Science of the Total Environment 408 (2010) 1403–1413

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An integrated approach to assess air pollution threats to cultural heritagein a semi-confined environment: The case study of Michelozzo's Courtyardin Florence (Italy)

S. Nava a,⁎, F. Becherini b, A. Bernardi b, A. Bonazza c, M. Chiari a, I. García-Orellana a,d, F. Lucarelli a,N. Ludwig e, A. Migliori a, C. Sabbioni c, R. Udisti f, G. Valli e, R. Vecchi e

a Department of Physics, Università degli Studi di Firenze and INFN, National Institute of Nuclear Physics, via Sansone 1, 50019, Sesto F.no (Fi), Italyb CNR-ISAC, National Research Council, c.so Stati Uniti 4, 35127, Padova, Italyc CNR-ISAC, National Research Council, via Gobetti 101, 40129, Bologna, Italyd Centro Nacional de Aceleradores, Universidad de Sevilla-CSIC-JA, av. Thomas A. Edison 7, 41092, Sevilla, Spaine Department of Physics, Università degli Studi di Milano and INFN, National Institute of Nuclear Physics, via Celoria 16, 20133, Milano, Italyf Department of Chemistry, Università degli Studi di Firenze, via della Lastruccia 3, 50019, Sesto F.no (Fi), Italy

⁎ Corresponding author. National Institute of NuclearSesto F.no (Fi), Italy. Tel.: +39 0554572727; fax: +39 0

E-mail address: nava@fi.infn.it (S. Nava).

0048-9697/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.scitotenv.2009.07.030

a b s t r a c t

a r t i c l e i n f o

Article history:Received 8 January 2009Received in revised form 7 July 2009Accepted 8 July 2009Available online 25 November 2009

Keywords:Cultural heritageSoilingMicroclimateThermographyParticulate matterCarbonaceous particles

An example of an integrated approach to assess air pollution threats to cultural heritage in a semi-confinedenvironment is presented in this work, where the monitoring campaign carried out at the Michelozzo'sCourtyard (in Palazzo Vecchio, Florence, Italy) is used as a case study. A wide research project was carriedout, with the main aim of obtaining the first quantitative data on air quality and microclimate conditionsinside the Courtyard, and, if possible, identifying the main causes of degradation and suggesting appropriateconservation strategies. The investigation adopted a holistic approach involving thermographic measure-ments on the wall paintings, microclimatic analysis, gaseous pollutant monitoring, atmospheric particlescharacterisation and dry deposition compositional analysis. Attention was focused on the wall paintingdepicting the city of Hall because of its anomalous and critical conservation conditions, which are visible at aglance, due to the contrast between a wide darker zone around the central subject of the painting andexternal lighter areas.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Atmospheric pollution is widely recognised as one of the majorthreats to cultural heritage, in particular when associated with waterabsorption phenomena (Brimblecombe, 2005). Atmospheric particledeposition on surfaces of artistic interest may cause an aesthetic impactinduced by soiling andmaterial deterioration due to chemical reactions.This can intensify surface damage as particles act as condensation nucleiformoisture or adsorb gaseouspollutants. Thus, informationonboth theair quality and microclimate are needed to effectively evaluateconservation methods.

There have been several investigations on the impact of air pollutionon indoor andoutdoor cultural heritage (see, for example, Camuffo et al.,2001; Sabbioni et al., 2003; Bonazza et al., 2005; Bernardi, 2008; Godoiet al., 2006).Works of art in outdoor environments are directly affectedby external pollution and climatic factors,while indoor they are stronglyinfluenced by cultural heritage management (i.e. tourists pressure

Physics, via Sansone 1, 50019,554572641.

l rights reserved.

regulation, air conditioning and heating systems, etc.), and by thestructural characteristics of the building, which affect the indoor/outdoor exchange of external pollutants. In contrast, the semi-confinedenvironment is thus far not well explored. Investigations on caves andhypogeum environments can be found in literature: we mention heresome earlier studies concerning the Buddhist cave temples at Yungangin China (Salmonet al., 1994; Christoforou et al., 1994), theworkson theAltamira cave in Northern Spain (see, for example, Sánchez-Moralaet al., 1999) and the investigations carried out within the CATS ECproject (Albertano et al., 2003). However, these environments could notbe directly compared to a historical building courtyard located in anurban area. A case study that is more comparable with our is the earlystudy concerning the historical building of Palazzo Ducale in Urbino,Italy (Bernardi et al., 1985), where a microclimatic investigation withinthe palace courtyard highlighted the major role played by water in thestone weathering process. In particular, the rain water percolation wasresponsible for the phenomena of dissolution and re-crystallization ofcalcium carbonate. However, Urbino is a relatively pollution-freeenvironment and no pollution measurements were carried out withinthat investigation.

PalazzoVecchio (14th century), located in the centre of Florence, is themost important historic building in the town, and is the headquarters of

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the communality. Its main Courtyard (Michelozzo's Courtyard) wasredecorated in 1565 by Giorgio Vasari, for the marriage betweenFrancesco I de' Medici and Johanna of Austria, with wall paintingsrepresenting views of themain Austrian cities and other decorations. Thissemi-confined environment is subject to both urban pollution and heavymicroclimatic conditions and, at present, all the wall paintings thatdecorate the portico show an advanced level of degradation.

In view of new restorations, a wide-ranging research project wascarried out in 2003–2004, with the objective of obtaining the firstquantitative data on air quality and microclimate conditions inside theCourtyard. The investigation adopted an integrated approach involvingthermographic measurements on the wall paintings, microclimaticanalysis, gaseous pollutant monitoring, atmospheric particle chemicalcharacterisation, and dry deposition compositional analysis.

Particular focus was given to the painting depicting the city of Hall,because of its critical conservation condition and the anomalouscontrast, which is visible at a glance, between a wide darker zonearound the central subject of the painting and external lighter areas.

The results of this field campaign are reported and discussed in thepaper. This database can be a reference for future studies, also in other

Fig. 1. Above, picture (a) and schematic map (b) of the Michelozzo's Courtyard (H: Hall, N: Npart of the employed instrumentation can be seen (c): aerosol samplers for the collection of(PS), microclimate probes (M1, M2, M3). Dry deposition sampling points are also shown (D

similar semi-confined environments. Starting from the obtained data,we tried tofindsome information on the possible causes of theobserveddegradation. This information is useful to conservators for the selectionof the most effective restoration and preservation strategies.

2. Experimental methods

2.1. Site description

Michelozzo's Courtyard is situated in the southwestern part of thePalazzo Vecchio. In Fig. 1, a picture and a map of the Courtyard areshown, together with a picture of the Hall view, where part of theemployed instrumentation can be seen. The Courtyard is delimited bya portico, which is fully decorated by wall paintings and plasters. Tocompletely cover the walls with paintings representing views of themain Austrian cities, in 1565 all the windows on the inner side of theCourtyard were plugged (two of them, Neustadt and Ebersdorf, arenow open again due to damage suffered during the World War II).Consequently, the structure underlying most of the views is stronglyheterogeneous, since it is constituted by both original walls, made

eustadt, K: Konstanz, E: Ebersdorf, I: Innsbruck). Below, picture of the Hall view, wheredaily samples (DS) and streaker samples (SS), passive samplers for gaseous pollutants1, D2, D3).

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mostly of stones more than 1m thick, and plugged walls, whichconsist of a 10–20 cm thick brick layer.

The wall paintings were completed in a few months; pigmentswere applied on the dry plaster primarily using the lime-washtechnique, which is considerably less durable than fresco, since thepainting layer is weakly linked to the underlying plaster.

Since the 19th century, several restorations and re-paintings havebeen performed, but documentation of these efforts is unfortunatelyvery scarce, and a detailed overview on the vicissitudes of the differentpaintings is not available. Important restorations took place in 1930sand 1950s; during the latter, re-paintings were removed and thepainting layer and substrate were strengthened. It is worth noting thatorganic bindersmay have been used during these restorations. After thecity flood of 1966, most of the views were detached from their originalsupport and fixed to an insulating Masonite support, to stop the saltmigration from the deeper layers of the wall to the surface.

The Hall view has not been detached and it partially lies over aplugged window, on the West side of the building facing the outside(see Fig. 1b). The outer side of the window was also plugged with athin layer of brick (visible from Piazza della Signoria); in this way, asort of sealed gap was made between the two plugged walls. Asmentioned above, the picture is in grave conservation condition, asthe entire surface exhibits a loss of fragments from the paint layer,chromatic alterations, and soiling. Moreover, an anomalous contrastbetween awide darker zone around the central subject of the painting

Fig. 2. Thermal images of the views of Hall (a) and Konstanz (b), registered inwinter (5th JanHall painting (c). Enlargement of a detail where the contrast between darker and lighter ar

and the external lighter areas can be observed. It is not clear, however,if this difference is due to a darkening of the central area or to a fadingof the external zones. In fact, the painting shows more details in thedarker area than in external zones (see for example the zoom of theview showed in Fig. 2d).

2.2. Thermographic analysis

Thermal images were recorded during three different periods ofthe year (5 January 2004, 7 June 2004, and 16–17 December 2004),and at different times of the day (morning, midday, sunset), in orderto detect the wall response to changes in environmental conditions.

Thermographic images were taken using a long-wave Avio TVS700thermal camera, equipped with a micro-bolometric MCT (HgCdTe)detector operating in the 8–14 μm infrared band with 0.07°Ctemperature resolution and 1.4mrad spatial resolution.

2.3. Microclimatic measurements

Microclimate monitoring was carried out during the winter (18December 2003–7 January 2004 and 16 December 2004–13 January2005), and the summer (24 June–21 July 2004).

In these periods, key thermo-hygrometric parameters were con-tinuously measured every 10min, in correspondence with three pointson the view (M1,M2 andM3 in Fig. 1c). Air temperature (T) and contact

. 2004, at about 10:00 a.m.). Superimposition of the picture and the thermal image of theeas can be clearly observed (d).

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temperature (CT) were measured with platinum thermoresistances(Pt100, accuracy 0.2 °C, time constant 1 s), and relative humidity (RH)wasmeasured using thin-film capacitive sensors (accuracy 2% RH in therange 10–95%). Other important parameters, such as air specifichumidity (SH) and dew point, were theoretically calculated (Bernardi,2008). It was not possible to measure CT on the painting area (pointM3), due to the advanced level of degradation, so two differentmeasuring points (M1 and M2) were chosen on the view frameaccording to the results of the thermographic analysis. Point M1 wasidentified on an area characterised by the same thermal level of pointM3, i.e. on the plugged zone of the window, while point M2was on theoriginal wall.

2.4. Particulate matter and gaseous pollutants

Airborne particle samplings were done during the winter (13December 2003–12 January 2004) and summer (22 June–21 July2004). Aerosol sampling inlets were placed at 3 m above the floor,near the Hall city view (Fig. 1c).

PM2.5 (particleswith aerodynamic diameterdaeb2.5 μm)samplingswere carried out daily from 00:00 to 24:00 local time, on Teflon andquartz fibre filters using two sequential CEN-equivalent samplers (flowrate: 2.3 m3h−1) working in parallel. The choice of collecting PM2.5, i.e.the fine aerosol fraction, was motivated by the fact that it has beenrecognised as an important fraction in the deposition mechanismoccurring on vertical surfaces (Nazaroff and Cass, 1989); the literatureindicates that the soilingper unit time, andunit surface, ismaximised forsub micrometric particles (Camuffo et al., 1999). Fine particles have along residence time in the atmosphere, which means they reach highelevations (the frescos in the Courtyard are at about 3 m above theground). Furthermore, carbon fractions (which cause the soiling andblackening of themonuments) and soluble acid aerosols, like sulphates,are generally abundant in fine particulate matter and, because of this,PM2.5 is a large fraction of PM10 in urban areas.

Since the aerosol composition may show variations on very shorttemporal scales in urban environments, on the order of the hours (forexample, some elements peak during traffic rush hours), andconsidering that the presence of tourists or particular events canproduce re-suspension and/or input of particles in the Courtyardduring specific periods of the day, time-resolved aerosol sampleswerealso collected. Fine (daeb2.5 µm) and coarse (2.5bdaeb10 µm) PM10fractions were sampled with an hourly resolution, using a streakersampler (Chiari et al., 2006). Further details on this device, its cut-offdiameters, and its control unit can be found elsewhere (Prati et al.,1998). One-hour resolution fine and coarse PM10 fractions wereanalysed by PIXE (Particle Induced X-ray Emission) at the LABEC-INFNaccelerator facility in Florence, Italy (Calzolai et al., 2006), and theconcentration of 19 elements (Na, Mg, Al, Si, S, Cl, K, Ca, Ti, Cr, Mn, Fe,Ni, Cu, Zn, Br, Sr, Ba, and Pb) was measured. It should be noticed thatthe mass concentration in the streaker samples is not measurable.

PM2.5 mass concentration was gravimetrically determined on 24-hour samples by an analytical balance (sensitivity 1 μg), in an air-controlled clean room (T=20±1 °C, RH=50±5%). Teflon filterswere analysed by ED-XRF (Energy Dispersive X-Ray Fluorescence) toobtain elemental concentrations for Mg, Al, Si, S, Cl, K, Ca, Ti, Cr, Mn,Fe, Ni, Cu, Zn, Br, and Pb (Marcazzan et al., 2004). Since this techniqueis not destructive, the same samples were also analysed by IC (IonChromatography) to determine the concentrations of water-solubleanions and cations (Fattori et al., 2005).

Samples collectedonquartzfibrefilterswere analysed by a thermal–optical method (Sunset Laboratory Inc., USA), following the NIOSHprotocol for elemental (EC) and organic (OC) carbon quantification(Birch and Cary, 1996).

The comprehensive chemical characterisationwasused to obtain theaerosol mass closure; in other words, it was possible to reconstruct themeasured PM mass by the sum of its main components: sulphate,

nitrate, ammonium, soil dust, EC and organic matter (OM). Sulphate,nitrate and ammonium were measured by IC. The soil component wascalculated from ED-XRF data using the sum of oxides algorithm(Marcazzan et al., 2001). The OC concentrations were converted inparticulate OM concentrations using an average mean molecular-to-carbon ratio of 1.4, as a compromise between the minimum ratioproposed in (Turpin and Lim, 2001) for urban aerosol and themaximumvalue to not exceed the PM mass for all sampling days.

During both campaigns, gaseous pollutants concentrations (NO2,SO2, O3) were measured on a weekly basis via passive samplers(Passam®) (Hangartner, 2006), with an experimental uncertainty ofabout 20%.

2.5. Dry deposition analysis

After a visual inspection to identify the most relevant zone for adiagnostic analysis, five dry deposition samples were collected on thehorizontal surface of the frames located immediately below the Hallview (D1, D2 and D3, as shown in Fig. 1c) and the contiguous Neustadtview (D4 and D5, at the same height of D1 andD2, respectively). Usinga paintbrush, a quantity ranging from about 200 to 600 mg for eachsample was collected. After the sampling, the specimens were storedin a nitrogen environment until analysis.

The samples, grounded into powder, were analysed by XRD (X-RayDiffraction), using a Philips PW 1730 diffractometer for crystallinephase identification, and by DTA–TGA (Differential Thermal Analysisand Thermal Gravimetric Analysis), using a Mettler Toledo TGA/SDTA851, to study their thermal behaviour and to identify the possiblepresence of carbon compounds. The soluble ions concentration wasmeasured by IC, using a Dionex 4500i chromatograph. The depositwas imaged using SEM–EDS (Scanning Electron Microscopy–EnergyDispersive Spectrometry), to detect atmospheric particles depositedon the frame surface. Finally, carbon fractions, that is total carbon(TC), carbonate carbon (CC) basically due to the underlying materials(plaster), non-carbonate carbon (NCC) originating from atmosphericdeposition, organic carbon (OC) and elemental carbon (EC), of bothnatural and anthropogenic origin, were quantified by the methodol-ogy described in Ghedini et al. (2006). Carbon measurements in drydeposition samples were carried out by means of combustion analysisusing a CHNSO EA 1108 Fisons analyser.

Compositional analyses (SEM–EDS and micro-PIXE) and opticalmicroscopy were also performed in the past on samples collectedfrom the Hall painting (Giuntini, 2008). To obtain transversal sectionsof the picture layer, samples were prepared using the cross sectiontechnique. Concentration maps with 10 μm spatial resolution wereobtained for all the elements with atomic numbers ZN10 by micro-PIXE measurements (Giuntini et al., 2007).

3. Results and discussion

Thermographic measurements, from different parts of the Miche-lozzo's Courtyard, confirmed the presence of wall plugs in correspon-dence with the original windows (which are still visible fromoutside), because of their lower thermal capacity, i.e., higher reactivityto the day–night cycle of temperature, in comparison with the thickerlateral walls on both sides of the plugs.

Fig. 2a–b illustrates thermal images of the Hall and Konstanzviews, taken in the winter (at about 10:00 a.m.). In this situation, thecolder zones were those lying over the plugged walls. For theKonstanz view, the shape of the window can be clearly seen as avertical blue element, and the sharp borders of the frame (thehorizontal rectangular element) are congruent with the documenteddetachment of this painting. A similar situation was observed forseveral other views painted on the walls of the Courtyard.

The Hall view has a different circumstance: the borders of the wallplug are very irregular, suggesting a toothing between the plug and

Fig. 3. Specific humidity (a) and contact temperature (b) temporal patterns.

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the original wall, and therefore a different building technique.Furthermore, the absence of a sharp gradient between the view areaand the frame confirmed that this painting is still on its originalsupport. It is worth noting that the plugged wall area fits the darkerzone of the painting exactly; differences in conservation conditions ofthe darker and lighter areas are therefore clearly connected todifferences in the wall structures (Fig. 2c–d).

Fig. 4. PM 2.5 daily concentrations (in μg/m3) measured in this study (Palazzo Vecchio),concentrations measured in some monitoring stations in Florence are also reported: siteGramsci) and site D (Via Ponte alle Mosse): traffic.

Finally, thermal images taken throughout the monitoring perioddid not give evidence for the phenomena of capillary rising in thewalls up to the paintings. Nevertheless, a colder area was sometimesidentified, which corresponded to the junction between the verticalwall of the Hall painting and the ground, suggesting the presence of anexternal source of water nearby the ground. It is worthwhile tomention that during the experimental campaigns traces of water on

during the winter (above) and summer (below) campaigns. For comparison, PM2.5A (Giardino di Boboli): urban park; site B (Via Bassi): urban background; site C (V.le

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the groundwere observed several times, probably due to thewateringof the plants in the courtyard.

Since the Courtyard is a semi-confined place, it is influenced byatmospheric conditions with daily and seasonal variations; thus, thethermo-hygrometric cycles measured near the paintings were directlycorrelated to the external ones. However, the presence of thesurrounding building dampened the cycles, which resulted reduced inamplitude, with lower maximum values.

During both seasons, air temperatures in correspondence to thethree different measuring points (M1, M2 and M3, see Section 2.3)resulted in quite good accordance. From the hygrometric perspective,the three points exhibited similar trends, with a higher variabilityduring summer, due to the typical atmospheric instability of thisseason, but point M3 was characterised by significantly lower values(especially in winter) (Fig. 3a). The contact temperature temporalpatterns in points M1 and M2 were similar (Fig. 3b); however, it canbe clearly noted that point M1 (plugged window) was characterisedby wider thermal excursions with respect to point M2 (original wall),because of the reduced thickness and different material of theunderneath structure.

In both seasons, surface condensation at points M1 and M2 wasnot favoured by the microclimatic conditions, because the differencebetween contact temperature and air dew point was always positive(N3 °C in winter and N7 ° C in summer).

In addition, the condensationwas less probable in the central area ofthe view with respect to the frame. In fact, the difference betweencontact temperature (indirectly evaluated via thermographic analysis)and air dew point at point M3 was higher than for points M1 and M2,because of the lower concentration of water vapour in the air inproximity of point M3, especially in winter.

Nonetheless, according to Kelvin's law (Camuffo, 1998; Bernardi,2008), internal condensation might have happened in the micro-pores with a diameter between 0.1 μm and approximately 1 nm. Thesmaller themicro-pore radius, the lower the critical RH value at whichthe condensation process happens.

Since RH in the external part of the viewwas generally higher thanin the central part, a greater range of micro-pores could have beenfilled by water in this area.

According to the results, the external zone that lies on the originalwall might have been more damaged by condensation and hygro-metric cycles, while the central area that lies on the plugged wallmight have been damaged primarily by thermal cycles.

For conservation purposes, it should be taken into account that thecumulative effect of the thermo-hygrometric cycles may lead todifferent, and often irreversible, degradation phenomena, such asageing processes on the colour coating of paintings as well asmechanical changes (i.e. dimensional variations or cracks) (Odlyhaet al., 1997; Bernardi, 2008).Moreover, thedamage strongly depends onthe type of material of which the object is made of (e.g. the organicmaterials are extremely sensitive to relative humidity changes) and onthe thermal inertia of the object itself (e.g. the central part of the Hallview has a low thermal inertia, so it is sensitive to thermal variations).

Table 1PM2.5 composition.

Winter Summer

Ammonium 1.5 (5 %) 1.6 (9 %)Sulphate 2.2 (8 %) 5.2 (27 %)Nitrate 3.9 (11 %) 0.1 (1 %)Elemental Carbon 1.8 (6 %) 0.7 (5%)OM 19 (55 %) 6.4 (38 %)Crustal/soil 0.6 (2 %) 0.8 (5 %)Not detected 3.7 (13 %) 2.5 (15 %)

Average concentrations (μg/m3) of the major aerosol components, in the two seasons.Averages of the percentage contributions to the weighted PM2.5 mass are also reported(in brackets).

It is noteworthy that, as a consequence of the wider thermal cycles,particle deposition processes due to thermophoresis are morefavoured in the central area of the view, where this behaviour couldexplain why this zone is darker than the external section. On the otherhand, the enhanced effects of condensation (superficial and internal)and hygrometric cycles could explain the greater loss of details in theexternal part of the painting.

PM2.5 mass concentrations registered in the Courtyard showed atemporal pattern and absolute values remarkably similar to thosemeasured at different urban background monitoring stations locatedin Florence (Fig. 4). PM2.5 showed the highest mass concentrations(up to 76 μg/m3) in winter during days with stable meteorologicalconditions (i.e. lower vertical atmospheric circulation), while thelowest PM levels were registered in the summer, mainly due to thehigher atmospheric mixing layer height. Winter and summer averagemass concentrations values were 32 μg/m3 and 17 μg/m3, respec-tively. Sometimes, PM2.5 levels inside the Courtyard were evenhigher than those recorded at a kerbside monitoring station; this canbe explained considering that the removal processes are less effectivein a semi-confined environment, and particulate matter accumulationprocesses may occur in the Courtyard, especially beneath the portico.

Gaseous pollutant monitoring gave average concentrations of SO2,NO2, and O3 of 1.4 μg/m3, 47 μg/m3, and 13 μg/m3, respectively, in thewinter. In the summer, values of 0.5 μg/m3, 30 μg/m3, and 56 μg/m3,respectively, were recorded. Similarly to PM2.5 mass concentrations,these values were similar to those measured at different monitoringstations by the air quality network in Florence.

A detailed chemical characterisation of atmospheric aerosols isimportant, because several chemical compounds (especially organicand elemental carbonaceous particles, and soluble ions) can beconsidered threatening to the preservation of wall paintings. ThePM2.5 chemicalmass closure results are summarised in Table 1 in termsof the absolute and percentage contributions of its major components.The PM2.5 composition was characterised by a relevant contribution oforganic matter (55% of the PM2.5 mass in winter and 38% in summer).OCwas themost abundant fraction (13.6 μg/m3 inwinter and 4.6 μg/m3

in summer), while EC was about 5% of the PM2.5 mass (1.8 μg/m3 inwinter and 0.7 μg/m3 in summer). Similar OC and EC values were foundin the centre of Florence during a previous study (July 2002–July 2003):~1.8 μg/m3 of EC and ~7.5 μg/m3 of OC (Cachier et al., 2004).

Carbon components are relevant to conservation studies; theirimportance in urban aerosols and their impact on cultural heritage iswell documented in the literature (Sabbioni et al., 2003; Bonazza etal. 2005, and references therein). EC originates from primary particleemissions, and consists of highly polymerised, graphitic substances(Cachier, 1998; Chow et al., 2001). Organic aerosols can be primary(i.e. directly emitted into the atmosphere) and/or secondary (i.e.originated by gas-to-particle conversion processes). EC plays animportant role in surface soiling and black crust formation (Lanting,1986; Sabbioni 2002; Bonazza et al. 2007); OC particles form acomplex evolved mixture, which could be the main agent respon-sible for the “laser yellowing” (Cachier et al., 2004). It is worthnoting that EC and other elements/compounds emitted chiefly bycombustion processes are generally found in the fine particlefraction, whose deposition occurs on surfaces of any orientation bya combination of advective diffusion and thermophoresis (Nazaroffet al., 1990). A study carried out in Florence (Monforti et al., 2004)showed that EC and organic compounds constitute more than 50% ofthe mass reaching monuments through particle deposition (exceptwater).

Currently, there is little quantitative data on the correlationbetween blackening and black crust composition; however, Bonazzaet al. (2005) demonstrated that damage layers are a record of theenvironmental changes over time, with their chemical compositionreflecting atmospheric combustion sources, and soiling due to OC willlikely prevail in the next future.

Fig. 5. Hourly concentrations of S in the fine fraction (a), Si and Ca in the coarse fraction (b), and Si in the fine fraction (c) during the winter period. Vertical lines indicate midnight.

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In this study, a high correlation between OC and K was measuredin PM2.5 samples during the winter (r=0.96), but not during thesummer, suggesting a significant contribution from biomass burning.Although K is also amajor component of soil dust, in this case it can beconsidered as a tracer of biomass burning for manifold reasons: it hasbeen detected in the fine fraction (while soil dust is mainly in thecoarse fraction); it showed higher concentrations in winter (winter tosummer ratio equal to ~3); the K/Al ratio resulted ~40 folds greaterthan typical values for the soil crust composition (so it is mainly non-soil K); finally, by a comparison between total K (detected by ED-XRF)and soluble K (detected by IC), we found that it was mostly (about90%) soluble. It is noteworthy that the contribution from biomassburning in Florence was highlighted also in a previous studyconcerning the aerosol composition near the Cathedral (Cachieret al., 2004).

Results for the ionic composition of PM2.5 samples showed thatmajor soluble ions were sulphate, nitrate, and ammonium. The ionicbalance of these compounds was quite good (with an average ~20%ammonium deficit), suggesting that they were mainly present in theammonium nitrate and ammonium sulphate forms. Since the anions/cations were well balanced (with a cationic deficit of ~10%), adverseeffects due to aerosol acidity should be negligible. However, the highsolubility of ammonium sulphate might cause SO4

2− release inaqueous solutions (e.g. in pores where water can be present).Moreover, formation of CaSO4•2H2O may occur, where this is a wellknown efficient medium for blackening by embedding soot duringcrystal growth (Sabbioni et al., 1996).

Atmospheric sulphate and nitrate exhibited very different seasonalbehaviour (Table 1). The strong decrease of nitrate in the summerwaslikely due to its volatile nature and to the lack of additional wintertime

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sources (i.e. domestic heating). In contrast, sulphate concentrationsdoubled in the summer because of a more efficient photochemicalactivity in the atmosphere, which favours the oxidation of SO2 intosulphate. It should be taken into account that nitrate has mainly localorigins (i.e. combustion sources), while sulphate is from regionalsources of SO2; consequently, nitrate concentrations were higher inreduced air circulation conditions, while the converse was observedfor sulphate.

The high concentrations of SO42− and SO2 measured in the

Courtyard are of concern because of their well known role in Ca-based material damage by sulphation processes (Sabbioni, 1995;Brimblecombe, 2002), which were detected in this study and arediscussed below.

Looking at the streaker samples, sulphur was mainly present in thefine fraction (~80–90% of total S). In this fraction, the hourly temporalresolution of the samplings indicated the presence of a dominant“background” S concentration, characterised by a slowly varyingtemporal pattern, having a few superimposed peaks (Fig. 5a). Thebackground concentration was likely due to a regional contribution ofSO2, while superimposed S peaks were mainly due to local emissions.This observation suggests that in the Courtyard the sulphate degrada-tion process is always active, and it is unlikely to be reduced by localmitigation actions.

Soil related elements (i.e. Al, Si, Ca, Fe, Ti andMn) were detected inhourly resolved samples mainly in the coarse fraction, suggesting thatthey were predominantly mechanically generated. Their temporalpatterns showed higher concentrations during the daytime (forexample, Si and Ca concentrations in the coarse fraction during thewinter period are shown in Fig. 5b), when the Courtyard wasaccessible to visitors, and when atmospheric turbulence was higher.Low levels were observed during the nighttime when the PalazzoVecchio was closed, and coarse particles deposition was favoured dueto the stable conditions (Chiari et al., 2006). On the 1st of January,Palazzo Vecchio was closed to visitors and no increase in theseelement concentrations was observed during the daytime. Thistemporal behaviour suggests the presence of a dust re-suspension

Fig. 6. Picture of a cross section of a sample collected from the Hall view, and elemental micprobably red ochre, characterised by Fe. The presence of S in the plaster indicates that the

effect attributed to the visitors, although it could be also partiallyascribed to a reduced anthropogenic activity in the city during thenighttimes and New Year's Day.

Hourly resolved analysis of Al and Si in the fine fraction (Fig. 5c)showed a temporal pattern similar to the coarse fraction. The PM10coarse fraction settles mainly on the horizontal surfaces under theinfluence of gravity, while the presence of these elements in the finefraction enhances the possibility of airborne particle deposition alsoon vertical surfaces, such as wall paintings in the Courtyard.

During the campaign some long-range transport phenomena weredetected. One example is a Saharan dust intrusion on the 29th ofDecember, between 3:00 and 5:00 a.m., identified by a simultaneousincrease in the concentration of all soil related elements, and confirmedby the analysis of air mass back trajectories (HYSPLIT, NOAA AirResource Laboratory, Draxler and Hess, 1998). Some marine aerosolepisodeswere also identified, by an increase in theNaCl concentrations,and confirmedbyback trajectory calculations. This is, in principle, highlysignificant from a conservation point of view because sea salt,particularly sodium chloride, may cause substantial damage via saltcrystallization onporousbuildingmaterials and surfaces ofmonuments.However, it should be noted that the absolute impact of thesecontributions (re-suspended soil, Saharan dust and marine aerosol) isquite small (see Table 1, the soil component is only ~5% of PM2.5).

Concerning the analysis of dry depositions (samples D1–D5), XRDanalysis showed that the major mineral phases were quartz, calciteand, to a lower extent, gypsum. Quartz was attributed to airborne soildust deposition, while calcite probably originated from plasterpowdering, since the substratum and the binder of the paintingswere made of calcium carbonate. Minor phases were plagioclase,orthoclase, illite and chlorite (both phyllosilicates). It is worth notingthat the presence of gypsum indicated that the surface is undergoing asulphation process. This result is in agreement with data previouslyobtained by SEM–EDS and micro-PIXE analysis on samples collectedfrom the Hall painting (Giuntini, 2008), which revealed the presenceof sulphur in the plaster, decreasing from the surface to the innerlayers (over a thickness of about 500 µm). As an example, elemental

ro-PIXE maps (S, Ca and Fe) measured on the highlighted red square. The paint layer issurface is undergoing a sulphation process.

Fig. 7. Scanning electron micrograph of spongy carbonaceous particle (a) and corresponding EDS analysis (b). Scanning electron micrograph of metallic particle (c) andcorresponding EDS analysis (d). Scanning electron micrograph of aluminosilicate particle, covered by minerals with irregular shape, mainly gypsum (e) and corresponding EDSanalysis (f). Scanning electron micrograph of phyllosilicate (g) and biological particle (h).

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maps obtained by micro-PIXE analysis of a painting sample crosssection, are shown in Fig. 6.

SEM–EDS observations of dry depositions revealed the presence ofdifferent kinds of particles (Fig. 7): carbonaceous/spherical (a–b),metallic (c–d), aluminosilicate/spherical (e–f), irregular/soil dust (g)and biological particles (h).

The IC analysis carried out on samples D1–D5 showed thatsulphate is the most abundant ion, with concentrations rangingfrom 1.4% to 4%, followed by nitrate (0.21–1.2%), acetate (0.20–0.9%)and chlorite (0.26–0.7%). IC analyses also indicated the presence ofsulphite (up to 800 ppm), with concentration values much lower thanother ions and well correlated to sulphate (correlation coefficient

Fig. 8. Percentages of carbonate carbon (CC), organic carbon (OC) and elemental carbon(EC) in the five collected samples. Total carbon (TC) is the sum of the three contributions,while NCC is the sum of EC and OC.

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over 5 samples: r=0.88). This highlights that the contribution tosulphation from gaseous SO2 dry deposition has an important roletogether with the deposition of particulate sulphate.

Percentage contributions in the dry deposition samples due tocarbon fractions are shown in Fig. 8. Total carbon percentage valueswere higher in deposition samples collected below the Neustadt view(~9% in samples D4 and D5), in comparison to those collected belowthe Hall painting (~5% in samples D1, D2 and D3). In addition, thepartitioning of TC into its components was different for the two sets ofsamples: the fraction of carbonate carbon (CC), mainly due to plasterpowdering, was higher in D1, D2 and D3 (Hall), while the contributionof non-carbonate carbon (NCC, i.e. EC+OC), linked to aerosoldeposition, was higher in D4 and D5 (Neustadt).

4. Summary and conclusions

An environmental characterisation of the Michelozzo's Courtyard(Palazzo Vecchio, Florence) was carried out for the first time. Thenovelty of this project was the use of a holistic monitoring programinvolving several techniques to study a particular and complexcultural heritage site: a semi-confined protected outdoor environ-ment. Results of our monitoring revealed that the air quality in theCourtyard is comparable to the urban environment, and that heavypollution loadings typical of the city likely affect the paintings. Itshould be noted that some effects of pollution might be enhanced inthis peculiar semi-confined environment: removal processes are lesseffective and particulate matter can accumulate. Particles depositedon the walls (soiling effect) in outdoor environments are easilyremoved by wind and rainfall, while under the portico atmosphericagents are less effective. Moreover, the PM2.5 composition wasdominated by chemical compounds that can be considered asthreatening to the preservation of wall paintings. These includecarbonaceous compounds, which cause aesthetic and soiling damage,and soluble ions, which produce salt crystallization and sulphation; inparticular, OC resulted the most abundant component and asignificant and always-present sulphate background was detected.The effect of visitors wasmeasured by the hourly resolution pattern ofsoil related elements, which showed temporal patterns in agreementwith visiting periods.

Thermographic analysis of the Hall view highlighted that thedistribution of the central darker zone in the painting perfectly fits theshape of the window-plugged wall, which was markedly moresensitive than the original walls to thermal changes in the environ-ment, thus suggesting that the peculiar conservation conditions areclearly related to differences in wall structure. This result, confirmedby microclimatic analysis, also implies that deposition processes dueto thermophoresis are favoured in the central area: this behaviourmay explain why this zone is darker than the external section. Otherdegradation phenomena, such as condensation (superficial and

internal) and hygrometric cycles, are likely at play in the externalzones, thus possibly explaining the greater loss of detail observed inthese areas of the painting. In fact, due to condensation the sulphationprocess can extend into the painting substrate, thus weakening it andthreatening its conservation. The analyses performed on the drydeposition samples confirmed that the surface is undergoing asulphation process. In particular, the presence of significant amountsof sulphite (SO3

2−), with concentration values well correlated to thesulphate ones, indicates that sulphate is also produced by theoxidation of sulphite at the painting surface.

In summary, this study identified the peculiar microclimate andpollution conditions of the Courtyard as possible causes of the observeddegradation. The quantitative information obtained by our project is anecessary first step in the elucidation of the relevant environmentalstresses.However, identifying clear cause–effect relationships, aswell assuggesting effective conservation strategies, is not trivial. An exhaustiveanalysis of these topics is not presently possible without furtherinvestigations and the attendance of expert conservators.

Nonetheless, some preliminary considerations can be done. Sincethe Courtyard is an open semi-confined environment, it is obviouslyvery difficult to protect it from adverse atmospheric agents. Theclosure of the door connecting the Courtyard to the outside couldreduce the inlet of particulate matter, but it is an invasive action forthe aesthetic of the building face (it is themain entrance to the PalazzoVecchio); moreover, it is not trivial to find an alternative entrance forboth tourists and people attending the communality services.Furthermore, based on our measurements, we cannot predict itseffectiveness (pollution may enter from the top of the courtyard, and/or from the other door that connects the courtyard to the rest of thebuilding). To reduce pollution and to achieve some microclimatecontrol, conservators should evaluate the possibility of closing boththe entrance and the top of the Courtyard. However, we recognisethat this is a very invasive action on the building architecture.Nonetheless, since a soil re-suspension effect was detected, someregulation of the access to the Courtyard, together with a moreaccurate removing of the pavement dust, is recommended to mitigatethe soiling effect.

We determined that the conservation conditions of theHall view arestrongly dependent on the heterogeneity of the substrate. From thisresult, conservators may evaluate the possibility of detaching thepainting and fixing it to an insulating support, in order to alleviate thedifferences in the preservation conditions of the two parts of the viewand to reduce thermophoretic and condensation processes (and hencedeposition and sulphation). Due to the different vicissitudes andrestorations of the different views, it is difficult to compare themdirectly to each other, although it is worth noting that all the paintingsthat have been detached in the past are in an improved condition.Conservators may also examine the possibility of acting on the pluggedwindow from the outside, but this operation may affect the buildingaesthetics, and, most importantly, it would not reduce the degradationof the painting external zones that lie on the original walls.

Finally, the confirmation of the presence of sulphation is animportant piece of information for conservators, so they can select themost appropriate restoration method.

The use of a holistic monitoring program resulted an effectiveapproach to study semi-confined cultural heritage environments,whose conservation cannot neglect the assessment of microclimateand atmospheric pollutants (which can deposit, accumulate andinteract with the surface of works of art). This approach can be thussuggested for similar investigations. However, based on this experi-ence, other measurements have been identified that should be madein future to refine the monitoring program. In general, an analysis ofsamples collected in areas with different degradation patterns shouldbe done; for example, a sampling in the darker and lighter areasof the Hall painting would help in determining the differentconservation conditions of these two zones. In addition, pore size

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measurements could verify our hypothesis on the condensation inmicro-pores.

To investigate the circulation patterns that transport particulatesinto the portico, and allow them to accumulate there, air circulationmeasurements should be done, as well as a simultaneous sampling ofparticulate matter inside and outside the courtyard. These measure-mentswould also help to investigate the different partitioning of TC intoits components found on theHall andNeustadt paintings by the analysisof dry deposition samples, since it might indicate a different contribu-tion of aerosol deposition on the two adjacent sides of the Courtyardfrom air circulation patterns.

Acknowledgements

The authors wish to thank the “Servizio Fabbrica Palazzo Vecchio eChiese” and, in particular, Mrs. Laura Corti, Dr. Ugo Muccini and Dr.Stefano Rinaldi, for permitting this research campaign in Michelozzo'sCourtyard. The skillful works of Dr. MartaMascalchi in thermal imagesediting and Dr. Francesca Giambi in the analysis of painting crosssections are kindly acknowledged.

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