Chemistry Research Group, Department of Geography, University of Toronto, 100 St. George Street, Toronto, Ontario, Canada M5S 3G3, and Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1
Organic films on an impervious surface (window glass) were sampled at paired indoor-outdoor sites in July 2000 and characterized for their paraffinic and polar organic compositions along an urban-rural transect. Four classes of polar compounds (C11-C31 aliphatic monocarboxylic, C6-C14 dicarboxylic, nine aromatic polycarboxylic, and five terpenoid acids) constituted between 81 and 95% (w/w) of the total organic fraction analyzed comprising n-alkanes (C10-C36), 46 PAH, 97 PCBs, and 18 OC pesticides. Concentrations of the polar compounds plus their precursors, n-alkanes, ranged from 8 to 124 µg m-2 and were dominated by monocarboxylic acids (67-89%, w/w). On outdoor windows, n-alkanes, aromatic acids, and terpenoid acids decreased in concentration along the urban-rural transect. The carbon preference index values and the interpretations of individual compounds indicate that the main sources of n-alkanes were plant waxes followed by petrogenic sources; monocarboxylic and dicarboxylic acids were from plant waxes and animal fats. Results of principal component analysis showed closer correspondence between outdoor and indoor signatures than among locations. In outdoor films, these compounds are suggested to play an important role in mediating chemical fate in urban areas by air-film exchange and facilitating “washoff” due to their surfactant-like properties. In indoor films, these compounds provide a medium for the accumulation of more toxic compounds.
Introduction Organic films have been found to develop on the exterior of an impervious surface (window glass) with the masses, thicknesses, and concentrations of semivolatile organic compounds (SOCs) decreasing along an urban-rural transect (1, 2). The film is derived from atmospheric sources, notably primary emissions of organic compounds and their polar transformation products that condense because of their lower vapor pressures (3). Once the film develops, particles accumulate at a greater rate due to the film’s “greasy” nature, which increases the particle dry deposition flux (4). Gas* Corresponding author telephone: (416)978-1586; fax: (416)9465992; e-mail:
[email protected]. † University of Toronto. ‡ McMaster University. 2340
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phase compounds partition into the organic phase of the film analogously to gas-particle partitioning in the atmosphere (1). Urban impervious surfaces with thicker layers of organic films are found to accumulate more gas- and particlephase chemicals than rural impervious surfaces that have thinner films (2, 4). Thus, the film acts as a sink for a wide range of chemicals. This film reservoir is highly transient and facilitates volatilization of higher vapor pressure compounds because of its high surface area to volume ratio (e.g., 1.4 × 107 m2 m-3) (5). Compounds and particles with lower vapor pressure are washed off by precipitation, and in urban areas, the washoff and these chemicals (stormwater) are rapidly conveyed to surface waters (6). The importance of the film in the transport of organic contaminants was revealed in a simulated rain experiment in which 65-75% of SOCs analyzed were washed off windows independent of solubility (7). The primary motivation of the present study was to characterize the polar organic compounds that are hypothesized to facilitate the nondiscriminatory wash-off of SOCs. This work relates to previous comprehensive analyses of the nonpolar and polar organic compounds associated with urban air particles (8-20) and recent studies of both gasand particle-phase organics (21-23). These studies reported that aliphatic monocarboxylic, dicarboxylic and aromatic acids constituted approximately 80% of the identified organic species in organic extracts of air-particulate materials. The acidic compounds have significantly lower vapor pressures than primary nonpolar emissions such as alkanes and will condense to form secondary organic aerosols (24, 25). By analogy, these polar compounds are expected to condense to form organic films on impervious surfaces. The sources of these compounds include outdoor biogenic sources (plant waxes, vegetative detritus, etc.) and anthropogenic sources, such as vehicle emissions and wood smoke, together with indoor sources, such as meat cooking (26), natural gas home appliances (15), and cigarette smoke (16). A second motivation for this work was to illustrate the use of the film as an integrative sampling method for complex mixtures of atmospheric constituents. The film accumulates over time, and organic extracts have been shown to produce a range of toxicological outcomes in a dose-response fashion (1, 27). The window film can be conveniently sampled, and because it integrates the atmospheric signal over time, it avoids temporal variability inherent in air sampling methods. As such, we suggest that the film can be used as a passive sampling method for deducing atmospheric concentrations of persistent compounds outdoors and a wider range of compounds in indoor settings where degradation within the film is less likely and unobtrusive methods are desirable. Although not of great toxicological concern, the importance of polar compounds lies in their abundance and ability to mediate the fate of more toxic compounds by providing a medium for accumulation. In the outdoor environment, the film, with its polar compounds, facilitates removal of contaminants by precipitation (7). In the indoor environment, the film accumulates gas- and particle-phase compounds. This paper reports concentrations of 71 compounds in organic films on paired indoor and outdoor impervious surfaces along an urban-rural transect. A method consisting of dual-solvent extraction, derivatization, solid-phase extraction, and GC-MS analysis was developed for the analysis of these compounds. Principal component analysis (PCA) was used to assist with data interpretation. Sources of these compounds are discussed in consideration of location along the urban-rural transect and indoor versus outdoor activities. 10.1021/es020848i CCC: $25.00
2003 American Chemical Society Published on Web 05/02/2003
Experimental Methods Sample Collection. Paired indoor and outdoor samples were collected from untinted windows at five sites in the Greater Toronto region along an urban-rural transect in July 2000 (Table 1). The rural site, an office/laboratory building, is located in Egbert, approximately 80 km north of Toronto. The suburban site is located approximately 10 km from downtown Toronto and is an office building close to an arterial road with heavy vehicle traffic. The three urban sites sampled in downtown Toronto were within 2 km of each other, with the restaurant being located on an arterial road. The residential sample is a composite from five 100-year-old houses located on the same block, just west of the campus of the University of Toronto where the urban laboratory site was located. The rural site is in an agricultural area where the main crops are monocotyledonous plants such as corn and grasses, while the suburban and downtown sites are surrounded by grassed areas together with mainly deciduous trees. Vegetation coverages within a 1 km distance of the urban, suburban, and rural sites were approximately 3, 20, and 90%, respectively, based on a 1999 satellite image with a pixel resolution of 30 m × 30 m (Burchfield, unpublished data). Wind directions along the south-north, urban-rural transect were predominantly from the west (Environment Canada, unpublished data). Temperatures were approximately 25-30 °C indoors and outdoors. Cigarette smoking occurred only at the restaurant site. Food cooking occurred routinely at the indoor residential (homes) and restaurant sites, while food handling occurred at the indoor rural site. Research within the urban laboratory has involved lipids and fatty acids. Furthermore, the tops of laboratory benches were painted in 1997 with an epoxy paint that contained resins and a hardener. Cleaning and waxing activities, including the use of furniture polish, have been ongoing since the 1960s at this site. Films develop rapidly initially (on a time scale of hours to days) followed by a much slower accumulation rate over months (Bahavar, unpublished data). Samples in this study were taken from sites where films had accumulated for 3-5 months after being washed with Kimwipes (laboratory tissues) wetted with dichloromethane (DCM). Outdoor samples were obtained by scrubbing windows to within 10 cm of the window edge with DCM-wetted Kimwipes (1). Indoor sites were sampled with the same method but using 2-propanol instead of DCM-wetted Kimwipes. A test comparing the two solvents showed no difference in efficiency of removal (Tieu, unpublished data). Kimwipes were precleaned by soaking in HPLC-grade DCM for 2 min, followed by air-drying in a clean fume hood and storage in a DCMcleaned glass jar with Teflon-lined caps. Most samples constituted 5-10 m2 of the window area. The numbers of Kimwipes used were proportional to the amount of “dirt” present. Field blanks were prepared by waving 10 precleaned Kimwipes in the air at each sampling site until they were dry (about 10-20 s). All sampled Kimwipes were stored at -20 °C prior to extraction and analysis. The total mass of material collected from windows was determined gravimetrically. Kimwipes were weighed before and after sampling following equilibration for 24 h at room temperature and 75% relative humidity (RH). A sample equivalent to an area of 0.09 m2 was collected for total carbon analysis. Samples were taken by scrubbing windows with DCM pre-extracted glass fiber filters (Gelman). Total organic carbon (TOC) and elemental carbon were measured by combustion with a Perkin-Elmer model 240-XA elemental analyzer at the Freshwater Institute, Winnipeg, MB. Organic matter (OM) content was calculated by multiplying TOC by 1.5 to convert carbon mass into an average organic mass. Organic film (O-film) thickness was calculated from the mass of organic material, assuming a density of organic carbon
(0.826 g/cm3) identical to that of 1-octanol. The masses of extractable organic materials (EOM) were measured by weighing aliquots of the organic extracts (DCM, 100-200 µL; methanol, 400 µL) after solvent evaporation and drying over P2O5 in a desiccator for 16 h. Sample Extraction and Preparation. Kimwipes were extracted first with 180 mL of DCM overnight in a Soxhlet apparatus followed by extraction with 180 mL of methanol overnight. The DCM and methanol extracts were concentrated separately to about 1 mL at 23 °C under N2 gas using a Zymark Turbovap II concentrator and then passed separately through columns packed with 3-5 g of anhydrous Na2SO4. Each column was then washed three times with 1 mL of DCM or methanol, and each sample was made up to 10 mL with correspondent DCM or methanol in a volumetric flask. DCM extracts were separated into three aliquots (4, 4, and 2 mL). Methanol extracts were separated into two aliquots (5 and 5 mL). The volume of each aliquot was measured, and the extracts were stored in glass vials with Teflon-lined caps at -18 °C. The following procedure was based on a derivatization/ analysis method reported previously (11). An aliquot of extract (400 µL) was spiked with a chemical conversion standard (deuterated myristic acid, C13D27COOH, 100 µL, ∼15 ng µL-1) and a recovery standard (1-phenyldodecane, 100 µL, ∼15 ng µL-1) and then treated with a DCM solution containing freshly generated diazomethane (∼20-fold excess) prepared using Diazald (N-methyl-N-nitroso-p-toluenesulfonamide) in a MNNG diazomethane generator (Aldrich). The reaction mixture (typically ∼500 µL) was shielded from light for 0.5 h and then reduced in volume to ∼200 µL using a stream of dry nitrogen gas to remove excess diazomethane. The resulting solution was made up to approximately 2 mL with DCM containing 5% methanol (v/v) and loaded onto an alumina solid-phase extraction cartridge (Waters Sep-Pak Alumina N Classic) and eluted with 6 mL of DCM that was concentrated to about 200 µL and then solvent-exchanged with 200 µL of toluene. Prior to gas chromatography-mass spectrometry (GC-MS) analysis, a toluene solution containing four internal standards (acenaphthene-d10, pyrene-d10, perylene-d12, and dibenz[a,h]anthracene-d14, 100 µL, ∼10 ng µL-1 each) was added. GC-MS Analysis. GC-MS analysis was performed using a HP 5890 gas chromatograph equipped with a HP 5971A mass selective detector and a 50% phenyl methyl silicone column (DB-17ht, J&W Scientific, Folsom CA, 30 m × 0.25 mm i.d. × 0.15 µm film thickness). The temperature program started at 90 °C and increased at 8 °C min-1 to 300 °C with a hold at 300 °C for 5 min. Helium (VitalAire) was used as the carrier gas at a linear velocity of 37 cm s-1. A selected-ion monitoring (SIM) program was developed to quantify all target analytes. Two ions were selected from each analyte’s full-scan mass spectrum, one for quantitation and the other for confirmation. Electron impact (EI) ionization at 70 eV in the EI positive mode was used for all analytes. Quantitation was done using the four internal standards with corrections for the response factors of individual compounds relative to the internal standards. A mixture of all 71 target analytes (n-alkanes and methyl esters) and six recovery and internal standards was run each time analyses were performed. Quality Control. Two lab blanks without Kimwipes and three field blanks with pretreated Kimwipes were analyzed to evaluate sampling and laboratory processes. Lab blanks without Kimwipes had n-alkanes that were 24 ( 12% of the average sample values and less than 1% of the sample concentrations of the polar compounds. Field blanks showed consistent levels of monocarboxylic acids, which were 27 ( 17% of the average sample values. Octadecane (C10) and undecane (C11) were the most abundant n-alkanes found in VOL. 37, NO. 11, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Total Mass (mg m-2), Percent Organic Matter (OM %), Percent Extractable Organic Matter (EOM %), Organic Film (O-Film) Thickness (nm), and Total Compound Concentrations (ng m-2) on Indoor and Outdoor Impervious Surfaces ∑ massa (mg m-2)
site rural suburban urbanshomes urbansrestaurant urbanslab range of ratio urban/rural
outdoor indoor outdoor/indoor outdoor indoor outdoor/indoor outdoor indoor outdoor/indoor outdoor indoor outdoor/indoor outdoor indoor outdoor/indoor outdoor indoor
OM %b
EOM %c
O-film thicknessd
53.1 17.1 5.4 90.7 nmk
10.0 25 0.4 9.2 nmk
7.3 12.2 0.3 nmk nmk
6.42 5.18 2.2 10.1 nmk
93.1 26.7 3.5 92.1 nmk
14.9 16.2 0.9 8.33 nmk
10.2 15.3 0.7 nmk nmk
15.5 5.24 3.0 9.29 nmk
107 55.2 1.9 1.0-1.2 1.6-3.2
12.6 28.4 0.4 0.8-1.5 0.6-1.1
7.9 17.8 0.4
17.8 19.0 0.9 0.8-1.6 1.0-3.7
∑ n-alkanee (C10-36)
∑ monoacids f (C11-31)
∑ diacidsg (C6-14)
∑ arom acidsh (9)
∑ terpene acidsi (5)
monoacids (%, w/w) j
1690 1020 1.6 1490 1370 1.1 4250 4170 1.0 7890 2910 2.7 1230 5030 0.2 0.7-4.7 2.8-4.9
8570 12 950 0.7 26 600 6500 4.1 21 600 33 400 0.6 31 100 18 300 1.7 10 000 100 300 0.1 1.2-3.6 2.6-7.7
163 2420 0.1 1390 85.3 16.3 4150 4550 0.9 6170 1970 0.7 149 15700 0.01 0.9-26 1.9-6.5
16.8 562 0.04 343 143 2.4 632 854 0.7 929 303 3.1 299 1660 0.2 18-55 1.5-3.0
34.9 47.1 0.7 190 130 1.5 240 109 2.2 619 16.3 37.9 179 1050 0.01 3.5-12 1.6-750
81.7 76.2 88.6 78.9 70.0 77.5 66.6 77.8 84.4 81.1
a Total mass was obtained by weighing. Kimwipes were conditioned at 75% RH for 6 h at ambient temperature (18-25 °C) before weighing. The humidity was controlled by super-saturated NaCl solution in a closed environment. b Organic matter percentage (OM %) was calculated based on the measured TOC data (Bahavar, unpublished data). OM % ) ∑OM/∑mass × 100%. c EOM % ) ∑EOM/∑mass × 100%. ∑EOM (ng m-2) was the sum of the DCM and methanol extracts, weighed separately. d Organic film (O-film) thickness (nm) ) mass (mg m-2) × 109 (nm m-1)/[0.826 × 109 (mg m-3)] × OM %. The number 0.826 is density of 1-octanol (g cm-3). Rural and suburban indoor OM % was assumed as the same as outdoor OM %. Average of measured Toronto urban OM % (13.75%) was used for the calculation of all urban O-film thicknesses. e n-Alkanes included the C -C alkanes. f Monocarboxylic acids included the C -C acids. g Dicarboxylic acids include adipic (6DA), pimelic (7DA), suberic (8DA), azelaic (9DA), sebacic (10DA), undecanedioic (11DA), 11 36 11 31 dodecanedioic (12DA), tridecanedioic (13DA), and tetradecanedioic (14DA) diacids. h Nine aromatic acids include phthalic (ph), terephthalic (tph), isophthalic (iph), 4-methyl phthalic (c-ph), 1,2,4-benzenetricarboxylic (124BA), 1,2,3-benzenetricarboxylic (123BA), 1,3,5-benzenetricarboxylic (135BA), 1,2,4,5-benzenetetracarboxylic (1245BA), and 2,3-naphthalenedicarboxylic (23NA) acids. i Five terpenoid acids include pimaric (PA), sandaracopimaric (SPA), isopimaric (IPA), dehydroabietic (DHA), and 7-oxodehydroabietic (ODA) acids. j % monocarboxylic acids ) ∑monocarboxylic acids/∑chemicals in this table × 100%. k nm, not measured.
FIGURE 1. Concentrations of n-alkanes on paired indoor and outdoor windows at (a) rural, (b) suburbanslight industry, (c) urbanshomes, (d) urbansrestaurant, and (e) urbanslaboratory sites in and near Toronto, Canada. lab and field blanks (50-80% of the total n-alkanes). The source of C10 and C11 n-alkanes in the blanks was the alumina solid-phase extraction cartridge (Sep-Pak). The source of the monocarboxylic acids was unknown but may have been from the Kimwipes or the sampling or extraction phase. The pattern of monocarboxylic acids in the field blanks was similar to that in the samples with the exception of “indoor lab”. Results were corrected for concentrations in the field blanks by determining (i) the concentration of each compound in the field blanks per Kimwipe, (ii) the number of Kimwipes used to obtain each sample, (iii) a corresponding blank value for each compound in each sample by multiplying the blank value per Kimwipe with the number of Kimwipes used for that sample, and (iv) the difference between sample and field blank concentrations. Concentrations that were less
than 50% of the total sample signal were reported as below detection. Compounds that were not detected in samples were assigned by 0.5 method detection limit (MDL). Recoveries for the methylation-cleanup-analysis procedure were 106 ( 5% for the chemical conversion standard (deuterated myristic acid) and 98 ( 4% for the recovery standard (1-phenyldodecane) in 33 samples. Analyte concentrations were not corrected for recoveries. Statistical Analysis. PCA, using SPSS Version 10.0 (Chicago, IL) was used to investigate factors related to polar compound concentrations in paired indoor-outdoor organic films at five sites (10 samples). Compounds that were detected in fewer than eight samples were treated as missing data and were excluded from the PCA. Compounds not detected in one or two samples were assigned values of 0.5 MDL. Since VOL. 37, NO. 11, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Concentrations of monocarboxylic acids on paired indoor and outdoor windows at (a) rural, (b) suburbanslight industry, (c) urbanshomes, (d) urbansrestaurant, and (e) urbanslaboratory sites in and near Toronto, Canada. the concentrations of compounds in the data set ranged over 4 orders of magnitude, two analyses were conducted: one with data normalized to the most abundant compound in a compound class and another on non-normalized data. Component loadings and score matrixes obtained from normalized and non-normalized data were very similar when PCA was performed on correlation matrixes. Therefore, PCA was performed with non-normalized data with correlated matrixes followed by orthogonal varimax rotation (28, 29).
Results and Discussion Film Masses and Thicknesses. Film masses decreased from urban to rural locations, consistent with previous results (2, 4) (Table 1). Film masses at the three urban sites were within 2344
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the range observed at nine urban sites sampled in Toronto in 1998 and 2001 (7-520 mg m-2) and were very close to the 1998 urban geometric mean (100 mg m-2). Film masses were consistently greater on outdoor windows as compared to indoor windows (Table 1), which we attribute to greater particle accumulation on outdoor surfaces. Conversely, the percentage of organic matter was greater on indoor windows than on outdoor windows, again suggesting that outdoor surfaces had a greater accumulation of inorganic material. The effective thicknesses of the organic layers were calculated from total film masses and measured percentages of organic material. These calculated thicknesses were less than those reported previously, which was due to the use of
FIGURE 3. Concentrations of dicarboxylic, aromatic, and resin acids on paired indoor and outdoor windows at (a) rural, (b) suburbanslight industry, (c) urbanshomes, (d) urbansrestaurant, and (e) urbanslaboratory sites in and near Toronto, Canada. Aromatic acids include 1,2-benzenedicarboxylic (phthalic, ph), 1,3-benzenedicarboxylic (isophthalic, iph), 1,4-benzenedicarboxylic (terephthalic, tph), 4-methyl1,2-benzenedicarboxylic (4-methylphthalic, c-ph), 1,2,4-benzenetricarboxylic (124BA), 1,2,3-benzenetricarboxylic (123BA), 1,3,5-benzenetricarboxylic (135BA), 1,2,4,5-benzenetetracarboxylic (1245BA), and 2,3-naphthalenedicarboxylic (23NA) acids. Resin acids include pimaric (PA), sandaracopimaric (SPA), isopimaric (IPA), dehydroabietic (DHA), and 7-oxo-dehydroabietic (ODA) acids.
measured rather than estimated organic material content (2). Chemical Composition. EOM at three rural and urban sites comprised 4-18% of the total film mass (Table 1), which is within the range observed for air particles (20; Harrad, unpublished data). The compounds reported here constituted 96.9-99.9% of the total organic compounds analyzed that included 46 PAH, 97 PCB congeners, and 18 organochlorine pesticides. Monocarboxylic acids (C11-C31, 67-89%) were the most abundant compound class, followed by n-alkanes (C10-C36, 4.1-17%), dicarboxylic acids (C6-C14, 1.3-14%),
nine aromatic acids (0.2-3.3%), and five terpenoid acids (0.11.5%) (Table 1). In comparison, Rogge et al. (11) analyzed 83 compounds extracted from air particles in Los Angeles in 1982; many compounds are the same as those analyzed here with the inclusion of one alkenoic acid (linoleic), one alkanal (C9), 14 PAHs, three polycyclic aromatic ketones and quinines, and four N-containing compounds. They reported the contribution of monocarboxylic acids (C9-C30) as 40-41%, dicarboxylic acids (C3-C9) as 29-31%, seven aromatic acids as 14-15%, n-alkanes (C23-C34) as 8-9%, and seven terpenoid acids as 4-5% of all analyzed compounds in air particles VOL. 37, NO. 11, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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from downtown and western Los Angeles. In this work, dicarboxylic acids comprised a lower percentage of the total because, in part, we did not analyze the C3-C5 dicarboxylic acids; however, the major differences between film compositions in Toronto in 2000 and air particle composition in Los Angeles in 1982 are more likely to be related to differences in sources, atmospheric chemistry, and vegetation. The relatively flat baseline in the chromatogram was free of the unresolved complex mixture (UCM) found in some of our DCM extracts and in typical urban samples (Figure 1 in Supporting Information and, e.g., ref 30). The dual extraction/ analysis approach allowed us to achieve better separations and detection limits for a number of acid esters, particularly those of the aliphatic dicarboxylic and aromatic acids. Detailed concentrations and patterns of each compound class are given in Figures 1-3. n-Alkanes. n-Alkanes were included because these compounds undergo atmospheric oxidation to form monocarboxylic acids and, to a much lesser extent, long-chain esters (11, 20). Concentrations of total n-alkanes on outdoor windows were lower at the rural site (1.7 µg m-2) and generally higher at the urban sites (1.2-7.9 µg m-2) (Table 1). The biogenic source of n-alkanes, in particular epicuticular waxes of vascular plants, was indicated by the dominance of oddcarbon-numbered compounds from C25 to C36, specifically C31 and C27 (Figure 1). This pattern is consistent with previous reports for organic films on outdoor window films (1, 2) and in urban and rural aerosols (30-32). A petrogenic signal from gasoline and diesel fuel combustion (Cmax ) C25 and C20) underlies the biogenic signal (e.g., ref 12). n-Alkane concentrations in indoor films were generally lower than in outdoor films, suggesting the importance of outdoor sources, but the concentrations also depended on building use. Indoor n-alkane concentrations were the lowest at the suburban site where the abundance pattern was similar to that in the outdoor films. The low concentrations are consistent with high ventilation rates in the building and its use as an office with few internal biogenic or petrogenic sources of n-alkanes. In comparison, the indoor restaurant and laboratory window films had higher concentrations than the paired outdoor samples. Indoor films from the laboratory had an odd-numbered preference that is consistent with the source being related to activities involving biogenic lipids and, notably, the practice of waxing the laboratory benchtops. On the other hand, the restaurant profile showed no carbon preference, which suggests sources such as meat cooking (Cmax ) C21) (26, 33), the use of natural gas appliances (Cmax ) C21, C22, and C23) (15), and cigarette smoke (Cmax ) C29) (16). Carbon preference index (CPI) values of n-alkanes at all sites were calculated as the ratio of ∑C11-31/∑C12-32 in order to assess the dominance of biogenic versus anthropogenic sources (Figure 2 in Supporting Information). Petroleum containing n-alkanes without carbon number preference yields a CPI of approximately 1, whereas the CPI for biogenically derived alkanes is considerably greater than 1 (34). In this study, the rural site had the highest CPI (11.5, outdoor; 6.5, indoor), followed by the suburban and urban sites (CPI ) 1.1-3.6), confirming the predominantly biogenic sources of these compounds, even at the urban sites. The CPI values at the rural and urban sites are comparable with the aerosol data from western rural sites (1.8-9.7) and urban sites (1.8-2.8) in the United States (30) and Britain (Harrad, unpublished data; 35). CPI values for outdoor films were higher than their paired indoor samples, indicating greater biogenic contributions to the outdoor window films. Aliphatic Monocarboxylic Acids. Concentrations of monocarboxylic acids did not vary consistently along the urbanrural transect (Table 1). Total concentrations and compound profiles were similar outdoors and indoors with the exception 2346
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FIGURE 4. Principal component analysis of n-alkane patterns (component score plots). (a) ∑ n-alkanes vs component 1 and (b) ∑ n-alkanes vs component 2. Sites: 1, rural outdoor; 2, rural indoor; 3, suburban outdoor; 4, suburban indoor; 5, urban homes outdoor; 6, urban homes indoor; 7, urban restaurant outdoor; 8, urban restaurant indoor; 9, urban laboratory outdoor; and 10, urban laboratory indoor (not shown in the plots).
of the laboratory site that had five times higher concentrations in indoor than outdoor films. Similarly to n-alkanes, the source of monocarboxylic acids at the laboratory site may be related to activities involving biogenic lipids and waxing the lab benches. All sites had a bimodal distribution of monocarboxylic acids, with maxima at C16 (palmitic acid) and C24 (lignoceric acid), and the predominance of even- over odd-numbered monocarboxylic acids (Figure 2). Stearic (C18) and behenic (C22) acids were also abundant in outdoor and indoor films. The bimodal distribution and the dominance of even-carbonnumbered acids are similar to that reported for urban air particles (11). CPI values for all sites, calculated as ∑C12-30/ ∑C11-31, were greater than 3.0 (Figure 2 in Supporting Information). These values are comparable to the CPI values of n-alkanoic acids in air particles from urban, rural, and tropical locations and suggest that the compounds are of biogenic origin (30, 36). Monocarboxylic acids with chain lengths less than 20 carbons are ubiquitous in plant and animal lipids, bacterial, and some algal detritus, while longer chain fatty acids are indicative of primary emissions or secondary reaction products of plant waxes (30). Concentrations of C24+ evenchain monocarboxylic acids are also found in green and dead leaves (14). Plant and animal lipids usually have a maximum at C16 (37). These sources are consistent with food preparation and consumption at indoor rural, residential, and restaurant sites.
FIGURE 5. Principal component analysis (∑ compound concentrations vs component score plots) of (a) monocarboxylic acids, (b) dicarboxylic acids, (c) aromatic acids, and (d) terpenoid acids. Sites: 1, rural outdoor; 2, rural indoor; 3, suburban outdoor; 4, suburban indoor; 5, urban homes outdoor; 6, urban homes indoor; 7, urban restaurant outdoor; 8, urban restaurant indoor; 9, urban laboratory outdoor; and 10, urban laboratory indoor (not shown in the plots).
In addition to biogenic sources, outdoor anthropogenic sources of monocarboxylic acids are fuel oil combustion (Cmax ) C16) (18), tire debris (Cmax ) C16, C18) (13), and hot asphalt roofing tar pot fumes (Cmax ) C16, C10) (17). Indoor sources are natural gas appliances (Cmax ) C9, C16) (15), meat cooking (Cmax ) C16, C18) (26, 33), and wood combustion (Cmax ) C16, C24) (23, 38). Aliphatic Dicarboxylic Acids. Similar to the monocarboxylic acids, dicarboxylic acids did not vary consistently along the urban-rural transect (Table 1). Concentrations of dicarboxylic acids were higher indoors than outdoors with the exception of the suburban office site. The most abundant dicarboxylic acid measured at all sites was azelaic acid (C9, Figure 3). The indoor laboratory site was anomalous for its very high concentrations of adipic (C6) and suberic (C8) acids, the reasons for which are not known. The predominance of azelaic acid (C9) and the pattern of other dicarboxylic acids (C6-C14) were consistent with the analyses of urban air particles (8, 11). One of the primary sources of azelaic acid is the oxidation of unsaturated acids, such as oleic (C18:1) and linoleic (C18:2) acids. These unsaturated acids are abundant in plant, microbial, and animal lipids (30, 39, 40). Gasoline and especially diesel exhaust emissions may be sources of short-chain dicarboxylic acids (41). These dicarboxylic acids, such as adipic acid (C6), may also be secondary reaction products as they have been found in secondary aerosols (11, 42). Long-chain (C10-C14) dicarboxylic acids are hypothesized to originate directly from biogenic
lipid residues and from the degradation of ω-hydroxy fatty acids found in vascular plant waxes (11). Finally, the pyrolysis of plants, trees, and organic soil constituents produces C4C9 dicarboxylic acids (43). While these sources may explain observed outdoor patterns, the greater accumulation of dicarboxylic acids in indoor rather than outdoor window films suggests indoor sources such as cooking (26, 33). Aromatic Carboxylic Acids. Total aromatic acid concentrations decreased along the urban-rural transect (Table 1), especially in outdoor films (Figure 3). Similar to the monocarboxylic and dicarboxylic acids, concentrations were higher on indoor than outdoor windows for the rural and three urban sites (Figure 3), suggesting the importance of indoor sources. Nine aromatic acids were quantified: two acids, 1,2,3-benzenetricarboxylic acid (123BA) and 2,3-naphthalenedicarboxylic acid (23NA), are the first to be measured in environmental studies. It is unclear whether 123BA and 1245BA are emitted from biogenic or anthropogenic sources. Phthalic (1,2-benzenedicarboxylic) acid was the most abundant aromatic acid in both indoor and outdoor films, particularly at urban sites (Figure 3), which is not surprising considering its widespread occurrence in the environment (44). Rogge et al. (11) have suggested that phthalic esters, which are widely used as plasticizers, are precursors of phthalic acid. Other anthropogenic sources of phthalic acid are automobile exhaust emissions (41, 45), tobacco smoke (46), and various PAH compounds via oxidative and photodegradative pathways (47-50). VOL. 37, NO. 11, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Terpenoid Acids. Similar to the n-alkanes and aromatic acids, concentrations of terpenoid acids decreased along the urban-rural transect (Table 1). The concentrations of terpenoid acids at all sites (except the urban lab) were higher outdoors than indoors (Figure 3). The high terpenoid acid concentrations inside the lab, especially for isopimaric (IPA), dehydroabietic (DHA), and 7-oxo-dehydroabietic (ODA) acids, may be related to the past practice of painting and waxing the laboratory benchtops (Figure 3). Dehydroabietic and 7-oxodehydroabietic acids were the most abundant terpenoid acids at all sites (Figure 3), similar to the pattern found in urban Los Angeles aerosols (9, 11, 30). Terpenoid acids are released mainly by conifers (51) and from pine wood combustion (23, 38, 51). These sources are consistent with concentrations being higher outdoors than indoors. PCA of Profiles. PCA was performed on the complete data set and yielded three components that accounted for 88% of the total variance in the compound profiles; component loadings above 0.5 were considered significant (Table 2 in Supporting Information). Component 1 accounted for 64% of the variance and was related to long-chain evennumbered n-alkanes (C28, C30, C32, C34), most monocarboxylic acids (C12-C29), all seven dicarboxylic acids, the four benzenedicarboxylic acids, and two major terpenoid acids (IPA and ODA). Component 2 accounted for 14% of the variance and had contributions from odd-chain n-alkanes (C21, C27, C29, C31, C33), four aromatic acids, and one terpenoid acid. This component may be related to plant waxes. Component 3 accounted for 10% of the variance and had high loadings for C21-C26 n-alkanes and three odd-chain monocarboxylic acids (C15, C19, C21). This component may be associated with petroleum emissions of lower molecular weight n-alkanes. The PCA was used to explore factors affecting organic component profiles. Total n-alkane concentrations were plotted against extracted scores of components 1 and 2, and ellipses were drawn around groups of sites (Figure 4). The groupings of sites in the two plots are similar. Indoor and outdoor organic films at rural and suburban sites were easily grouped together (Figure 4 a,b), indicating the resemblance of the sites and the close correspondence between indoor and outdoor films. Urban homes indoor and outdoor were in another group because there were higher n-alkane concentrations at this site than at rural and suburban sites. The indoor laboratory was separated by PC1, and the outdoor restaurant was separated by PC2. There was no correspondence between indoor and outdoor patterns at these sites due to the large differences of their component scores (Figure 4 a,b) and concentrations (Figure 1d,e). These differences are presumably related to the significant emissions indoors for the laboratory and outdoors for the restaurant. Plots of PC1 against the total monocarboxylic, dicarboxylic, and aromatic acid concentrations consistently separated sites into three groups: the restaurant outdoorsindoors, the urban homes outdoors-indoors, and the remaining sites (rural and suburban outdoors-indoors and the laboratory outdoors) (Figure 5). The indoor laboratory site was an outlier in all cases because of its high values and unusual patterns and is well off-scale in these plots. Overall the PCA results suggest that indoor and outdoor profiles at one site are more similar than all the outdoor or all indoor sites. Thus the sources that dominate at a site, either indoor or outdoor, have the greatest influence on film composition. These results support observations of the influence of outdoor sources on indoor air composition (52).
Discussion The compounds reported here are of little toxicological concern (27). However, the relevance of these compounds, and in particular the monocarboxylic acids that contributed 2348
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the greatest mass to the films, lies in their ability to provide a medium into which more toxic and/or persistent compounds can partition and/or adhere. Their abundance in the film is consistent with the hypothesis of film development in which atmospheric compounds with lower vapor pressures first condense on surfaces followed by particle accumulation and gas-film partitioning. In the outdoor environment, the film is presumed to coat all impervious surfaces and act as a transient sink that influences air concentrations through air-film exchange processes (6). Outdoors, the film affects surface water concentrations by first accumulating atmospherically deposited chemicals beyond that expected in the absence of the film (4) and, second, by conveying them to surface waters via precipitation (i.e., stormwater). Here, the polar compounds act as surfactants that solubilize the nonpolar constituents in the film and facilitate wash-off. Thus, the high proportion of polar compounds in the film explains the nondiscriminatory wash-off of nonpolar compounds in a simulated rain event (7). In the indoor environment, the accumulation of surface films, which are presumed to develop on nonglass surfaces as well, is suggested to be important in terms of indoor air quality, again mediated by air-film exchange processes. Film profiles provide evidence of the importance of outdoor sources to indoor films and, by extension, indoor air composition. The film provides a route of exposure to more toxic compounds, such as PAH and PCBs, that accumulate in the predominantly carboxylic acid matrix. The exposure can occur through hand-to-mouth activities, especially for infants and toddlers that are in close and frequent contact with surfaces (53). This is particularly important indoors where many compounds have lower degradation rates than outdoors (e.g., through breakdown due to photolysis and removal by precipitation) (54).
Acknowledgments The Toxic Substances Research Initiative of Health Canada and Environment Canada provided funding (Project 227). J. Truong, T. Tieu, and D. Lapierre assisted with sample preparation. S. Harrad (Birmingham University, U.K.), M. Burchfield, and B. Bahavar provided unpublished data. M. Stainton (Freshwater Institute, Winnipeg) was responsible for the TOC analyses.
Supporting Information Available One table and two figures. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review July 25, 2002. Revised manuscript received March 17, 2003. Accepted March 25, 2003. ES020848I
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