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Cenacolo env sci tech

  1. 1. ARTICLE pubs.acs.org/estChemical Characterization and Source Apportionment of Fine andCoarse Particulate Matter Inside the Refectory of Santa Maria DelleGrazie Church, Home of Leonardo Da Vinci’s “Last Supper”Nancy Daher,† Ario Ruprecht,‡ Giovanni Invernizzi,‡ Cinzia De Marco,‡ Justin Miller-Schulze,§Jong Bae Heo,§ Martin M. Shafer,§ James J. Schauer,§ and Constantinos Sioutas†,*† Department of Civil and Environmental Engineering, University of Southern California, Los Angeles, California, United States‡ LARS Laboratorio di Ricerca Ambientale SIMG/ISDE, Milan, Italy§ Environmental Chemistry and Technology Program, University of Wisconsin-Madison, Madison, Wisconsin, United Statesb Supporting InformationS ABSTRACT: The association between exposure to indoor particulate matter (PM) and damage to cultural assets has been of primary relevance to museum conservators. PM-induced damage to the “Last Supper” painting, one of Leonardo da Vinci’s most famous artworks, has been a major concern, given the location of this masterpiece inside a refectory in the city center of Milan, one of Europe’s most polluted cities. To assess this risk, a one-year sampling campaign was conducted at indoor and out- door sites of the painting’s location, where time-integrated fine and coarse PM (PM2.5 and PM2.5À10) samples were simulta- neously collected. Findings showed that PM2.5 and PM2.5À10 concentrations were reduced indoors by 88 and 94% on a yearly average basis, respectively. This large reduction is mainly attrib- uted to the efficacy of the deployed ventilation system in removing particles. Furthermore, PM2.5 dominated indoor particle levels, with organic matter as the most abundant species. Next, the chemical mass balance model was applied to apportion primary and secondary sources to monthly indoor fine organic carbon (OC) and PM mass. Results revealed that gasoline vehicles, urban soil, and wood-smoke only contributed to an annual average of 11.2 ( 3.7% of OC mass. Tracers for these major sources had minimal infiltration factors. On the other hand, fatty acids and squalane had high indoor-to-outdoor concentration ratios with fatty acids showing a good correlation with indoor OC, implying a common indoor source.1. INTRODUCTION outdoor sources.7,8 An accurate characterization of airborne Damage to cultural assets has been of growing interest to PM in museums is therefore essential for conserving the ex-museum conservators and curators. There is mounting evidence hibited artifacts.correlating indoor air pollution, biological contamination, mass An emerging concern is with PM-induced damage to thetourism, and variability in microclimate conditions with material “Last Supper” painting, one of Leonardo da Vinci’s mostdeterioration.1,2 A major concern is damage by particulate mat- famous artworks, located in the refectory of Santa Maria delleter (PM) to masterworks of art displayed in museums. Poten- Grazie Church in Milan, Italy. Although this painting hastial hazards include “soiling” (perceptible degradation of visual survived many challenges, including bombing during Worldqualities) due to deposition of airborne particles, particularly War II, it is yet facing another challenge. The “Last Supper”elemental carbon, and soil dust.3 Further damage can be induced painting, which was majorly restored in the 20th century, is atby chemically reactive species, such as ammonium sulfate and risk with air pollution arising from its surrounding Milan area.organic acids.2,4 Milan is one of the most polluted areas in Western Europe9 with Typically, indoor PM consists of outdoor-infiltrating and PM10 air quality standards frequently exceeded.10 In an attemptindoor-emitted particles in addition to indoor-formed particles to protect the painting, a sophisticated heating, ventilation, andthrough reactions of gas-phase precursors emitted both indoorsand outdoors.5,6 Moreover, the level and composition of indoor Received: August 5, 2011PM are governed by a myriad of factors. These mainly consist of Accepted: November 9, 2011the ventilation system, filtration effect of the building envelope, Revised: October 31, 2011deposition rate of particles as well as the intensity of indoor and r XXXX American Chemical Society A dx.doi.org/10.1021/es202736a | Environ. Sci. Technol. XXXX, XXX, 000–000
  2. 2. Environmental Science & Technology ARTICLEair conditioning (HVAC) system equipped with particle filtra- to maintain constant conditions. Large variations in the thermo-tion has been installed. hygrometric factors in the refectory, enhanced by the presence of To assess the effectiveness of this control measure, we con- visitors, may lead to an increase in the deposition rate ofducted a one-year sampling campaign at indoor and outdoor airborne pollutants on the painting, as previously demonstratedsites of the refectory. At both locations, fine and coarse PM by Camuffo and Bernardi.12 The temperature of the backside(PM2.5 and PM2.5À10, respectively) samples were simultaneously room is also maintained at about 2 °C higher than the refectory’scollected then analyzed for their chemical properties. In the temperature to avoid PM deposition on the painting due topresent article, the indoor-to-outdoor relationship of key tracers thermophoresis effects.3,13 Finally, it should be noted that theof PM sources is investigated in order to evaluate the impact of HVAC system failed for few days during one week of April.indoor and outdoor sources on indoor particle levels. Further- PM concentration substantially increased during that week,more, the chemical mass balance model is applied to identify and compared to the remaining weeks with noninterrupted systemestimate sources contributions to indoor PM2.5 concentration. functioning. This effect was more noticeable for PM2.5À10, forResults of this study provide a quantitative understanding on which a nearly 9-fold increase was observed, as shown in SIthe composition, origin and level of PM inside the refectory. Figure S3. This occurrence, however, has minor impacts on theUltimately, these findings can be used as guidelines for the results where monthly averages are reported throughoutimplementation of additional, and particularly source-specific, this study.control strategies to mitigate the concentration of particle 2.2. Chemical Analyses. To conduct the chemical analyses,components potentially detrimental to the “Last Supper” paint- the Teflon and quartz filters were sectioned into portions. Theing. They can also be used as a benchmark in future studies aimed fractions used for elemental and organic carbon (EC and OC,at protecting indoor artworks and antiquities. respectively) quantification were grouped into weekly samples and quantified using the NIOSH thermal optical transmission2. MATERIALS AND METHODS method.14 All remaining fractions, with the exception of few PM2.5À10 sections, were composited monthly. Given their low 2.1. Sampling Description. To characterize PM inside the mass loading, February/March, April/May, and October/No-refectory, PM2.5 and PM2.5À10 were simultaneously sampled at vember coarse samples were composited bimonthly. Theseindoor and outdoor sites of the refectory. The sampling cam- monthly and bimonthly fractions were analyzed for water-solublepaign lasted from December 2009 to November 2010. During OC (WSOC) and ions using a Sievers 900 Total Organic Carbonthis period, 24-hour size-segregated PM samples were collected Analyzer15 and ion chromatography, respectively. Total elementalon a weekly basis by means of two sets of Sioutas personal content of these composites was also measured using highcascade impactor samplers (Sioutas PCIS, SKC Inc., Eighty Four, resolution magnetic sector inductively coupled plasma massPA11). Every set consisted of two collocated PCIS loaded with spectrometry (Thermo-Finnigan Element 2).16 Additionally,37 and 25 mm filters for fine and coarse PM analyses, respectively. organic speciation was conducted on PM2.5 filter sections usingEach of the PCIS was placed at the indoor or outdoor site and gas chromatography mass spectrometry (GC-6980, quadrupoleoperated at a flow rate of 9 lpm. For the purpose of chemical MS-5973, Agilent Technologies). PM2.5À10 lacked sufficientanalysis, one set of the PCIS was loaded with Teflon filters (Pall mass for this analysis. Details of these analyses are provided inLife Sciences, Ann Arbor, MI), whereas the other one was loaded the SI.with quartz microfiber filters (Whatman International Ltd., 2.3. Source Apportionment. A molecular marker chemicalMaidstone, England). PM mass concentration was determined mass balance model (MM-CMB) that was mathematically solvedfrom the mass loadings of the weekly Teflon filters as described in with the U.S. Environmental Protection Agency CMB (EPA-the Supporting Information (SI). CMB8.2) software was used to estimate primary and secondary The indoor sampling location was inside the refectory of Santa source contributions to indoor fine OC on a monthly basis. TheMaria delle Grazie Church, where da Vinci painted the “Last effective variance weighted least-squares algorithm was appliedSupper” on one of its walls. Samples were collected at approxi- to apportion the receptor data to the source profiles.17mately 1 m directly below the painting and a few centimeters MM-CMB was conducted using primary molecular sourcefrom the wall surface. The site is equipped with a newly deployed tracers that were quantified in the PM2.5 samples. Markers thatHVAC system, supplying 4000 m3/h total air flow, of which are chemically stable and secondary organic aerosol (SOA)2000 m3/h are external fresh air. This system is operated continu- tracers that are unique to their precursor gases were selected asously. The air-flow rate inside the refectory, whose volume is 3130 m3, fitting species.18 These included levoglucosan, αββ-20R&SÀis 3000 m3/h, resulting in an air exchange rate of roughly 1 hÀ1. C27-cholestane, αββ-20R&S-C29-sitostane, 17α(H)-22,29,30-This relatively low air change rate helps avoid convective air trisnorhopane, 17α(H)-21β(H)-hopane, 17β(H)-21α(H)-30-velocities on the painting to the degree possible. The remaining norhopane, benzo(b)fluoranthene, benzo(k)fluoranthene, indeno-air flow rate goes into two 130 m3 isolating zones, located at the [c,d]pyrene, benzo(ghi)perylene, EC, aluminum (Al), andentrance and exit of the refectory, through which visitors pass for titanium (Ti).isolation and decontamination from outdoor pollution. Further- The input source profiles were based on the observed molec-more, the air is filtered with plane, pocket and absolute filters as ular markers and assumed representative of sources in Milan.well as chemical filters (Purafil, Inc.); more details about these These profiles included wood-smoke,19,20 urban soil, gasolinefilters as well as the design and operation of the HVAC system vehicles,21 and diesel emissions.21 Biogenic-derived SOA was notcan be found in the SI. The number of visitors and duration of included in the model but its contribution was estimated usingvisit are limited to 25 persons and 15 min at any time between fixed tracer-to-OC ratios.22 Moreover, the selected urban soil8:15 a.m. and 6:45 p.m. Visits are allowed each day, except for profile is not specific to Milan. However, the choice of this profileMonday, with number of visitors averaging 1000 visitors/day. is not critical for the overall apportionment of fine OC as itsThe temperature and relative humidity are automatically controlled contribution to total OC mass is small23. Its selection was B dx.doi.org/10.1021/es202736a |Environ. Sci. Technol. XXXX, XXX, 000–000
  3. 3. Environmental Science & Technology ARTICLEFigure 1. Monthly average indoor concentration (compared to outdoor concentration) and bulk composition for (a, c) PM2.5 and (b, d) PM2.5À10.Error bars represent one standard error.nonetheless based on a comparison of the elemental ratios of the 3. RESULTS AND DISCUSSIONmeasured data to those of available soil profiles, where the urbansoil profile of St. Louis (Missouri)23 provided a best fit. In contrast, 3.1. IndoorÀOutdoor Relationship. 3.1.1. Particulate Massnatural gas, coal soot and toluene-derived (anthropogenic) SOA and Composition. Indoor and outdoor monthly average PM2.5sources were not considered in the model, as their molecular and PM2.5À10 mass concentrations are shown in Figure 1(a, b).markers were not detected in the samples. Furthermore, contribu- Indoor concentrations were substantially lower than those out-tions from vegetative detritus were not apportioned because doors for both particle modes. This significant reduction in PM2.5n-alkanes (C29ÀC33) did not exhibit an odd-carbon preference and PM2.5À10 concentration (88 ( 7% and 94 ( 3% on a yearlyindicative of modern plant material. Lastly, the CMB model average ((standard deviation) basis, respectively) can be largelyresults were considered valid if they met specific acceptance attributed to the efficacy of the HVAC system in removingcriteria as outlined in the SI. infiltrating outdoor PM. Moreover, coarse PM exhibited C dx.doi.org/10.1021/es202736a |Environ. Sci. Technol. XXXX, XXX, 000–000
  4. 4. Environmental Science & Technology ARTICLEextremely low indoor levels ranging between 0.12 and 0.83 μg/m3 (MarchÀMay), summer (JuneÀAugust), and fall (SeptemberÀwith no specific seasonal trend. PM2.5 was the dominant indoor November). Data for PM2.5À10, a minor component of indoorPM component with a concentration range of 1.7À4.9 μg/m3. It PM, is reported in SI Table S2. In consistence with its extremelyalso followed a pattern dissimilar to that of its outdoor compo- low indoor mass, coarse PM exhibited small I/O ratios (e8%).nent, indicating that indoor sources may have major contribu- For a given species, the combination of the I/O ratio andtions to fine PM indoors. Finally, it is noteworthy that currently correlation provides an estimate of its I/O source relationship. Thethere are no regulations for PM levels in museums, galleries, and latter can be described by the following categories. First, a low I/Oarchives. However, the American Society of Heating, Refrigerat- ratio accompanied by a good positive I/O correlation indicates aing and Air-Conditioning Engineers (ASHRAE)24 as well as the low but non-negligible infiltration factor and the lack of substantialCanadian Conservation Institute,25 provide recommendations indoor sources for a given species. Sulfate, which is a classic tracerfor PM2.5. They suggest concentration limits of <0.1 and 1À10 μg/m3 of atmospheric outdoor aerosols with no known indoorfor sensitive materials and general collections, respectively. PM2.5 sources29,30, presents a similar I/O source relationship, despitelevels in the refectory are within the limit values for general its usual association with high I/O ratios. However, in the currentcollections, but greater than those for sensitive materials. How- study, species that are usually considered to originate from out-ever, it is important to recognize that attaining such limits doors (e.g., sulfate, EC in nonsmoking environments)29À31, haverequires controls that may not be feasible and realistic. low I/O ratios due to their effective removal by the HVAC system. To determine the monthly bulk composition of indoor PM2.5 Second, a species associated with a low I/O ratio and poor I/Oand PM2.5À10, a chemical mass balance was conducted as correlation has a very low infiltration factor and no significantillustrated in Figure 1(c, d). PM chemical species were classified sources impacting its indoor levels. Furthermore, a species with ainto water-insoluble organic matter (WIOM), water-soluble high I/O ratio and a poor I/O correlation has a relatively loworganic matter (WSOM), EC, ions, crustal material (CM), and infiltration ratio but important indoor sources. For instance, OC,trace elements (TE). WIOM and WSOM were determined by which is commonly associated with indoor sources29,31, exhibits amultiplying both WSOC and water-insoluble OC (WIOC = OC- similar behavior. Lastly, a species with a high I/O ratio and goodWSOC) concentrations by a factor of 1.7.26À28 Further details positive I/O correlation displays high infiltration efficiency andabout the reconstruction are provided in the SI. lacks important indoor sources. WIOM was the dominant component of PM2.5 and PM2.5À10, As can be inferred from Table 1, the I/O ratios were generallyaccounting for an average ((one standard deviation) of 70.9 ( greater for PM2.5 than PM2.5À10 species, reflecting the lower16.4% and 61.3 ( 29.7% of total PM mass, respectively. PM2.5- infiltration efficiency and larger deposition velocity of coarseWIOM concentrations exhibited some variation, ranging between particles.32,33 Moreover, I/O ratios for fine PM mass were below1.7 μg/m3 in December and 3.1 μg/m3 in July. Conversely, coarse unity and presented some seasonality reaching a minimum (0.04 (mode WIOM displayed lower concentrations, varying between 0.03) in winter and a maximum (0.18 ( 0.06) in summer. These0.10 μg/m3 in December and 0.38 μg/m3 in April/May. WSOM ratios were also accompanied by a negative I/O correlationwas the next most abundant component of PM2.5 but only a minor (R = À0.31), indicating that indoor sources have major contribu-fraction of OM, comprising 17.8 ( 4.4% and 19.9 ( 2.9% of their total tions to PM2.5. On the other hand, ionic species, sulfate, nitrate andmass, respectively. Moreover, WSOM only accounted for 8.8 ( 4.9% ammonium, originated from outdoors (R = 0.66À0.76) but withof coarse PM mass. EC, on the other hand, only contributed to PM2.5. very low infiltration factors (I/O ratio e2%). Similarly to fine PMIts concentration and relative proportion were however minimal mass, OC and WIOC exhibited peak I/O ratios in summer(<0.05 μg/m3 and 1.5%). CM accounted for 8.2 ( 2.2% and 14.6 ( (0.61 and 1.07, respectively) and were negatively correlated to their4.7% of PM2.5 and PM2.5À10, respectively. Lastly, ions accounted for outdoor components, implying the existence of significant indoor3.1 ( 1.8% of PM2.5 and 5.4 ( 3.5% of PM2.5À10. OM sources. Furthermore, WSOC, an indicator of SOA formation The agreement between the reconstructed and gravimetric processes and biomass burning,34 showed a weak I/O associationmass is overall good, averaging 104 ( 20% for PM2.5 and 88 ( and slightly higher I/O ratios in spring and summer (∼0.20).29% for PM2.5À10. The observed discrepancy could be related to These results reflect a possible formation of SOA indoors. EC,uncertainties in the conversion factors from WSOC to WSOM, a key tracer for diesel exhaust,35 displayed a weak I/O correlationWIOC to WIOM, and metals to oxides as well as uncertainties in (R = 0.20) although it is expected to originate from outdoors giventhe measured mass, particularly for coarse PM. the prohibition of smoking inside the refectory. This could be 3.1.2. Infiltration Ratios of Tracer Species. To investigate the related to its efficient removal by ventilation, where EC was presentinfluence of outdoor and indoor sources on PM levels inside the at levels less than 0.05 μg/m3 with low I/O ratios (∼3%). Theserefectory, seasonal average indoor-to-outdoor (I/O) mass ratios findings suggest a nominal influence of outdoor diesel emissions onand their standard deviations were determined for key tracers of indoor PM levels. Typical crustal metals, for example, Al, calciummajor PM sources as summarized in Table 1 for PM2.5. I/O (Ca), Ti and iron (Fe), exhibited I/O ratios ranging from 0.03 toSpearman correlation coefficients (R) were also evaluated to 0.33 with similar peak occurrence in fall. Although these elementsdetermine whether an indoor species is attributable to infiltration are expected to originate from outdoors, they were weakly to mod-from outdoors. This analysis, coupled with CMB results, pro- erately related to their outdoor components (R = 0.17À0.45), whichvides a quantitative assessment of the infiltration of PM from indicates their dependence on indoor sources, likely particlespecific outdoor sources. It should also be noted that concentra- resuspension during visiting hours. Moreover, in spite of its weaktions that are below or comparable to the limit of detection I/O correspondence, potassium (K) was strongly associated with(LOD), increase the uncertainty associated with the data, but Ti and Al (R = 0.98 and 0.62, respectively) at the indoor site,should not cause a large overprediction. Concentration values indicating their common outdoor crustal source. Its poor I/Othat were below the LOD were assumed as half the detection correlation may be attributed to its mixed outdoor origin.limit. LODs of all measured species are listed in SI Table S1. The Marcazzan et al.36 reported that K is associated with motorseasons were segregated as winter (DecemberÀFebruary), spring vehicles in Milan. Conversely, Nickel (Ni), a marker of fuel oil D dx.doi.org/10.1021/es202736a |Environ. Sci. Technol. XXXX, XXX, 000–000
  5. 5. Environmental Science & Technology ARTICLETable 1. Indoor-to-Outdoor (I/O) Seasonal Average (One Standard Deviation) Mass Ratios and Spearman CorrelationCoefficients (R) for key species in PM2.5 average (standard deviation) I/O ratio winter spring summer fall R(I/O) mass 0.04(0.03) 0.14(0.10) 0.18(0.06) 0.11(0.07) À0.31 SO42‑ 0.01(0.003) 0.02(0.02) 0.01(0.001) <1% 0.76 NO3À <1% <1% 0.01(0.001) <1% 0.72 NH4+ <1% <1% <1% <1% 0.66 OC 0.11(0.03) 0.38(0.25) 0.61(0.17) 0.25(0.12) À0.13 WIOC 0.11(0.04) 0.51(0.29) 1.07(0.45) 0.30(0.18) À0.64 WSOC 0.11(0.01) 0.20(0.05) 0.22(0.04) 0.15(0.03) À0.10 EC 0.03(0.007) 0.03(0.01) 0.03(0.01) 0.02(0.01) 0.20 Al 0.09(0.02) 0.13(0.07) 0.06(0.03) 0.19(0.03) 0.17 Ca 0.17(0.04) 0.26(0.12) 0.15(0.06) 0.33(0.06) 0.43 Ti 0.18(0.06) 0.27(0.11) 0.21(0.1) 0.31(0.12) 0.45 Fe 0.03(0.01) 0.06(0.03) 0.05(0.02) 0.06(0.01) 0.39 K 0.03(0.02) 0.14(0.09) 0.14(0.05) 0.12(0.07) À0.45 Ni 0.04(0.01) 0.07(0.04) 0.04(0.01) 0.05(0.01) 0.66 Cu 0.04(0.02) 0.06(0.02) 0.05(0.01) 0.04(0.01) 0.31 Zn 0.02(0.02) 0.07(0.03) 0.10(0.05) 0.05(0.02) À0.23 benzo(e)pyrene 0.01(0.01) 0.03(0.03) n.d n.d n.q benzo(a)pyrene <1% 0.01(0.02) n.d n.d n.q indeno(l,2,3-cd)pyrene 0.02(0.005) 0.03(0.06) n.d n.d n.q benzo(ghi)perylene 0.01(0.004) 0.03(0.03) n.d <1% n.q coronene 0.01(0.003) 0.02(0.03) n.d <1% n.q picene n.d n.d n.a n.a n.q 17β(H)-21α(H)-30-norhopane 0.14(0.06) 0.23(0.01) 0.69(0.36) 0.13(0.07) 0.61 17α(H)-21β(H)-hopane 0.22(0.14) 0.34(0.04) 0.85(0.7) 0.11(0.06) 0.73 22S-homohopane 0.11(0.05) 0.15(0.02) 0.36(0.22) 0.08(0.03) 0.53 22R-homohopane 0.10(0.05) 0.16(0.05) 0.32(0.11) 0.08(0.03) 0.62 nonacosane 0.13(0.05) 0.17(0.03) 0.24(0.06) 0.14(0.05) 0.38 triacontane 0.20(0.08) 0.38(0.15) 0.76(0.25) 0.28(0.14) 0.30 hentriacontane 0.09(0.04) 0.11(0.04) 0.17(0.05) 0.08(0.02) 0.59 dotriacontane 0.24(0.17) 0.32(0.15) 0.65(0.21) 0.32(0.1) 0.47 tritriacontane 0.20(0.17) 0.14(0.04) 0.23(0.05) 0.18(0.10) 0.69 squalane a 167(71) 62(55) 58(6.4) 76(30) tetradecanoic acid 3.2(1.0) 5.8(0.62) 12(3.3) 11(5.0) À0.68 pentadecanoic acid 1.1(0.35) 2.3(0.22) 6.1(1.7) 4.4(1.9) À0.88 hexadecanoic acid 0.49(0.14) 1.0(0.11) 2.1(0.83) 0.94(0.52) À0.66 heptadecanoic acid 0.42(0.09) 0.82(0.17) 1.3(0.23) 0.89(0.41) 0.57 octadecanoic acid 0.40(0.10) 0.62(0.08) 1.2(0.42) 0.52(0.22) À0.20 nonadecanoic acid 0.15(0.05) 0.45(0.19) 0.73(0.17) 0.53(0.4) À0.20 palmitoleic acid a 182(58) 315(34) 413(98) 346(55) oleic acid 2.2(0.8) 11(5.9) 37(17) 6.7(4.7) -0.78 tributyl phosphate 67(9.2) 32(8.5) 9.1(2.5) 37(36) 0.53 phthalic acid 0.04(0.02) 0.13(0.07) 0.07(0.02) 0.05(0.01) 0.71 methylphthalic acid 0.01(0.01) 0.03(0.03) n.d 0.03(0.01) 0.40 suberic acid 0.06(0.06) 0.06(0.11) 0.17(0.02) 0.18(0.02) À0.11 azelaic acid 0.06(0.02) 0.13(0.04) 0.21(0.04) 0.14(0.05) À0.30 2-methylthreitol n.a n.d n.d n.a n.q 2-methylerythritol n.a n.d n.d n.a n.q 2-hydroxy-4-ispropyladipic acid 0.26(0.21) 0.51(0.44) 1.07(0.48) 0.85(0.51) À0.43 pinonic acid n.a n.d n.d n.a n.q 2,3-dihydroxy-4-oxopentanoic acid n.a n.d n.d n.a n.q levoglucosan 0.12(0.06) 0.09(0.10) 0.03(0.03) <1% 0.66n.a, non detected indoor and outdoor compound. n.d, non detected indoor compound. n.q, Not quantified due to insufficient number of data points (<6).R in bold is statistically significant at a 0.05 level. a non detected outdoor concentration is replaced by 1/2 detection limit for I/O ratio computation. E dx.doi.org/10.1021/es202736a |Environ. Sci. Technol. XXXX, XXX, 000–000
  6. 6. Environmental Science & Technology ARTICLEcombustion,37,38 strongly correlated with its outdoor compo- palmitoleic and oleic acids, normally associated with cooking,51nent (R = 0.66), signifying its mostly outdoor origin. Ni also exhibited I/O ratios much greater than 1, also indicating theirdisplayed comparable I/O ratios (4À7%), indicating a stable primarily indoor origin. However, given that cooking is prohib-but rather small outdoor influence on its indoor levels. Anthro- ited inside the refectory, the most probable source is biogenicpogenic metals, copper (Cu) and zinc (Zn), displayed I/O material45 potentially emitted from waxes.52,53 Emissions fromratios ranging from 0.02 to 0.10 and were weakly dependent on skin surface lipids of visitors are also a likely source of oleic acid.48their outdoor components (R = 0.31 and À0.23, respectively), These fatty acids are ubiquitous indoors and may be sorbed toindicating a potential but small indoor influence such as their indoor airborne particles or also settled dust54 that is subse-accumulation in indoor dust.39 This build up may in turn be quently resuspended by human/cleaning activities. Tributyldetermined by indoor and outdoor emissions.40 phosphate, a phosphate ester used in plasticizers and flame Among the listed polycyclic aromatic hydrocarbons (PAHs), retardants,55 persistently displayed I/O ratios exceeding unity,picene, a molecular marker for coal soot,41 was not measured in thereby indicating its indoor source, most likely wall or ceilingthe indoor samples despite its detection outdoors, implying that paint. In contrast, phtalic and methylphtalic acids, which exhib-coal soot is not a source contributor to indoor PM levels. The ited low but non-negligible I/O ratios (e13 and 4%, re-remaining PAHs, common products of incomplete combustion spectively), infiltrated from outdoors (R = 0.71 and 0.40,including fossil fuel and biomass combustion,42 were mostly respectively) with emissions from mobile sources and associationundetected indoors, especially in summer, and exhibited extre- with SOA formation45 as possible sources. The I/O ratios ofmely low infiltration factors (e3%), indicating that their sources suberic and azelaic acids, which are photo-oxidation products ofdo not significantly impact indoor PM levels. Hopanes, which are biogenic unsaturated fatty acids,56 suggest the presence of SOApredominantly associated with engine lubricating oil of mobile indoors. Nonetheless, the indoor levels of SOA are probably low,sources,43 fairly correlated with their outdoor levels (R = as WSOM only constitutes ∼20% of OM, as aforementioned.0.53À0.73), confirming their outdoor origin. Excluding summer, Tracers for biogenic-derived SOA include photo-oxidationduring which high I/O ratios were observed for 17β(H)- products of α-pinene and isoprene. These comprise pinonic21α(H)-30-norhopane and 17α(H)-21β(H)-hopane (0.69À0.85), acid, 2-hydroxy-4-isopropyladipic acid, 2-methylthreitol, andseasonal infiltration factors ranged from 0.08 to 0.36. These 2-methylerythritol.22 Conversely, tracers for anthropogenic-ratios highlight a year-long influence from vehicular sources on derived SOA include 2,3-dihydroxy-4-oxopentanoic acid,hopanes levels indoors. The peak summertime infiltration ratio photo-oxidation product of toluene.22 Among these SOAmay be a result of measurement uncertainties associated with the tracers, only 2-hydroxy-4-isopropyladipic acid, derivative oflow outdoor hopanes levels (0.02À0.06 ng/m3). To investigate α-pinene, was detected indoors with peak I/O ratio about unitythe origin of indoor n-alkanes (C29ÀC33), the carbon preference (1.07 ( 0.48) in summer. This secondary compound alsoindex (CPI), defined as the concentration ratio of their odd-to- presented a poor I/O correlation (R = À0.43) suggesting itseven numbered homologues, was estimated. A CPI about 1 indoor formation. A likely pathway is gas-phase reactionsindicates a dominance of anthropogenic sources, whereas a CPI involving α-pinene constituents and oxidants. The higher sum-greater than 2 indicates a prevalence of biogenic sources.44 These mertime I/O ratio reflects an enhanced production of SOA,indoor compounds did not exhibit a discernible odd-to-even possibly promoted by an increase in infiltrating oxidants.carbon number preference (CPI = 1.01 ( 0.14 on a yearly Lastly, the low I/O ratio and high I/O correlation foraverage basis), indicating their anthropogenic outdoor source, levoglucosan (R = 0.66), a tracer for biomass burning,57 indicatessuch as fossil fuel utilization and wood-smoke.45 Nonetheless, the its low but non-negligible indoor intrusion, mainly in winterlow I/O correspondence for some of these n-alkanes can be (12%).related to the primarily biogenic nature of their outdoor compo- In summary, these findings show that key tracers of majornents, which exhibited a CPI of 2.53 ( 0.61. Moreover, n-alkanes outdoor sources generally have small infiltration factors. Addi-displayed highest I/O ratios (0.17À0.76) in summer, possibly tionally, it is particularly interesting that fatty acids were mainly ofrelated to condensation of infiltrating gas-phase n-alkanes onto indoor origin with palmitoleic and oleic acids exhibiting I/Oindoor particles as a result of I/O temperature differences. In ratios >1.contrast, squalane existed in higher indoors than outdoor amounts. 3.2. CMB Results. 3.2.1. Source Apportionment of Fine OC.This undoubtedly suggests that indoor sources significantly con- The monthly contributions of primary and secondary sources totribute to its presence indoors. Squalane is a naturally occurring indoor fine OC as estimated by the CMB model are shown incompound in humans and plants as well as a compound used in Figure 2a and summarized in SI Table S3a. Three sources,skin care products,46,47 suggesting visitors as a possible source including wood-smoke, gasoline vehicles, and urban soil weregiven the absence of plants in the refectory. identified. Contributions to OC from biogenic-derived SOA were n-alkanoic acids, C14ÀC19, were uncorrelated with their out- not statistically significant (<2 Â standard error) with an utmostdoor components and displayed relatively high I/O ratios with value of 0.015 μg/m3. Similarly, diesel emissions were notsome greater than unity, indicating their predominantly indoor statistically significantly different from zero. The three sourcesorigin. Their I/O ratios also demonstrated a seasonal pattern collectively contributed to 6.3À20.7% of measured fine OC,with greatest ratios occurring in summer (0.73À12.4), in accor- with the remainder representing unidentified sources, likelydance with those of OC and WIOC. Moreover, these fatty acids including biogenic SOA. The largest contributor to OC massdisplayed a yearly average CPI of 7.55 ( 0.92, indicative of their consisted of gasoline vehicles, which accounted for 4.9À16.7%biogenic origin. For n-alkanoic acids, CPI is estimated as the of OC. The largest percent contributions and source estimatesconcentration ratio of their even-to-odd numbered homologues. occurred in cold months (DecemberÀMarch) with highestPotential indoor sources include skin emissions from visitors48 average ((standard error) levels of 0.23 ( 0.024 μg/m3and wax49 emissions from the painting itself. Waxes were used attained in February. The next most contributing source wasduring the restoration process of the painting.50 Furthermore, wood-smoke during winter, while urban soil during the F dx.doi.org/10.1021/es202736a |Environ. Sci. Technol. XXXX, XXX, 000–000
  7. 7. Environmental Science & Technology ARTICLEFigure 2. Sources contribution to indoor (a) fine organic carbon (OC) and (b) PM2.5 estimated using the chemical mass balance (CMB) model.remaining seasons. Wood-smoke contribution was only statis- Accordingly, unapportioned WIOC was determined as thetically significant during DecemberÀApril and peaked in Feb- difference between total WIOC and the sum of all primaryruary to reach 0.039 ( 0.015 μg/m3 (i.e., 2.8% of OC mass). source estimates (excluding wood-smoke) and WIOC fromThis seasonal pattern suggests wood burning for domestic wood-smoke. These estimates are reported in SI Table S3a.heating during cold months. On the other hand, urban soil As can be deduced, unattributed OC is largely water-insolublelacked any discernible seasonal trend and contributed to 1.14 ( (77.9 ( 3.2%), which indicates that uncharacterized OC sources0.46% (0.018 ( 0.007 μg/m3) of OC mass on a yearly based are mostly primary. Given that major outdoor sources of OCaverage ((standard deviation). have been included in the CMB model, and considering their low Lastly, unidentified source contributions, denoted as “other infiltration factors and contribution estimates, unknown primaryOC”, were estimated as the difference between measured OC sources are likely dominated by indoor sources such as dustand the contributions from the modeled sources. The percent of of biogenic origin, or PM emissions from the visitors and theresidual mass was higher during JulyÀOctober and could be painting itself.attributed to uncharacterized primary and SOA sources. The relative importance of fine OM to indoor PM and the 3.2.2. Source Apportionment of Total PM2.5. Source contribu- likelihood of its predominantly indoor source warrant furthertions to total PM2.5 were assessed by converting CMB results for investigation of this aerosol component. Accordingly, monthlyfine OC to PM2.5 apportionment using reported fine OC-to-PM variations of indoor fine WSOC, WIOC, fatty acids with I/Omass ratios for each source.19À23 In addition to the sources ratio >1 and CPI of n-alkanoic acids are examined as illustrated inidentified in OC apportionment, “other OM” as well as sulfate, Figure 3(aÀd).nitrate, and ammonium ion concentrations were considered in WSOC, attributed to SOA formation processes and biomassPM2.5 apportionment as displayed in Figure 2b and SI Table S3b. burning,34 mainly originated from indoors as previously noted. In“Other OM” was estimated by multiplying “other OC” by a factor fact, the contribution to WSOC from wood-smoke source wasof 1.7.26À28 These sources collectively accounted for 96.2 ( only significant in winter and early spring when it averaged18.7% of the measured PM2.5 mass, on a yearly based average ((standard deviation) 0.02 ( 0.006 μg/m3. Total WSOC, on((standard deviation). Some of the inconsistency in apportion- the other hand, maintained a stable average concentration ofment could be due to uncertainties associated with the conver- 0.31 ( 0.02 μg/m3 throughout the year, which further confirms asion factor from OC to OM and geographical differences of the continual and prevalent contribution to WSOC from indoorsources compositions. Finally, the most significant contributions SOA formation processes. This contribution to overall OC waswere from “other OM” (80.5 ( 17.4%), followed by urban soil however minor, as WSOC only comprised 20 ( 3% of indoor(6.9 ( 1.7%), gasoline vehicles (6.5 ( 2.8%), wood-smoke OC across the year.(1.2 ( 0.51%), then sulfate, nitrate and ammonium ions with WIOC, on the other hand, was a major component of indoorcontributions less than 1%. fine OC, accounting for 80 ( 3% of its mass on a yearly average 3.3. Comparison of OC to Organic Acid Species in PM2.5. basis. It also follows closely unapportioned OC, which could notTo elucidate the nature of uncharacterized OC, WSOC con- be assigned to outdoor sources, supporting the likelihood thattribution to unapportioned OC was estimated as WSOC that OC is mostly insoluble of indoor primary origin. Furthermore,is not associated with wood-smoke emissions. This calculation WIOC was present at a yearly average concentration of 1.25 (estimates wood-smoke contribution to OC as 71% water-soluble.58 0.25 μg/m3, with higher levels observed during MayÀOctober G dx.doi.org/10.1021/es202736a |Environ. Sci. Technol. XXXX, XXX, 000–000
  8. 8. Environmental Science & Technology ARTICLEFigure 3. Monthly time series of (a) water-soluble organic carbon (WSOC), (b) water-insoluble organic carbon (WIOC) and un-apportioned organiccarbon (OC), (c) palmitoleic and oleic acids, (d) carbon preference index of n-alkanoic acids (C14ÀC29), in fine PM in the indoor environment.and peak occurrence of 1.82 μg/m3 in July. These variations dust, waxes used in the painting and human skin emissions asimply an increase in the source strength of indoor primary potential sources.emissions during these months. Indoor n-alkanoic acids, C14ÀC29, exhibited a year-long To characterize potential indoor sources of OC, the monthly strong even carbon preference, with an annual average CPI oftrend of palmitoleic and oleic acids, which were present in 6.62 ( 0.55 and limited monthly variation. Carbon numbergreater indoors than outdoor amounts, was investigated. These maxima occurred at C16 and C14 (SI Table S4), which arecomponents exhibited a temporal distribution fairly similar to commonly found indoors59 and associated with human skinthat of WIOC and unapportioned OC, reaching a collective surface lipids.48 These findings are indicative of the consistentpeak of 41.9 ng/m3 in July and low of 14.4 ng/m3 in January. biogenic source of these n-fatty acids such as waxes used inParticularly, palmitoleic and oleic acids, biogenic components painting the “Last Supper” and skin emissions, as aforemen-sorbed to indoor airborne PM or dust as aforementioned, were tioned. Moreover, these components highly correlate withsignificantly and well correlated to WIOC (R = 0.75 and 0.76, WIOC (R = 0.73), a dominant component of OC, suggestingrespectively). Thus, this temporal correlation suggests their their shared origin with indoor dust of biogenic nature, emis-common source with biogenic material associated with indoor sions from the painting or human skin as likely sources. H dx.doi.org/10.1021/es202736a |Environ. Sci. Technol. XXXX, XXX, 000–000
  9. 9. Environmental Science & Technology ARTICLE Overall, the results of this study showed that outdoor sources Jones, J.; Farrar, C.; Maberti, S. Influence of ambient (outdoor) sourceshave small infiltration factors and contribution to PM2.5. OM, on residential indoor and personal PM2.5 concentrations: Analyses ofwhich could not be apportioned to any major outdoor sources, RIOPA data. J. Exposure Anal. Environ. Epidemiol. 2005, 15 (1), 17–28.accounted for most of PM2.5 (80.5 ( 17.4%), and was largely (6) Wallace, L. Indoor particles: A review. J. Air Waste Manage. Assoc.water-insoluble (77.9 ( 3.2%) with indoor dust of biogenic 1996, 46 (2), 98–126. (7) Salmon, L. G.; Nazaroff, W. W.; Ligocki, M. P.; Jones, M. C.;origin as a potential source. Consequently, it can be concluded Cass, G. R. Nitric acid concentrations in southern California museums.that controls to prevent infiltration of outdoor PM into the Environ. Sci. Technol. 1990, 24 (7), 1004–1013.refectory, where the “Last Supper” painting is housed, are very (8) Nazaroff, W. W. Indoor particle dynamics. Indoor Air 2004, 14effective. However, additional measures targeting the reduction (Suppl 7), 175–183.of fine OM should be implemented. Particularly, these controls (9) Putaud, J.-P.; Raes, F.; Van Dingenen, R.; Br€ggemann, E.; ushould address indoor sources of biological material that is likely Facchini, M. C.; Decesari, S.; Fuzzi, S.; Gehrig, R.; H€glin, C.; Laj, P.; uassociated with indoor dust. Lastly, we should note that findings Lorbeer, G.; Maenhaut, W.; Mihalopoulos, N.; M€ller, K.; Querol, X.; uof this study are characteristic of the specific site location, climatic Rodriguez, S.; Schneider, J.; Spindler, G.; Brink, H. t.; Tørseth, K.;conditions inside the refectory, visitors’ pattern and specifica- Wiedensohler, A. A European aerosol phenomenology—2: Chemicaltions of the HVAC system. These results, therefore, cannot be characteristics of particulate matter at kerbside, urban, rural and back- ground sites in Europe. Atmos. Environ. 2004, 38 (16), 2579–2595.directly extrapolated to other exhibits. (10) Lonati, G.; Giugliano, M.; Butelli, P.; Romele, L.; Tardivo, R. Major chemical components of PM2.5 in Milan (Italy). Atmos. Environ.’ ASSOCIATED CONTENT 2005, 39 (10), 1925–1934. (11) Misra, C.; Singh, M.; Shen, S.; Sioutas, C.; Hall, P. M. Devel-bS Supporting Information. Figures S1ÀS3 and Tables opment and evaluation of a personal cascade impactor sampler (PCIS).S1ÀS4. This material is available free of charge via the Internet J. Aerosol Sci. 2002, 33 (7), 1027–1047.at http://pubs.acs.org. (12) Camuffo, D.; Bernardi, A. The Microclimate of Leonardo’s “Last Supper. Joint Edition European Cultural Heritage Newsletter on’ AUTHOR INFORMATION Research and Bollettino Geofisico, Special Issue 1991, 14 (3), 1–123. (13) Nazaroff, W. W.; Cass, G. R. Mass-transport aspects of pollu-Corresponding Author tant removal at indoor surfaces. Environ. Int. 1989, 15 (1À6), 567–584.*E-mail: sioutas@usc.edu. (14) Birch, M. E.; Cary, R. A. Elemental carbon-based method for occupational monitoring of particulate diesel exhaust: Methodology and exposure issues. The Analyst 1996, 121 (9), 1183–1190.’ ACKNOWLEDGMENT (15) Stone, E. A.; Hedman, C. J.; Sheesley, R. J.; Shafer, M. M.; Schauer, J. J. Investigating the chemical nature of humic-like substances This research was supported by Southern California Particle (HULIS) in North American atmospheric aerosols by liquid chroma-Center, funded by US EPA and the University of Southern tography tandem mass spectrometry. Atmos. Environ. 2009, 43 (27),California (USC) Viterbi School of Engineering. We would 4205–4213.like to thank the superintendent of fine arts and culture in (16) Zhang, Y.; Schauer, J. J.; Shafer, M. M.; Hannigan, M. P.;Lombardy for his willingness to accept this study. We also wish Dutton, S. J. Source apportionment of in vitro reactive oxygen speciesto acknowledge the support of USC Provost’s Ph.D. fellowship. bioassay activity from atmospheric particulate matter. Environ. Sci.We thank Jeff DeMinter, Brandon Shelton and the staff at the Technol. 2008, 42 (19), 7502–7509. (17) Watson, J. G. Overview of receptor model principles. Air Pollut.Wisconsin State Laboratory of Hygiene for their assistance with Control Assoc. 1984, 34 (6), 619.the chemical measurements. We also wish to thank SIMG-Italian (18) Schauer, J. J.; Rogge, W. F.; Hildemann, L. M.; Mazurek, M. A.;College GPs and ISDE-International Doctors for the Environ- Cass, G. R.; Simoneit, B. R. T. Source apportionment of airbornement for managerial support, Eng. Franco Gasparini, designer of particulate matter using organic compounds as tracers. Atmos. 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