ChemicalâThermal Quantitative Methodology for ... - ACS Publications

Environ. Sci. Technol. 2006, 40, 939-944

Chemical-Thermal Quantitative Methodology for Carbon Speciation in Damage Layers on Building Surfaces N A D I A G H E D I N I , * ,‡ C R I S T I N A S A B B I O N I , † ALESSANDRA BONAZZA,† AND GIANCARLO GOBBI§ Institute of Atmospheric Sciences and Climate, National Research Council (CNR), Via Gobetti, 101, 40129 Bologna, Italy

The issue of environment protection, including the conservation of the monumental heritage worldwide, is related to atmospheric pollution, and its future therefore depends on air pollutant reduction. Carbonaceous particles emitted by combustion processes are the main factors responsible for the blackening of buildings. The identification and evaluation of the carbon species constituting the noncarbonate fraction of total carbon in damage layers, particularly in urban areas, are required in order to investigate atmospheric deposition on building surfaces. Since noncarbonate carbon contains organic and elemental carbon originating from various human activities, its measurement and speciation are crucial to the protection and conservation of monuments and ancient masonry, playing an important role both in the proposal of mitigation strategies and in the definition of conservation treatments. The availability of a correct, accurate, and reproducible analytical method for a complete carbon balance is essential in studying the effects of atmospheric pollutants on the environment, including those affecting cultural heritage. A chemical-thermal methodology was set up, and its sensitivity, accuracy, repeatability, and reproducibility were tested on appropriate standard samples of composition similar to the black crusts on stones and mortars. The results indicate that the technique satisfactorily distinguishes among carbon species, particularly those of anthropogenic origin, allowing a reliable evaluation of their quantities in damage layers. In view of the difficulties encountered in applying the thermo-optical methods adopted for the measurement of carbon filters, the proposed methodology contributes to filling the current gap in suitable and reliable analytical procedures in the field of cultural heritage protection.

Introduction Anthropogenic multipollutants are the main factors responsible for the accelerating damage observed on cultural artifacts exposed to the atmosphere over recent decades. Monuments and historic buildings located in urban areas * Corresponding author phone: +39-051-6399577; fax: +39-0516399649; e-mail: [email protected]. † National Research Council (CNR), Bologna (I). ‡ Bologna University (I). § Technical University of Marche, Ancona (I). 10.1021/es050164l CCC: $33.50 Published on Web 12/24/2005

 2006 American Chemical Society

and industrial sites are particularly subject to rapid damage processes (1). The weathering of stone and mortar surfaces causes the growth of damage layers, presenting a characteristic dark color, which give rise to more serious stone decay processes, such as material loss and blackening. Such phenomena engender the need for repeated restoration works, thus incurring high costs for society and cultural heritage management agencies as well as risks to the safeguard and maintenance of the built heritage and monuments. Buildings in urban and industrial areas are “blackened” because of the accumulation of atmospheric pollutants from human activity, mainly carbonaceous particles originating from the incomplete combustion of fossil fuels. This damage, often referred to as soiling, gives rise to visual impairment and leads to a loss of architectural value through the darkening of exposed visible surfaces or areas observed by the public (2). Air quality in Europe, especially in urban areas, has undergone several radical changes since the beginning of industrialization and is likely to experience further modification in the future because of the combined effects of population pressure, development strategies, and environmental policies. Pollution has originated, and continues to originate, chiefly from combustion processes, which produce and release a variety of gases and aerosols into the atmosphere (3). Carbonaceous particles are one of the main components of combustion emissions, and over the last century, their concentration in urban areas has risen drastically. While the threat of atmospheric pollution is increasing worldwide, it is also changing in character because of variations in types of sources (2, 4). In urban areas, up to the 1960s, coal combustion was the main agent of atmospheric pollution. Subsequently, the escalating demand for energy favored a quasi-exponential use of liquid fuel and gas. Although fuel consumption is still increasing, atmospheric pollution from industrial and heating installations has now dramatically decreased thanks to improved combustion in industrial settings, the sharp reduction of heavy petrol fractions in domestic use, the recommended consumption of low S-content fuel, and mitigation policies adopted at the local level. Expected future variations will be mainly related to the increase in traffic caused by the dense concentrations of population in urban centers. At present, in most European cities, traffic exhaust is the main source of atmospheric pollution, and its contribution is difficult to regulate for political, economic, and technical reasons. Since the surfaces of buildings and monuments are the areas where airborne pollutants deposit, accumulate, and interact with the substrate, the chemical composition of black crusts reflects the surrounding atmospheric environment. Thus, the identification and evaluation of the anthropogenic chemicals present in damage layers on monuments are prerequisites for the proposal of air quality levels necessary for the sustainable protection and conservation of the built cultural heritage. Over recent years, important advances have been made in the knowledge on air pollutants-building materials interaction. Many studies have been performed on sulfur and nitrogen compounds in stone deterioration. Particular attention has been focused on the role of atmospheric sulfur in gypsum formation on carbonate architectural structures (5). However, only a few recent works have reported quantitative measurements on the carbon content of the atmospheric particles that accumulate in the damaged layers, addressing the issue of their possible role in gypsum VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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formation (6). Data on the ubiquitous and abundant carbon particles in black crusts remain rather scarce despite their fundamental role in damage layer formation. The main environmental damage effect on the carbonate structural components and binders used in historic buildings is the transformation of calcium carbonate into gypsum because of wet and dry deposition of SO2 (5). Gray-to-black crust formation is produced by gypsum crystals and atmospheric deposition, including carbonaceous particles which, because of their high specific surface and heavy metal content, act as catalytic support to the heterogeneous oxidation of SO2. The interaction between carbonaceous particles and stones has been confirmed by studies involving simulation experiments in a laboratory exposure system (6). Despite the key role of soot, existing data are generally limited to the identification of the different types of embedded particles, on the basis of their morphology and elemental composition observed by optical microscopy and scanning electron microscopy (7). Analyses of carbon species in black crusts performed over recent years (8) have shown that, in order of abundance, carbon is the second most important anthropogenic element, after sulfur, contained in damage layers. Total carbon (TC) is composed of the following fractions:

TC ) CC + NCC and NCC ) OC + EC where CC is carbonate carbon, basically because of the underlying materials (stones and mortars), and NCC is noncarbonate carbon, originating from atmospheric deposition. The NCC is in turn composed of organic carbon (OC), of both primary or secondary natural and anthropogenic origin, and of elemental (EC) because of the deposition of combustion-derived aerosol (soot). Thus, the measurement of noncarbonate carbon, and particularly of EC, is necessary for ascertaining whether the carbon derives from the stone substrate or is due to anthropogenic particle deposition. Such considerations underscore the pressing need for an analytical procedure able to supply satisfactory measurements. Different analytical methods are used to measure OC and EC concentrations in investigating atmospheric samples (9, 10). Optical and thermal techniques are the most widely used for distinguishing and measuring carbon species. Photoacoustic methods (11) are also reported in the literature for the quantification of aerosol elemental carbon, although such procedures are not frequently adopted. Optical methods, based principally on measurements in reflectance or transmittance (12) of a visible light beam through a sample, are essentially used to evaluate the dark component content of carbonaceous materials, which some researchers also refer to as black (12), elemental (13), or graphitic carbon (14). Thermal oxidation (15) and thermaloptical, in transmittance (9) and reflectance (16), methodologies are the analytical procedures most often applied in the case of atmospheric carbonaceous particles. Thermal oxidation methods comprise two oxidation steps: the preliminary removal of the organic material by precombustion at a relatively low temperature, with the detection of the remaining carbon content by heating to a high temperature, followed by the measurement of the CO2 or methane obtained by several different methods. Some authors replace the first step with chemical oxidation, in carbon determination on marine and lake sediments (17). Concerning these methods, considerable disagreement exists on oxidation temperatures, operating conditions, treatment times for removing organics in the first step, and complete combustion of the remaining carbon fraction in the second. Measurements of carbon dioxide, derived from thermal carbon oxidation, adopting various nonhomogeneous procedures reported in the literature, for example, coulometric 940

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titration or GC (17, 18), give rise to difficulties in the comparison of the respective quantitative data. While the analytical problems associated with the discrimination and measurement of carbon fractions in atmospheric particles have been more widely studied (9-19) and a large number of different techniques have been tested, analogous investigations on black crusts are scarce. This is because the application of thermal and optical methodologies is yet to be tested on damage layers on building and monument surfaces. To date, very few quantifications of carbonaceous aerosol deposited on monument surfaces are available. In addition, a quantitative correlation between blackening and black crust composition has not been carried out because of a lack of carbon measurements for damage layers. The quantification of carbon in black crusts represents a complex problem. The carbonaceous matter in damage layers on building surfaces is a rather complex mixture of organic materials, frequently highly polymerized. No clear discontinuity is observed between an elemental carbon fraction and an organic carbon fraction. Furthermore, with respect to atmospheric carbonaceous particles, the presence of an abundant quantity of carbonates in black crusts, at times over 50% of TC, as reported in Ghedini et al. (8, 18), constitutes a critical challenge in the measurement of carbon species in damage layers on building materials, considerably increasing the analytical problems related to their removal. The organic materials in damage layers constitute a system of particular complexity, as they result not only from the deposition of carbonaceous particles but also from the adsorption of volatile organic compounds over the predeposited material. An accurate discrimination of the organic and elemental fractions and their reliable quantification are also problematic analytical stages. The objective of this study is to address the above issue by presenting a specific method of TC, CC, NCC, OC, and EC determination, suitable for damage layers on buildings. The proposed method optimizes experimental conditions so that a satisfactory cutoff for carbonaceous fractions can be obtained. The procedure was validated using standard samples of composition similar to the typical black crusts of the damage layer encountered on building surfaces in urban areas. With the purpose of completing the information obtained on the standard samples, the analytical method was also applied on natural crusts from some European urban centers.

Experimental Section Analytical Method. On the basis of experience gained in carbon analyses on black crusts with the analytical procedure of Ghedini et al. (18), characterized by a complex preliminary chemical treatment of the specimens for NCC and EC determination, a new simplified methodology was set up to discriminate and quantify the carbon fractions in damage layers on the building surfaces exposed to atmospheric pollution. All the experimental measurements of carbon were carried out by flash combustion/gas chromatographic analysis using a conductibility detector (CHNSO EA 1108 FISONS Instruments). The quantity of powdered black crust required for the overall procedure is about 1 g. The methodology consists of three distinct stages (Figure 1), each performed on a different part of the same damage layer specimen. First, total carbon is quantified by burning one part of the bulk sample. Second, noncarbonate carbon is obtained by the combustion of a second part after carbonate decomposition and the complete removal of carbon dioxide. Finally, elemental carbon is measured by oxidation of the residue obtained after eliminating the inorganic matrix and organic species by means of a chemical treatment. Carbonate carbon and organic carbon are then calculated.

FIGURE 1. Schematic diagram of the analytical procedure developed for the measurement of TC and its fraction speciation and quantification (NCC, CC, EC, OC) in damage layers on the built environment. The procedure is summarized in the following steps: (1) Total Carbon. About 10 mg of bulk ground sample, exactly weighed, is directly placed into a silver capsule, where it is fully oxidized and quantified by flash combustion/gas chromatographic analysis. (2) Noncarbonate Carbon. For the measurement of NCC, about 10 mg, exactly weighed, of ground sample is placed in a silver capsule, where it undergoes acidification to remove carbonates by means of maintenance in an atmosphere of HCl concentrate solution until microeffervescence stops. It is then preserved for 12 h in a KOH drier to eliminate CO2, HCl, and H2O before being analyzed by the instrument for noncarbonate carbon quantification. Carbonate carbon is calculated as the difference between TC and NCC. (3) Elemental Carbon. The procedure set up for elemental carbon quantification involves the following six stages. (a) An exactly weighed quantity of ground black crust, ranging between 200 mg and 1 g, depending on its EC content and the type of preparation equipment, is placed in an airtight tube, and 2 mL of Na2CO3 saturated solution is added. The tube is then hermetically sealed and heated at 120 °C for 2 h. The cooled sample is centrifuged at 5.000 revs/min for about 10 min and, after liquid-phase removal, is washed with 2 mL of tepid distilled water and is recentrifuged. (b) Subsequently, the residue is treated, under agitation in an open tube, in steps of 20 mL, with a concentrated solution of HCl (37%), until the effervescence stops; the complete decomposition of carbonates and the removal of CO2 are then obtained by heating the suspension to 40-50 °C. After cooling, the residual sample is centrifuged and rinsed as described above. The treatment with the Na2CO3 saturated solution (a) leads to the solubilization of lowest soluble salts (e.g., CaHPO4, SrSO4, CaC2O4) because of the formation of carbonates, which are then removed, together with the carbonates present as mineralogical components of the original sample, using the HCl treatment (b).

(c) The residual sample undergoes five alternate digestion steps at 120 °C for 1 h performed with 2 mL HCl 37% and at 120 °C for 30 min with 2 mL KOH 30%. Each digestion step is followed by centrifugation and washing with 2 mL of distilled water after the acid digestion and with 2 mL of distilled water, acidified with HCl solution, after the basic one. The HCl treatments allow the dissolution of basic materials and the decomposition of silicates, while the repeated KOH attacks produce the complete dissolution of the acid substances, the quartz, and the amorphous silica derived from the HCl action on silicates. The five alternate attacks with HCl and KOH concentrated solutions are indispensable for isolating elemental carbon from particularly complex matrixes. (d) If, after the above steps, the residual sample still reveals the presence of silicates, their complete removal can be obtained by means of a final treatment with NH4F‚HF, at 120 °C, until the dissolution of salts, followed by centrifugation and washing with distilled water. (e) For each sample, the liquid phases derived from all treatments, including washing, are collected, mixed, pH adjusted to about 10, added to 25-30 mg of Zn2+ (as ZnCl2), and stirred. The soft jellylike mass resulting from the formation of zinc hydroxide, which contains EC, is isolated by centrifugation, is washed, and is added to the residue of the previous treatment. (f) The sample is finally dried at 180 °C until reaching constant weight, after which it is analyzed by combustion (CHNSO) to evaluate the elemental carbon content. Organic carbon is then calculated as the difference between NCC and EC. Standard Samples Preparation. To verify accuracy and repeatability, the developed chemical-thermal methodology for TC, NCC, and EC discrimination and measurement was applied to four standard samples, which simulated black crusts specimens, particularly in their carbon fraction amounts. This validation procedure was considered essential for checking the applicability of the methodology to a sample as complex as damage layers on monuments. The representative standard mixtures, indicated as standard (Std.) 1, 2, 3, and 4, were prepared; their composition was similar to that of black crust specimens collected from building material surfaces, such as marble, limestone, sandstone, and mortar, exposed to polluted atmosphere, using 16 components of maximum commercial purity (Table 1). The composition of the standards was defined on the basis of the data reported in the literature relating to damage layers from monuments and buildings located in urban areas, taking into account the mean values found for gypsum/carbonate ratio, salts, oxides, organics, and elemental carbon (8, 18, 20-24). With the aim of reproducing CC, OC, and EC contents that are representative of those found in black crusts, four standards with different concentrations of CC, OC, and EC, and consequently of NCC and TC, were prepared, as reported in Table 2. In particular, Std. 1 (base) reflected the mean composition in calcium carbonate and gypsum (CaSO4‚2H2O) measured in black crusts sampled on low porosity carbonate stones, such as marbles and limestones (20). In this base standard, the amount of the carbon species was the mean value reported in the literature (8, 18, 23). With respect to Std. 1, the others standards were prepared by changing the percentage of only one of the carbonaceous components. In Std. 2, the amounts of CC and calcium sulfate were changed using the same quantity of CaCO3 and CaSO4 (about 35%), simulating, on average, the situation typically observed in damage layers on high porous carbonate building materials, such as sandstone and mortars (21, 24); the amounts of organic and elemental carbon remained unaltered with respect to Std. 1. Std. 3 and Std. 4 were prepared by modifying VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Mass Percentage of Various Components in Standards Used for Carbon Method Validationa Std. 1 Component

%

calcium carbonate calcium sulfate quartz kaolin titanium(IV) oxide magnesium oxide barium sulfate iron(III) oxide lead(II) oxide graphite calcium oxalate sodium formate potassium acetate potassium hydr. phthalate sodium palmitate 1,2-benzanthracene

10.03 59.92 13.65 4.96 0.34 2.04 0.49 2.98 0.33 1.52 1.07 0.17 0.90 0.60 0.71 0.29

a

Std. 2 C%

%

0.20 0.03 0.22 0.28 0.49 0.27

34.97 34.91 13.75 5.07 0.32 1.98 0.45 2.96 0.35 1.48 1.01 0.23 0.94 0.57 0.70 0.31

%

0.19 0.04 0.23 0.27 0.48 0.29

10.04 59.85 12.91 4.71 0.32 2.04 0.35 2.99 0.31 0.50 1.39 0.28 1.59 0.83 1.39 0.50

Std. 4 C%

%

C%

0.26 0.05 0.39 0.39 0.96 0.47

9.97 59.94 14.89 5.02 0.34 2.17 0.65 3.01 0.35 2.48 0.21 0.11 0.29 0.17 0.30 0.10

0.04 0.02 0.07 0.08 0.21 0.09

For the organic compounds, the corresponding carbon percentage (C%) is also reported.

TABLE 2. Carbon Fraction Content (%) in the Prepared Standards total carbon (TC) carbonate carbon (CC) noncarbonate carbon (NCC) organic carbona (OC) elemental carbonb (EC)

Std. 1

Std. 2

Std. 3

Std. 4

4.21 1.20 3.01 1.49 1.52

7.18 4.20 2.98 1.50 1.48

4.23 1.21 3.02 2.52 0.50

4.19 1.20 2.99 0.51 2.48

a OC was calculated from the carbon content of organic anions (Caoxalate, Na-formate, K-acetate, K-hydrogen phthalate, and Na-palmitate) and PAH (1,2-benzanthracene). b EC indicates the graphite content.

only the percentages of OC and EC with respect to Std. 1: in Std. 3, organic carbon was increased to 2.52% at the expense of elemental carbon (0.50%), while in Std. 4 the OC (0.51%) and EC (2.48%) amounts were inverted with respect to Std. 3. Consequently, Std. 1, Std. 3, and Std. 4 were composed of the same amount of TC and NCC. The complete homogenization of the standards was obtained by grinding the mixtures in an agate mortar immediately prior to analysis, since the standards contained calcium sulfate anhydrous, which tends to absorb water very easily. In addition, to overcome the problem of the complete combustion of the synthetic graphite, lead dioxide was added to all the standard samples after weighing. Black Crust Sampling. Specimens of black crusts were collected in different European cities from monuments clearly damaged by soiling and blackening, that is, the Vittoriano Monument in Rome (RO), the Corner Palace in Venice (VE), Milan Cathedral (MI), St. Eustache Church in Paris (PA), and the Cathedral of Seville (SE). Sampling was performed by carefully scraping away only the dark surface deposit using a fine, but strong, bistoury. Underlying building materials mixed with the surface crust specimen brings about a change in the carbon concentration. Standards and Damage Layer Analyses. The chemicalthermal analytical procedure setup for carbon fraction discrimination and measurement in damage layers was applied on four portions (a-d) of each of the four prepared standards (1-4). The methodology was then also applied on damage layers sampled at the different urban sites with the purpose of testing it on real specimens. Three portions of each collected specimen were treated according to the method described.

Results and Discussion The data on TC, NCC, and EC determination, obtained by applying the chemical-thermal analytical methodology on 942

Std. 3 C%

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the prepared standard mixtures, are reported in Table 3. The table also includes (i) the means of the four measurements (a-d) of total, noncarbonate, and elemental carbon obtained for each standard (Std. 1-4); (ii) the standard deviations (SD); (iii) the relative standard deviation (RSD[%]), that is, the percentage ratio between standard deviation and mean; (iv) the theoretical value of each standard as reported in Table 2; and (v) the percentage error of the measured mean with respect to the true quantity of TC, NCC, and EC in the four standards (Table 2). On the basis of the mean concentrations of TC, NCC, and EC resulting from the standards analyses, the corresponding quantities of CC and OC were calculated. The results prove that the procedure satisfactorily distinguishes and measures the carbon fractions. The good agreement between the mean values of TC, NCC, and EC measured in the four samples and their theoretical values is evidence of the high accuracy of the method. In fact, the maximum absolute difference is -0.04% for EC in Std. 3, which has the lowest amount of elemental carbon. SD and RSD show, in all standards, very low values for all the carbon fractions measured, underscoring the high repeatability of the procedure. The highest values of SD are found for NCC and EC in Std. 2 (e.g., 0.02% and 0.03%, respectively), which contains the highest amount of CC (4.20%), and for EC in Std. 3 (0.03%), characterized by the lowest quantity of EC (0.50%). The RSD (%) reflect the same trends shown by the standard deviations indicating that high contents of CC and low EC concentrations are the key factors affecting the accuracy and repeatability of carbon fraction measurement in black crusts. Regarding the elemental carbon measurements in Std. 1-4, all the EC values measured show a negative error with respect to the true values, proving that the elimination of the organic fraction, one of the critical steps of the methodology, is effective using the developed procedure. The error of the measurements, evaluated as the difference between the experimental mean data and the theoretical value expressed as a percentage, is very small in all the standards, which is further evidence of the good reproducibility of the method. Elemental carbon presents, in absolute terms, the highest percentage error, which is due to the numerous treatments required for EC measurement, compared to the other carbon fractions. The lowest percentage error is in Std. 4 (-0.40%), which has the highest EC concentration (2.48%), while the highest is in Std. 3 (-8.00%), which has the lowest EC concentration (0.50%). Medium values are obtained for Std. 1 and Std. 2, both of which have an intermediate EC content (about 1.5%). Of Std. 1 and Std. 2, the higher percentage

TABLE 3. Total, Noncarbonate, and Elemental Carbon (%) Obtained from the Four Measurements Performed on Std. 1-4a Standard 1

Standard 2

Standard 3

Standard 4

1a 1b 1c 1d mean std. dev. rel. std. dev. theoretical error

4.21 4.23 4.20 4.22 4.22 0.01 0.24 4.21 0.24


Total Carbon 7.20 3a 7.19 3b 7.18 3c 7.20 3d 7.19 mean 0.01 std. dev. 0.14 rel. std. dev. 7.18 theoretical 0.14 error


3.03 3.05 3.05 3.03 3.04 0.01 0.33 3.01 1.00


Noncarbonate Carbon 2.99 3a 3.00 3b 3.04 3c 3.00 3d 3.01 mean 0.02 std. dev. 0.66 rel. std. dev. 2.98 theoretical 1.01 error

3.05 3.02 3.03 3.04 3.04 0.01 0.33 3.02 0.66


3.01 3.03 3.02 3.03 3.02 0.01 0.33 2.99 1.00


1.50 1.47 1.51 1.49 1.49 0.02 1.34 1.52 -1.97


Elemental Carbon 1.44 3a 1.49 3b 1.45 3c 1.43 3d 1.45 mean 0.03 std. dev. 2.07 rel. std. dev. 1.48 theoretical -2.03 error

0.47 0.42 0.45 0.48 0.46 0.03 6.52 0.50 -8.00


2.46 2.45 2.47 2.48 2.47 0.01 0.40 2.48 -0.40

4.23 4.24 4.25 4.26 4.25 0.01 0.24 4.23 0.47


4.21 4.21 4.20 4.22 4.21 0.01 0.24 4.19 0.48

a Mean, standard deviation, percentage relative standard deviation and percentage error vs the theoretical value of standard compositions (Table 2) are reported.

TABLE 4. Concentrations (%) and Standard Deviation (SD) of TC, NCC, and EC Measured in the Samples of Black Crust Analyzeda Sample

TC

SD

NCC

SD

CC

EC

SD

OC

RO VE MI PA SE

5.56 4.25 3.14 4.08 4.62

0.01 0.02 0.02 0.02 0.02

5.08 3.72 2.21 3.37 3.23

0.01 0.01 0.02 0.02 0.01

0.48 0.53 0.93 0.71 1.39

2.11 1.53 1.58 1.35 1.07

0.02 0.01 0.01 0.01 0.01

2.97 2.19 0.63 2.02 2.16

a

The calculated CC and OC values are reported.

error is in Std. 2 (-2.03%) because of the higher carbonate content (about 35%). The methodology was also applied to real damage layers sampled from different urban sites. The results are reported in Table 4, including the mean values obtained on the three samples analyzed for each specimen and the SD. The low SD obtained show that the analytical method is suitable for specific application in the complex measurements of the carbon fractions in the damage layer on building surfaces. The specific analytical procedure setup constitutes a fundamental step in the study of the interaction between atmospheric pollutants and building materials with the aim of attaining the safeguard of the built heritage against anthropogenic pollutants, such as combustion emissions, and filling the existing gap in the discrimination and quantification of carbonaceous species in black crusts on monuments.

Acknowledgments The present study was supported by the EC, 5° FP Research, within the Environment and Sustainable Development Program, Project Carbon content and origin of damage layers in European monumentssCARAMEL (Ct. No. EVK4-CT-200000029).

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(14) Horvath, H. Atmospheric absorption-A review. Atmos. Environ. 1993, 27(A), 293-317. (15) Cachier, H.; Bremond, M. P.; Buat-Menard, P. Determination of atmospheric soot carbon with a simple thermal method. Tellus 1989, 41(B), 379-390. (16) Cao, J. J.; Lee, S. C.; Ho, K. F.; Zhang, X. Y.; Zou, S. C.; Fung, K.; Chow, J. C.; Watson, J. G. Characteristics of carbonaceous aerosol in Pear River Delta Region, China during 2001 winter period. Atmos. Environ. 2003, 37, 1451-1460. (17) Lim, B.; Cachier, H. Determination of black carbon by chemical oxidation and thermal treatment in recent marine and lake sediments and Cretaceous-Tertiary clays. Atmos. Environ. 1996, 131, 143-154. (18) Ghedini, N.; Gobbi, G.; Sabbioni, C.; Zappia, G. Determination of elemental and organic carbon on damaged stone monuments. Atmos. Environ. 2000, 34, 4383-4391. (19) Crow, J. C.; Watson, J. G.; Crow, D.; Lowenthal, D. H.; Merrifield, T. Comparison of IMPROVE and NIOSH Carbon Measurements. J. Aerosol Sci. Technol. 2001, 34, 23-34.

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(20) Sabbioni, C.; Zappia, G. Atmospheric-derived element tracers on damaged stone. Sci. Total Environ. 1992, 126, 35-48. (21) Brimblecombe, P. The Effects of Air Pollution on the Built Environment; Air Pollution Reviews Vol. 2; Imperial College Press: London, 2003. (22) Saiz-Jimenez, C. Deposition of airborne organic pollutants on historical buildings. Atmos. Environ. 1993, 27(B), 77-85. (23) Sabbioni, C.; Ghedini, N.; Bonazza, A. Organic anions in damage layers on monuments and buildings. Atmos. Environ. 2003, 37, 1261-1269. (24) Sabbioni, C.; Zappia, G.; Ghedini, N.; Gobbi, G.; Favoni, O. Black crusts on ancient mortars. Atmos. Environ. 1997, 32, 215-223.

Received for review January 25, 2005. Revised manuscript received October 10, 2005. Accepted November 1, 2005. ES050164L

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