Call for Data “Inventory and Condition of Stock of Materials at UNESCO World Cultural Heritage Sites”

Home , Aachen Cathedral, Lorsch Abbey, Old City (Bern), Palazzo Madama, Speyer Cathedral

Report No 83: Call for Data “Inventory and condition of stock of materials at UNESCO world cultural heritage sites”. Part II – Risk assessment

September 2018 PREPARED BY THE SUB-CENTRE FOR STOCK OF MATERIALS AT RISK AND CULTURAL HERITAGE

Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Rome, Italy

CONVENTION ON LONG-RANGE TRANSBOUNDARY AIR POLLUTION

INTERNATIONAL CO-OPERATIVE PROGRAMME ON EFFECTS ON MATERIALS, INCLUDING HISTORIC AND CULTURAL MONUMENTS (ICP Materials)

Report No 83

Call for Data “Inventory and condition of stock of materials at UNESCO world cultural heritage sites”

Part II – Risk assessment

Pasquale Spezzano1, Johan Tidblad2, Mirna Bojić3, Zrinka Radunić3, Vanja Kovačić3, Sonja Vidić4, Nina Zovko5, Stefan Brüggerhoff6, Markus Faller7, Ulrik Hans7, Terje Grøntoft8, Jessica Andersson2

1ENEA, Italy 2Swerea KIMAB AB, Sweden 3Ministry of Culture, Croatia 4Meteorological and Hydrological Service, Croatia 5Croatian Agency for Environment and Nature 6Deutsches Bergbau – Museum Bochum, Germany 7Swiss Federal Laboratories for Materials Testing and Research (EMPA), Switzerland 8Norwegian Institute for Air Research (NILU), Norway

ENEA, Rome, Italy September 2018

http://www.enea.it/

Contents

1. Introduction ...... 4 2. Cultural objects ...... 5 3. Assessment of air pollution risks for corrosion of materials ...... 21 3.1 Limestone ...... 21 3.2 Sandstone ...... 27 3.3 Copper ...... 30 3.4 Bronze ...... 33 4. Assessment of air pollution risks for soiling of materials ...... 38 4.1 Limestone ...... 38 4.2 Glass...... 40 5. Discussion of air pollution risks to materials ...... 45 6. Comparison of predicted degradation rates: local data vs. EMEP model data...... 48 7. Conclusions ...... 55 8. References ...... 56

1. Introduction

The International Co-operative Programme on Effects on Materials, including Historic and Cultural Monuments (ICP Materials) was launched in 1985 within the scope and the activities of the Convention on Long-range Transboundary Air Pollution. The aim of the Programme is to fill some of the major gaps in scientific knowledge in the area of materials corrosion influenced by atmospheric pollutants by performing a quantitative evaluation of multi-pollutant effects on atmospheric corrosion on both technically important materials and materials used in historic and cultural monuments.

Many of the materials used in the construction of historic and cultural monuments are very sensitive to air pollution resulting in corrosion and soiling of the materials that were used to create the artefacts. The Programme, through the Sub-Centre for stock of materials at risk and cultural heritage, agreed to launch a Call for Data on “Inventory and condition of stock of materials at UNESCO world cultural heritage sites”. UNESCO sites are considered of outstanding universal value and are therefore ideal for illustration and dissemination of effects of air pollution on materials. The Call was approved at the 1st joint session of the Steering Body to the EMEP and the Working Group on Effects (Geneva, 14-18th September 2015) and then launched in October 2015.

Documents provided by the Call included the Call text, the reporting template, an explanatory note with guidance on the use of the reporting template, and a brochure on a pilot study “Inventory and condition of stock of materials at risk at five UNESCO world cultural heritage sites”, exemplifying the approach. In addition, a page dedicated to the Call for Data was added to the ICP Materials website (http://www.corr-institute.se/icp-materials), where all documents and some examples of the reporting template were available for downloading. The call was closed in June 2017.

Six Parties to the Convention; Croatia, Germany, Italy, Norway, Sweden and Switzerland, provided qualitative and quantitative data on both historic/cultural monuments and on concentrations of main air pollutants and meteo-climatic parameters measured at monitoring stations close to the selected UNESCO world cultural heritage sites. Taken together, the twenty-one cultural heritage objects included in the call have a total external surface area of about 430,000 m2 and cover a wide range of materials (natural stone, artificial stone, copper, bronze, glass and others) as well as a wide range of environmental conditions. Detailed information on monuments/sites and their selection can be found in a thematic report, ICP Materials Report No 80: Call for Data “Inventory and condition of stock of materials at UNESCO World Cultural Heritage sites”. Part I - Status Report, released in September 2017.

The purpose of this report (Part II) is to assess the potential risk of damage due to air pollution on the materials of which these twenty-one cultural objects are made by using dose-response functions established by ICP Materials. The degradation rates predicted on the basis of these functions and using the environmental parameters collected during the Call for data, calculated for the materials constituting the twenty-one cultural objects, were compared with the target (tolerable corrosion rate) suggested by ICP Materials for protecting cultural heritage monuments for 2050. This comparison was then used to assess the relative risks due to air pollution for different pollutants and different materials.

Future reports planned to be launched are Part III on Economic Evaluation and Part IV on the relationship between the environment and the artefact.

2. Cultural objects

The list of the twenty-one cultural objects considered in the call for data is shown in Table 1. The beauty and uniqueness of these monuments can be appreciated from the Figures 1-21. Table 2 summarizes the occurrence of the different materials in these monuments of outstanding universal value. A more detailed description of the materials can be found in the previous ICP Materials report (ICP Materials, 2017).

Taken together, the twenty-one cultural heritage objects have a total external surface area of about 430,000 m2. Not surprisingly, a high percentage of the external surfaces of this sample of built heritage, about 60%, consists of natural stone, mainly several varieties of sandstone and limestone but also talc-schist, tufa, slate (mainly for roofing) and other ornamental stone varieties, which reflect the geo-diversity and the availability of stone resources in a determined region or territory. The different stone materials that are found in built heritage are an important element in the cultural heritage of people. The exterior of the considered monuments is often finely decorated and adorned by statues, bas-reliefs, columns, capitals, etc. Carved stone is very delicate and more vulnerable to damage than bulk masonry.

Artificial stone materials such as ceramic (bricks, “terracotta” tiles), plasters, mortars, and cement- based concrete also have a large presence in the exteriors of the monuments covered by the study (about 17% of the total external surface). Artificial stone materials have different characteristics with respect to natural stones. Sometimes, roofs of buildings are covered with waterproof materials based on asphalt, bitumen or tar (about 2.5% of the total external surface of the monuments considered in the present study).

In some cases a certain fraction (a little less than 2%) of the outer surfaces of the monuments is painted. Most of the decorations and paintings are inside the buildings and therefore not directly exposed to outdoor concentrations of atmospheric pollutants and other external deteriorating parameters (wind, rain, salt crystallization, presence of lichens or moss, cycles of wetting and drying, etc.).

Regarding the use of metals, copper and bronze are widely present in the monuments considered. In addition, decorative objects of historic and artistic interest may include other metals such as iron and lead. Copper is a constituent element of the roof in eight cultural buildings and accounts for about 3% of the total outer surface. Bronze is mainly present in belfries in the form of bells. However, it was not possible to evaluate their overall surface. Historic bells and the frames they hang in are an important part of our heritage and should be cared for appropriately.

The twenty-one cultural heritage objects characterized in the present call for data are particularly rich in windows containing glass and especially stained glass windows. Glass is present in fourteen monuments and accounts for about 6.5% of the surface exposed to the outdoor environment. Historic glass is one of the most fragile materials and is particularly susceptible to soiling due to the airborne particulate contamination.

Table 3 summarizes the environmental parameters in the surroundings of the studied monuments. The twenty-one cultural objects are located in different environments that can be characterized as urban, suburban or rural. Monuments located in very clean air areas have been deliberately excluded during site selection as the relative risk of damage due to air pollution is definitely lower.

3 In areas close to the investigated monuments, SO2 concentrations range from 0.1 to 7 μg/m , NO2 is 3 3 3 between 5 and 65 μg/m , O3 in the range 19-96 μg/m and PM10 in the range 8-40 μg/m . It should be

noted that nowadays SO2 is measured only at a limited number of air quality monitoring stations. SO2 has been stable at a low level for years and its significance for air quality health assessment is in most cases no longer relevant. In addition, air quality monitoring networks prefer background stations to traffic stations for ozone measurement. Therefore, for many monuments located in urban areas data on nearby ozone concentrations are unavailable. Missing SO2 and O3 data were obtained from the EMEP/MSC-W model for the year 2015 at the new resolution of 0.1º x 0.1º longitude- latitude.

Atmospheric HNO3 concentrations are not measured regularly by air quality networks. Annual HNO3 concentrations were approximated by using the empirical equation:

-3400/(T+273) 0.5 [HNO3] = 516 e ([NO2] [O3] RH) developed by the MULTI-ASSESS project (MULTI-ASSESS, 2015), where [HNO3], [NO2] and [O3] are the annual concentrations of nitric acid, nitrogen dioxide and ozone, respectively, RH is the relative humidity (%) and T is the temperature (°K). pH of rainwater was also missing for some sites. These missing data were obtained by interpolating the hydrogen ion concentration calculated from pH values collected in a database with values from approximately 200 sites, including stations of the EMEP and the ICP Materials network. The interpolation was performed using the Open Source Geographic Information System QGIS 2.18 software (QGIS, 2018) according to the Inverse Distance Weighting (IDW) algorithm. An output raster layer at a resolution of 0.1° x 0.1° longitude-latitude was produced.

Regarding climate factors in the surroundings of the twenty-one cultural heritage objects, the average annual temperature ranges between 4.5 and 19.1 °C, relative humidity is between 56 and 85 % while the amount of precipitation ranges between 400 and 1343 mm per year.

Table 1. List of the cultural objects.

Country Name of the UNESCO World Cultural Heritage Site Name of the cultural object Location Latitude Longitude Croatia Historical Complex of Split with the Palace of Diocletian Cathedral of Saint Domnius Split 43.5094 16.4433 Germany Aachen Cathedral Aachen Cathedral Aachen 50.7744 6.0844 Germany Speyer Cathedral Speyer Cathedral Speyer 49.3167 8.4430 Germany Würzburg Residence with the Court Gardens and Residence Square Würzburg Residence Würzburg 49.7928 9.9389 Germany Roman Monuments, Cathedral of St Peter and Church of Our Lady Porta Nigra Trier 49.7500 6.6333 in Trier Germany Town Hall and Roland on the Marketplace of Bremen Town Hall of Bremen Bremen 53.0759 8.8075 Germany Wartburg Castle Wartburg Castle (palace and keep) Eisenach 50.9668 50.9668 Germany Bergpark Wilhelmshöhe Hercules Monument Kassel 51.3158 9.3931 Germany Abbey and Altenmünster of Lorsch The Gatehouse of Lorsch Abbey Lorsch 49.6537 8.5686 Italy Historic Centre of Rome, the Properties of the Holy See in that City The Colosseum Rome 41.8902 12.4923 Enjoying Extraterritorial Rights and San Paolo Fuori le Mura Italy Piazza del Duomo, Pisa The Tower of Pisa Pisa 43.7230 10.3964 Italy Residences of the Royal House of Savoy Palazzo Madama Turin 45.0725 7.6857 Italy Cathedral, Torre Civica and Piazza Grande, Modena Ghirlandina Tower Modena 44.6462 10.9257 Italy 18th-Century Royal Palace at Caserta with the Park, the Aqueduct Royal Palace Caserta 41.0733 14.3264 of Vanvitelli, and the San Leucio Complex Norway Rjukan-Notodden Industrial Heritage Site Hydroparken Notodden 59.8786 8.5936 Norway * Nidarosdomen Trondheim Sweden Royal Domain of Drottningholm Drottningholm Palace Theatre Ekerö 59.3231 17.8833 Sweden Church Town of Gammelstad, Nederluleå church Luleå 65.6461 22.0286 Sweden Hanseatic Town of Visby Wall of the Hanseatic Town of Visby Gotland 57.6417 18.2958 Switzerland Abbey of St Gall Towers of the cathedral of the Abbey St Gall 47.4233 9.3778 of St. Gall Switzerland Old City of Berne Bern Minster Bern 46.9481 7.4503 * Nidarosdomen is not a UNESCO site. It is considered to be the world’s northern most medieval cathedral. As the most monumental and well known stone heritage building in Norway it was considered to be interesting in this context.

Table 2. Occurrence of materials in the twenty-one cultural objects.

Name of the cultural object Total Limestone Sandstone Render, Brick Glass Copper Bronze Other surface marble (m2) mortar, (m2) (m2) (m2) (m2) (m2) (m2) plaster (m2) Cathedral of Saint Domnius 1960 1385 - 42.99 492.08 13 - Yes Wood, steel Aachen Cathedral 17300 3287 7698 - 17 1557 - Yes Greywacke, trachyte, tuff, granite, lead, slate Speyer Cathedral 26000 16900 - - 1040 7800 Yes Slate Würzburg Residence 41100 1027 19522 5959 - 3493 - - Painted surfaces, slate Porta Nigra 5500 4840 - - - 660 - - Town Hall of Bremen 4060 1868 41 690 244 1218 - Painted surfaces, Wartburg Castle (palace and keep) 4300 201 2165 - - 120 765 - Wartburg-Konglomerat Hercules Monument 15100 - 0 - - - 151 - Tuff The Gatehouse of Lorsch Abbey 570 17 120 200 - Yes - - Slate The Colosseum 22750 19450 - - Yes - - - Roman concrete, tuff The Tower of Pisa 7735 7735 - - - - - Yes - Palazzo Madama 7300 2700 - 900 3500 200 - - Gneiss, terracotta tiles Ghirlandina Tower 2650 2623 27 - - - - Yes Trackyte, lead Royal Palace of Caserta 149800 54700 - 4000 28500 17400 - - Steel, terracotta tiles, wood Hydroparken 40460 - - 11220 1460 2820 - - Painted surfaces, Tar paper/ruberoid, concrete Nidarosdomen 4430 60 60 - - 94 1165 Yes Talc-schist, slate Drottningholm Palace Theatre 4205 - 55 1577 - 288 - - Painted surfaces, wood Nederluleå church - - 2603 - 175 2 Yes Painted surfaces, wood, 3166 tarred shingles Wall of the Hanseatic Town of Visby 62000 58900 - 3100 - - - - - Towers of the cathedral of the Abbey 3150 158 2918 - - - 75 Yes - of St. Gall Bern Minster 8980 240 8740 Yes - Yes - Yes Lead

Table 3. Environmental parameters in the surroundings of the twenty-one cultural objects.

Name of the UNESCO cultural SO2 NO2 O3 PM10 HNO3* T RH Rain Rain object µg/m3 µg/m3 µg/m3 µg/m3 µg/m3 °C % mm pH Cathedral of Saint Domnius 4 24 96 19 1.53 17.5 56 798.9 5.6 Aachen Cathedral 1.5** 14 46 14 0.71 10.3 79.1 826.5 5.2 Speyer Cathedral 3.1** 29 37 18 1.03 13.3 73.3 431.4 5.2 Würzburg Residence 0.9** 42 62** 23 1.43 10.3 77.9 550.8 5.2 Porta Nigra 1 30 64** 18 1.21 10.2 79.8 751.3 5.2 Town Hall of Bremen 2 23 46 17 1.06 10.1 80.6 546.7 5.2 Wartburg Castle (palace and keep) 0.5** 15 44 15 0.69 9.3 79.3 511.7 5.2 Hercules Monument 2 22 42 17 0.84 9.2 79.1 645.3 5.2 The Gatehouse of Lorsch Abbey 1.4** 33 59** 19 1.29 11.4 78.3 680.1 5.2 The Colosseum 2.3** 65 50** 31 2.15 19.1 68.4 735 5.8*** The Tower of Pisa 1.4** 37 62** 29 1.75 16.7 78.3 887.6 5.6*** Palazzo Madama 7 53 24** 40 1.19 14.8 74.7 961.6 5.4*** Ghirlandina Tower 0.8** 32 43 31 1.21 14.5 75 592.4 5.6*** Royal Palace of Caserta 2.8** 26.4 53.7 21.3 1.53 18.5 83.5 873 5.5*** Hydroparken 1 5 55 8 0.38 5.3 80 737 5.0 Nidarosdomen 1 51 18.9 12 0.73 6.5 76 751 5.3 Drottningholm Palace Theatre 2 15 30 12 0.56 8.7 79.6 656 5.0 Nederluleå church 1.9 11.6 54.7 12.4 0.57 4.5 83.5 824 5.0 Wall of the Hanseatic Town of Visby 0.7 5.7 64.1 37.4 0.48 7 85 399 4.9 Towers of the cathedral of the Abbey 1.8 29.5 44.2 12.8 0.97 9.66 76.13 1343 5.4*** of St. Gall Bern Minster 1.7 25 38 16 0.86 10.05 80.15 1034 5.4***

-3400/(T+273) 0.5 * Calculated with the empirical function (MULTI-ASSESS, 2005): [HNO3] = 516 e ([NO2] [O3] RH) ** Modelled data (EMEP/MSC-W model for the year 2015 at the resolution of 0.1º x 0.1º longitude-latitude), see Table 10. *** Interpolated values, for further explanation see text

Figure 1. The Cathedral of Saint Domnius, Split

Figure 2. Aachen Cathedral (copyright: Domkapitel Aachen, Dombauhütte)

Figure 3. Speyer Cathedral (Image by Alfred Hutter, source: www. wikipedia.de)

Figure 4. Garden facade of Würzburg Residence (Image by Rainer Lippert, source: www. wikipedia.de)

Figure 5. South-facade of the Porta Nigra in Trier (Image by Michael Auras)

Figure 6. South-facade of the Town Hall of Bremen (copyright: Landesamt für Denkmalpflege Bremen)

Figure 7. Palas (with Knights bathhouse in front) and castle keep of Wartburg Caste (copyright: Wartburg-Stiftung)

Figure 8. Hercules-Monument (image by Hendrik Thole, source: www. wikipedia.de)

Figure 9. West-facade of the Gatehouse of Lorsch Abbey (image by Hans Joosten, 2014)

Figure 10. Colosseum exterior, inner and outer walls (Photo by Adrian Pingstone1)

1 https://commons.wikimedia.org/wiki/File:Colosseum.rome.arp.jpg This work has been released into the public domain by its author. 14

Figure 11. View of the Leaning Tower and the Duomo in Pisa (Photo by Yair Haklai2)

Figure 12. Palazzo Madama, Turin (Photo by Adelina Rossano3)

2 https://commons.wikimedia.org/wiki/File:Leaning_Tower-Pisa.jpg By Yair Haklai (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons. 3 https://commons.wikimedia.org/wiki/File:Palazzo_Madama_facciata.jpg By Adelina Rossano (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons 15

Figure 13. The Cathedral in Modena with the Ghirlandina Tower (Photo by Biancamaria Rizzoli4)

Figure 14. The main façade of the Royal Palace of Caserta (Photo by Antonio Gentile5)

4 https://commons.wikimedia.org/wiki/File:Emilia-Romagna_Modena_Duomo_Abside_e_Ghirlandina.JPG By Biancamaria Rizzoli (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons 5 http://www.reggiadicaserta.beniculturali.it/wp/ 16

Figure 15. Aerial view of Hydroparken

Figure 16. A panorama of Nidarosdomen, Trondheim (Photo by Eikern6)

6 https://commons.wikimedia.org/wiki/File:NidarosdomenPanorama.jpg By Eikern (Own workby uploader) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons 17

Figure 17. The south facade of Drottningholm Palace Theatre (Photo by Arild Vågen7)

Figure 18. Nederluleå church in Gammelstad church town (Photo by Lars Falkdalen Lindahl8)

7 https://commons.wikimedia.org/wiki/File:Drottningholm_June_2013_07.jpg By Arild Vågen (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons 8 https://it.wikipedia.org/wiki/File:Nederlule%C3%A5_church_October_2011.jpg By Lars Falkdalen Lindahl (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons 18

Figure 19. Visby town wall (Photo By En-cas-de-soleil9)

Figure 20. The towers of the cathedral of the Abbey of St. Gall (Photo by Petar Marjanovic10)

9 https://commons.wikimedia.org/w/index.php?curid=27992087 By En-cas-de-soleil (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons. 10 https://commons.wikimedia.org/wiki/File:Stiftskirche_St.Gallen.jpg By Petar MARJANOVIC (edit von Image:StiftskircheSt.Gallen.jpg) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC- BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY 2.5 (http://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons. 19

Figure 21. The Cathedral (Münster) of Bern (Photo by Maksym Kozlenko11).

11 https://commons.wikimedia.org/wiki/File:M%C3%BCnster_(Bern).jpg By Maksym Kozlenko (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons. 20

3. Assessment of air pollution risks to materials – surface recession and corrosion

In this report the collected environmental data presented in table 3 have been used to calculate surface recession of two stone materials (limestone and sandstone) and corrosion of two metals (copper and bronze). The calculations are based on dose-response functions from ICP Materials that include the effect of different pollutants, for example SO2, PM10 and HNO3. Thus, it is possible not only to calculate the total degradation but to estimate the relative importance of these different pollutants. There are some limitations with this approach, in particular the lack of an available dose- response function for sandstone in the multipollutant situation and the long term importance of ozone for the corrosion of copper, discussed more in depth below under the individual material.

3.1 Limestone

Fourteen of the twenty-one cultural objects considered in the call for data contain calcareous stones in their outer surface. The list is shown in Table 4, which gives the total area considered and the surface composed of limestone, in absolute amount (the area of stock at risk) and as a percentage. The percentage of limestone varies greatly in these cultural objects, going from a few percentage points (for example Nidarosdomen in Norway, Würzburg Residence in Germany and Bern Minster in Switzerland) up to represent a large percentage (the Cathedral of Saint Domnius in Croatia, the Colosseum in Italy, the Hanseatic Town of Visby in Sweden) or even the totality of the external surface of the monument (the Tower of Pisa in Italy).

Table 4. List of the UNESCO cultural objects with limestone.

Object Total surface Limestone Limestone (m2) (m2) (%) Cathedral of Saint Domnius 1 960 1 390 70.9 Aachen Cathedral 17 300 3 290 19.0 Würzburg Residence 41 100 1 030 2.5 Wartburg Castle 4 300 200 4.7 The Gatehouse of Lorsch Abbey 570 17 3.0 The Colosseum 22 750 19 450 85.5 The Tower of Pisa 7 740 7 740 100.0 Palazzo Madama 7 300 2 700 37.0 Ghirlandina Tower 2 650 2 620 98.9 Royal Palace of Caserta 149 800 54 700 36.5 Nidarosdomen 4 430 60 1.4 Wall of the Hanseatic Town of Visby 62 000 58 900 95.0 Towers of the Abbey of St. Gall 3 150 160 5.1 Bern Minster 8 980 240 2.7

The methodology used for the estimation of the damage of limestone due to attack of atmospheric pollutants at the selected UNESCO world cultural heritage sites is based on the use of the time- dependent dose-response function for Portland limestone in the multi-pollutant situation:

+ R = 3.1 + {0.85 + 0.0059[SO2]Rh60 + 0.054Rain[H ] + 0.078[HNO3]Rh60 + 0.0258PM10} t

where

R = surface recession, µm Rh60 = Rh – 60 when Rh > 60, 0 otherwise (Rh = relative humidity, % annual average) Rain = amount of precipitation (mm year-1) -3 [SO2] = annual average concentration, g m [H+] = annual average concentration, mg l-1 -3 [HNO3] = annual average concentration, g m -3 PM10 = annual average concentration, g m t = time, years

The multi-pollutant dose-response function relates damage to limestone, expressed as surface recession, to a range of atmospheric pollutants: sulphur dioxide (SO2), nitric acid (HNO3), total + acidity of rainfall (H ), and particulate matter (PM10). Climatic parameters also play a role, as reflected by the presence in the dose-response function of the two terms amount of precipitation (Rain) and relative humidity (Rh).

For the first year of exposure (t = 1) the equation becomes:

+ R = 4.0 + 0.0059[SO2]Rh60 + 0.054Rain[H ] + 0.078[HNO3]Rh60 + 0.0258PM10

Figure 22 shows the estimated recession rates for limestone, in the first year of exposure, for the fourteen cultural objects where this material is present. The figure also shows the contribution of the different pollution factors to the recession rates and compares the estimated recession rates with the target (tolerable corrosion rate) suggested for protecting cultural heritage monuments in 2050 and indicated as 2.0 times the background corrosion (WGE, 2009). Within ICP Materials the background corrosion rate was defined as the lower 10 percentile of the observed corrosion rates in the materials exposure programme, which started in 1987 and ended in 1995 (CLRTAP, 2014). For limestone, the background corrosion rate was calculated as 3.2 m/year. Therefore, the target of the tolerable recession rate for limestone for the year 2050 is equal to 6.4 m/year. The target value set for 2020 of 8.0 m/year corresponds to 2.5 times the background value but is not considered in this context due to the imminence of this year.

In general, a large majority of the cultural objects taken into account by the Call shows recession rates below or close to the target of 6.4 m/year for 2050. In particular, the Croatian Cathedral of Saint Domnius stands out with low recession rate for limestone, 4.6 m/year, close to the background recession rate of 3.2 m/year. The low recession rate predicted for the limestone of the Cathedral of Saint Domnius is a consequence of the low relative humidity (56%): according to the dose-response function, at values of Rh <60%, any corrosive effects of SO2 and HNO3 do not occur (Rh60 = 0).

For some of the cultural objects, the estimated recession rates of the limestone are higher than the target of 6.4 m/year. This happens for the Würzburg Residence and the Gatehouse of Lorsch Abbey in Germany and for the Italian monuments Tower of Pisa, Palazzo Madama and the Royal Palace of Caserta. The other two Italian monuments, the Colosseum and Ghirlandina Tower, have limestone recession rates equal to the target of 6.4 m/year. For these sites, it is important to reduce the concentrations of atmospheric pollutants to limit the limestone recession rate to the recommended target for 2050. This is particularly critical for the Italian sites, where limestone is the dominant part.

Figures 23a and 23b show the percentage distribution of the contribution of the different doses in determining the recession of the limestone due to atmospheric pollutants. The calculations predict that HNO3 is the pollutant most responsible for the deterioration of the limestone in all the sites 22

considered, contributing for 41-72% (average 60%), excluding the Cathedral of Saint Domnius in Split, for which the contribution is calculated equal to zero. PM10 is the second most critical pollutant for the deterioration of limestone, with a contribution of 14-42% (average 24%) to the total deterioration of limestone attributable to the air pollution (always excluding the Cathedral of Saint Domnius where, due to the absence of SO2 and HNO3, the PM10 contribution to the corrosion attributable to air pollution is 82%). The Wall of the Hanseatic Town of Visby in Sweden is the only site where HNO3 and PM10 have a similar weight, 41% and 42% respectively, reflecting the low 3 3 concentrations of NO2 (5.7 µg/m ) and the relatively high concentrations of PM10 (37.4 µg/m ). The acidity of precipitation and the concentrations of SO2 are still important deterioration agents for limestone used in cultural heritage but no longer dominant factors: the contributions are, on average, 9% and 6%, respectively.

As an illustration, it is also possible to calculate theoretical maintenance times based on the estimated corrosion rates and a corrosion attack after which it is considered necessary to make an intervention. For ornament limestone representing fine structures of the monument that needs special care, a value of 100 µm was considered appropriate before maintenance action was needed (Irwin et al., 2009). Figure 24 shows calculated maintenance times based on this value (100 µm). Apart from the limestone of the Cathedral of Saint Domnius in Split, which has a 67-year time interval due to moderate recession rate, maintenance time interval for the limestone of the other cultural objects varies from about 20 to 41 years.

Figure 22. Estimated recession rates for limestone, first year of exposure

Figure 23a. Contribution of the different terms in determining the recession of the limestone.

Figure 23b. Contribution of the different terms in determining the recession of the limestone

Figure 24. Times between maintenance for a tolerable corrosion depth before action of 100 m

3.2 Sandstone

Sandstone is present in twelve of the twenty-one cultural objects taken into consideration by the Call for data. The list of the cultural objects is shown in Table 5, which gives the total area considered and the surface composed of sandstone, in absolute amount (the stock at risk) and as a percentage. Sandstone is practically almost the only material present in the outer surface of the two monuments in Switzerland (the Towers of the Abbey of St. Gall and Bern Minster) and is a dominant part of the monuments in Germany (in seven of the eight monuments, sandstone is present in percentages varying between 21% and 88%). It is present in very limited quantities in the Ghirlandina Tower in Italy, Nidarosdomen in Norway and in the Drottningholm Palace Theatre in Sweden.

Table 5. List of the UNESCO cultural objects with sandstone.

Object Total surface Sandstone Sandstone (m2) (m2) (%) Aachen Cathedral 17 300 7 700 44.5 Speyer Cathedral 26 000 16 900 65.0 Würzburg Residence 41 100 19 520 47.5 Porta Nigra 5 500 4 840 88.0 Town Hall of Bremen 4 060 1 870 46.1 Wartburg Castle 4 300 2 170 50.5 The Gatehouse of Lorsch Abbey 570 120 21.1 Ghirlandina Tower 2 650 30 1.1 Nidarosdomen 4 430 60 1.4 Abbey of St. Gall 3 150 2 920 92.7 Bern Minster 8 980 8 740 97.3 Drottningholm Palace Theatre 4 205 55 1.3

The methodology used for the estimation of the damage of sandstone due to attack of atmospheric pollutants at the selected UNESCO world cultural heritage sites is based on the use of the time- dependent dose-response function for White Mansfield dolomitic sandstone (CLRTAP, 2014) in the SO2 dominating situation:

0.52 0.91 + 0.91 R = 2.0[SO2] exp{f(T)}t + 0.028Rain[H ]t

f(T) = 0 when T≤10°C, otherwise -0.013(T-10) where

R = surface recession, µm Rain = amount of precipitation, mm year-1 -3 [SO2] = annual average concentration, g m [H+] = annual average concentration, mg l-1 T = temperature, °C – annual average t = time, years

The SO2-dominating dose-response function relates damage to sandstone, expressed as surface + recession, to sulphur dioxide (SO2) and total acidity of rainfall (H ). Climatic parameters also play a

role, as reflected by the presence in the dose-response function of the two terms amount of precipitation (Rain) and temperature (T).

For the first year of exposure (t = 1) the equation becomes:

0.52 + R = 2.0[SO2] exp{f(T)} + 0.028Rain[H ]

f(T) = 0 when T≤10°C, otherwise -0.013(T-10)

Figure 25 shows the estimated recession rates for sandstone, first year of exposure, for the twelve cultural objects where this material is present. The figure also shows the contribution of the dry 0.52 + (2.0[SO2] exp{f(T}) and wet (0.028Rain[H ]) deposition terms to the recession rates and compares the estimated recession rates with the target (tolerable recession rate) suggested for protecting cultural heritage monuments in 2050 and indicated as 2.0 times the background corrosion (WGE, 2009). Within ICP Materials the background corrosion rate was defined as the lower 10 percentile of the observed corrosion rates in the materials exposure programme which started in 1987 and ended in 1995 (CLRTAP, 2014). For sandstone, the background corrosion rate was calculated as 2.8 m/year. Therefore, the target of the tolerable recession rate for limestone for the year 2050 is equal to 5.5 m/year. The target value set for 2020 of 7.0 m/year corresponds to 2.5 times the background value but is not considered in this context due to the imminence of this year.

Unfortunately, and in contrast with limestone, there is no function available for sandstone for the multi-pollutant situation. The effect of HNO3 and PM10, which was the most important pollution terms for limestone, is not included in the calculation for sandstone. Therefore it is not surprising that all the cultural objects taken into account by the Call show recession rates lower than the target of 5.5 m/year for 2050, taking into account only current levels of acidity of precipitation and concentrations of SO2. Figure 25 indicates that SO2 is still more important than acid rain; representing 92-98% (average: 95.7%) of the recession rate, while the pH of the precipitations is only responsible for 1.4-7.5% (average: 4.2%).

Figure 26 shows calculated maintenance times for ornament sandstone, also based on a 100 µm corrosion depth before action (Irwin et al., 2009). Lifetimes for the sandstone of the twelve cultural objects taken into consideration by the Call for Data range from 33 years for the Aachen Cathedral to 99 years for Wartburg Castle, both in Germany, depending on the estimated recession rate.

Figure 25. Predicted recession rates for sandstone, first year of exposure

Figure 26. Times between maintenance for a tolerable corrosion depth before action of 100 m 29

3.3 Copper

In the cultural objects taken into consideration in the context of the Call for data, copper is mainly present as a roof material in different buildings (Speyer Cathedral, Porta Nigra, Town Hall of Bremen and Wartburg Castle in Germany, the Towers of the Abbey of St. Gall in Switzerland and Nidarosdomen in Norway). Copper details are also present in Nederluleå church in Sweden (door) and in Hercules Monument in Germany (the statue of the Greek demigod Hercules on its top). In total, eight of the twenty-one cultural objects have copper exposed to the outdoor environment. The list of these eight cultural objects is shown in Table 6, which gives the total area considered and the surface composed of copper, in absolute amount (stock at risk) and as a percentage.

Table 6. List of the UNESCO cultural objects with copper.

Object Total surface Copper Copper (m2) (m2) (%) Speyer Cathedral 26000 7800 30.0 Porta Nigra 5500 660 12.0 Town Hall of Bremen 4060 1220 30.0 Wartburg Castle 4300 765 17.8 Hercules-Monument 15100 151 1.0 Nidarosdomen 4430 1165 26.3 Abbey of St. Gall 3150 75 2.4 Nederluleå church 3170 2 0.1

The methodology used for the estimation of the damage of copper due to attack of atmospheric pollutants at the selected UNESCO cultural world heritage sites is based on the use of the time- dependent dose-response function for copper in the multi-pollutant situation (MULTI-ASSESS, 2005):

0.4 f(T) + ML = 3.12 + {1.09 + 0.00201[SO2] [O3]Rh60·e + 0.0878Rain[H ]}t

f(T) = 0.083(T-10) when T<10°C, -0.032(T-10) otherwise where

ML = mass loss, g m-2 Rh60 = Rh – 60 when Rh > 60, 0 otherwise (Rh = relative humidity, %, annual average) Rain = amount of precipitation, mm year-1 -3 [SO2] = annual average concentration, g m -3 [O3] = annual average concentration, g m [H+] = concentration, mg l-1, annual average T = temperature, °C, annual average t = time, years

The multi-pollutant dose-response function relates damage to copper, expressed as mass loss, to a + range of atmospheric pollutants: sulphur dioxide (SO2), ozone (O3), and total acidity of rainfall (H ). Environmental parameters also play a role, as reflected by the presence in the dose-response function of the terms amount of precipitation (Rain), relative humidity (Rh) and temperature (T).

For the first year of exposure (t = 1) the equation becomes:

0.4 f(T) + ML = 4.2 + 0.00201[SO2] [O3]Rh60·e + 0.0878Rain[H ]

f(T) = 0.083(T-10) when T<10°C, -0.032(T-10) otherwise

Mass loss (ML, g m-2) can be converted into corrosion (R, m) by dividing by the density of copper, equal to 8.93 g cm-3.

Figure 27 shows the estimated corrosion of copper, first year of exposure, for the eight cultural objects where this material is present. The figure also shows the contribution of the different terms to the corrosion and compares the estimated corrosion with the target (tolerable corrosion rate) suggested for protecting cultural heritage monuments in 2050 and indicated as 2.0 times the background corrosion (WGE, 2009). Within ICP Materials the background corrosion rate was defined as the lower 10 percentile of the observed corrosion rates in the materials exposure programme which started in 1987 and ended in 1995 (CLRTAP, 2014). For copper, the background corrosion rate was calculated as 0.32 m/year. Therefore, the target of the tolerable corrosion rate for copper for the year 2050 is equal to 0.64 m/year. The target value set for 2020 is 0.80 m/year and corresponds to 2.5 times the background value but is not considered in this context due to the imminence of this year.

Except for Nidarosdomen in Norway, the corrosion of copper for all the monuments is equal or higher than the target of 0.64 m/year for 2050. In all sites except Nidarosdomen, the contribution of dry deposition, which reflects the atmospheric concentrations of SO2 and O3, dominates over wet deposition, expressed by the term 0.0878Rain[H+], accounting for 70 – 90 % (average: 83%) of the total contribution of air pollutants to the corrosion of copper. Nidarosdomen differs in that the contributions of the two terms are almost similar, 58% and 42%, respectively. A high contribution of the dose due to wet deposition (30%) is also found for Nederluleå church in Sweden.

Figure 28 shows the times between maintenance in years, calculated according to the time-dependent dose-response function and assuming a tolerable corrosion depth before action of 10 m for ornamented copper (Irwin et al., 2009). The copper roof of Nidarosdomen is not ornamented and therefore the 46-year lifetime given in figure 28 is not relevant. The lifetime for the copper present in the other cultural objects varies from 21 to 33 years but again, the calculation is only valid for ornamented structures.

Figure 27. Predicted corrosion rates for copper, first year of exposure

Figure 28. Times between maintenance for a tolerable corrosion depth before action of 10 m

3.4 Bronze

Although the amount of bronze being affected by air pollution (stock at risk) has not been quantified in terms of surface area, the occurrence of this material has been detected in nine of the twenty-one cultural objects considered in the Call for data, mainly in the form of bells in bell towers but also as a constituent of an entrance door (the Wolf’s Doors of the Aachen Cathedral in Germany). The monuments where the presence of bronze has been found and the main elements made with this material are listed in Table 7.

Table 7. List of the UNESCO cultural objects with bronze.

Object Elements made in bronze Cathedral of Saint Domnius 9 bells in the belfry Aachen Cathedral 8 bells in the belfry (total weight 13 850 kg). Bronze doors at the west portal (Wolf’s Doors) Speyer Cathedral 9 bells in the belfry (total weight 12 264 kg) The Tower of Pisa 7 bells in the belfry (total weight 10 496 kg) Ghirlandina Tower 5 bells in the belfry (total weight 5 085 kg) and 1 small bell (250 kg) on display along the route Nidarosdomen 4 bells Abbey of St. Gall 9 bells Bern Minster 7 bells distributed between two belfries (total weight ca. 28 000kg) Nederluleå church 2 bells

The methodology used for the estimation of the damage of bronze due to attack of atmospheric pollutants at the selected UNESCO world cultural heritage sites is based on the use of the time- dependent dose-response function for cast bronze in the multi-pollutant situation:

f(T) + ML = 1.33 + {0.00876[SO2]Rh60·e + 0.0409Rain[H ] + 0.0380PM10}t

f(T) = 0.060(T-11) when T<11°C, -0.067(T-11) otherwise where:

ML = mass loss, g m-2 Rh60 = Rh – 60 when Rh > 60, 0 otherwise (Rh = relative humidity, %, annual average) Rain = amount of precipitation, mm year-1 -3 [SO2] = annual average concentration, g m -3 [PM10] = annual average concentration, g m [H+] = concentration, mg l-1, annual average T = temperature, °C, annual average t = time, years

The multi-pollutant dose-response function relates damage to bronze, expressed as mass loss (g m-2), + to a range of atmospheric pollutants: sulphur dioxide (SO2), total acidity of rainfall (H ), and particulate matter (PM10). Environmental parameters also play a role, as reflected by the presence in the dose-response function of the terms amount of precipitation (Rain), relative humidity (Rh) and temperature (T). 33

Mass loss (ML, g m-2) can be converted into corrosion (R, m) by dividing by the density of bronze, equal to 8.8 g cm-3. For the first year of exposure (t = 1) the equation then becomes:

f(T) + R = 0.15 + 0.000985[SO2]RH60e + 0.00465Rain[H ] + 0.00432PM10

f(T) = 0.060(T-11) when T<11°C, otherwise f(T) = -0.067(T-11) where R is the corrosion (µm).

Figure 29 shows the estimated corrosion for bronze, first year of exposure, for the nine cultural objects where this material is present. The figure also shows the contribution of the different doses to the corrosion and compares the estimated corrosion with the target (tolerable corrosion rate) suggested for protecting cultural heritage monuments in 2050 and indicated as 2.0 times the background corrosion (WGE, 2009). Within ICP Materials the background corrosion rate was defined as the lower 10 percentile of the observed corrosion rates in the materials exposure programme which started in 1987 and ended in 1995 (CLRTAP, 2014). For bronze, the background corrosion rate was calculated as 0.25 m/year. Therefore, the target of the tolerable corrosion rate for bronze for the year 2050 is equal to 0.5 m/year. The target value set for 2020 is 0.6 m/year and corresponds to 2.5 times the background value but is not considered in this context due to the imminence of this year.

All the cultural objects taken into account by the Call show corrosion rates for bronze lower than the target of 0.5 m/year for 2050 and close to the background corrosion rate of 0.25 m/year, suggesting that current levels of air pollutants do not represent a significant risk for cultural objects made of bronze and exposed to the outdoor environment.

Figure 30 shows the percentage distribution of the contribution of the different terms in determining the corrosion of bronze due to the atmospheric pollutants. The degradation of the bronze is strongly influenced by the concentration of PM10, which contributes on average to 66% of the corrosion attributable to atmospheric pollutants, while SO2 and the acidity of rainfall both contribute, on average, to about 17%. The two Italian monuments, the Tower of Pisa and Ghirlandina Tower, are exposed to higher concentrations of PM10. The bronze bells in their bell towers are subject to a slightly higher corrosion rate and the contribution of PM10 to this corrosion rate reaches 80-90%.

Figure 31 shows the times between maintenance (lifetime) in years, calculated according to the time- dependent dose-response function and assuming a tolerable corrosion depth before action of 10 m (Irwin et al., 2009) for ornamented structures. Lifetimes for the bronze of the nine cultural objects taken into consideration by the Call for Data range from 64 years (the Tower of Pisa, Italy) to 121 years (Nidarosdomen, Norway).

Figure 29. Predicted corrosion rates for bronze, first year of exposure

Figure 30. Contribution of the different terms in determining the corrosion of bronze.

Figure 31. Times between maintenance for a tolerable corrosion depth before action of 10 m

4. Assessment of air pollution risks to materials - soiling

Soiling affects both non-transparent and transparent materials which here are represented by limestone and glass. For non-transparent materials the loss of reflectance is typically expressed as a function of only the particulate matter concentration, allowing only for an estimation of this parameter. For glass a function for modern glass have been used despite possible differences in composition but this function also includes SO2 and NO2 in addition to PM10 allowing for a comparative assessment of their relative contribution.

4.1 Limestone

The list of the fourteen cultural objects containing calcareous stones in their outer surface is reported in Table 4. The methodology used for predicting the soiling of limestone due to atmospheric pollution at the selected UNESCO world cultural heritage sites is based on the use of the dose- response function for Portland limestone:

-6 R/Ro = 1 – exp(-PM10 x t x 6.5 x 10 ) where

R/Ro = relative loss of reflectance -3 PM10 = average annual concentration of PM10, g m t = time, days 6.5 x 10-6 = soiling constant for limestone.

The dose-response function for the soiling of limestone due to air pollution exposure relates the blackening of the surface to the ambient PM10 concentrations to which the material is exposed.

This dose-response function for soiling was developed within the MULTI-ASSESS project (MULTI- ASSESS, 2005), but was not presented by Watt et al. in a subsequent paper (Watt et al., 2008) because of the high variability of the soiling constant for limestone. However, considering that for the other materials for which a dose-response function was considered valid (painted steel, white plastic and polycarbonate membrane) the structure of the function is identical except for the value of the constant (3.96 x 10-6, 4.43x 10-6 and 3.47 x 10-6 respectively), this function has been applied in this context while keeping its limitations in mind. These are the uncertainty of the soiling constant for limestone, the complex nature of particulate matter and its deposition, the expected site-specific variability, and the extrapolation beyond the time frame during which the experimental data were collected to derive the dose-response function. Therefore the results obtained must be considered only relative and indicative of the possible degradation due to atmospheric pollution and not sufficiently robust for a decision making process.

The “tolerable soiling before action” represents the threshold when significant adverse public reaction of what constitutes tolerable soiling is triggered. This is generally set at 35% loss in reflectance (MULTI-ASSESS, 2005). This value, combined with the dose-response function and values of PM10 makes it possible to estimate the necessary cleaning interval ("lifetime"), which is the period of time for which the cultural object can remain without cleaning before generating the perception of degradation no longer tolerable in the public. For cultural heritage objects a period of 10-15 years seems appropriate and therefore times shorter than this could be considered not tolerable.

Figure 32 shows the predicted time between cleaning for limestone for the fourteen cultural objects where this material is present. Despite the decrease in atmospheric concentrations of PM10 in recent

years, the times between cleaning still seems unacceptably short. Only for Nidarosdomen, Norway, a cleaning interval higher than 15 years can be estimated. The cleaning interval for which most of the Italian sites and the wall of the Hanseatic Town of Visby could remain without cleaning would not exceed 4-6 years.

Figure 32. Estimated cleaning interval for limestone to maintain a loss of reflectance less than 35%

4.2 Glass

Glass is an important material of historical and cultural buildings and is present in fourteen of the twenty-one cultural objects located in the UNESCO sites considered in the Call for data. The list is shown in Table 8, which gives the total area and the surface composed of glass, in absolute amount (stock at risk) and as a percentage. The percentage of the surface area composed by glass varies greatly in these cultural objects, reaching or exceeding around 10% for the Aachen Cathedral in Germany and the Royal Palace of Caserta in Italy.

Table 8. List of the UNESCO cultural objects with glass.

Object Total surface Glass Glass (m2) (m2) (%) Cathedral of Saint Domnius 1 960 13 0.7 Aachen Cathedral 17 300 1 560 9.0 Speyer Cathedral 26 000 1 040 4.0 Würzburg Residence 41 100 3 490 8.5 Town Hall of Bremen 4 060 240 5.9 Wartburg Castle 4 300 120 2.8 The Gatehouse of Lorsch Abbey 570 Yes - Palazzo Madama 7 300 200 2.7 Royal Palace of Caserta 149 800 17 400 11.6 Hydroparken 40 460 2 820 7.0 Nidarosdomen 4 430 95 2.1 Drottningholm Palace Theatre 4 205 290 6.9 Nederluleå church 3 170 180 5.7 Bern Minster 8 980 Yes -

The methodology used for the prediction of the soiling of glass due to dry deposition of pollutants at the selected UNESCO cultural world heritage sites is based on the use of the dose-response function for the soiling of silica–soda–lime glass (Lombardo et al., 2010):

1.86 Haze = (0.2529⋅[SO2] + 0.1080⋅[NO2] + 0.1473⋅ [PM10]) ⋅1/(1 + (382/t) where

Haze = haze12, % -3 [SO2] = annual average concentration, g m -3 [NO2] = annual average concentration, g m -3 [PM10] = annual average concentration, g m t = time, days

The dose-response function relates soiling of glass (using haze as a response), to a range of air pollutants: sulphur dioxide (SO2), nitric dioxide (NO2), and particulate matter (PM10). Figure 33 shows the estimated values of haze after one year of exposure for the fourteen cultural objects where glass is present. The figure also compares the estimated values of haze with the target

12 Defined as the ratio between the diffuse transmitted light (Td) and the direct transmitted light (TL). 40

of 1%. Haze above 1% causes a visual discomfort and aesthetical impairment perceived by human eyes leading to the feeling of a "dirty" glass plate. With the exception of Hydroparken, Norway, in all sites a one-year exposure causes a value of haze much higher than 1%.

Figure 34 shows the estimated cleaning interval for glass, i.e. the time, expressed in days, necessary for the haze to reach a value of 1%. During this period of time the glasses of the cultural heritage object can remain without cleaning without generating the perception of degradation no longer tolerable. For most of the cultural objects studied in this Call for data, this time ranges between four and seven months. Notable exceptions are represented by Hydroparken, Norway, which seems to require annual clean-up and Palazzo Madama in Turin, Italy, for which a much higher cleaning frequency, roughly quarterly, seems necessary. A greater frequency of maintenance and cleaning interventions implies higher costs as well as further possible unnecessary damage resulting from cleaning operations.

Figure 35 shows the percentage distribution of the contribution of the different terms in determining the degradation of the optical properties of the glass and its loss of transparency, expressed as haze. NO2 and PM10 are the pollutants most responsible for the soiling of glass in all the sites considered, contributing to 27-73% (average 45%) and 23-60% (average 45%), respectively. SO2 is an important factor in the soiling of glass but not a dominant factor, contributing to 3-16% (average 9%).

Figure 33. Estimated haze for glass, first year of exposure

Figure 34. Estimated cleaning interval to maintain a haze level below 1% for glass

Figure 35a. Contribution of the different doses in determining the soiling of glass.

Figure 35b. Contribution of the different terms in determining the soiling of glass.

5. Discussion of air pollution risks to materials

Table 9 summarizes observations from previous chapters and highlights the main risks (corrosion/soiling) and for which materials. A risk is indicated if the tolerable level estimated from dose-response functions are exceeded, regardless of how much it is exceeded. For soiling of limestone a risk is indicated if the calculated maintenance time needed to maintain a decrease in reflectance below 35% was shorter than 10 years. Table 10 summarizes the risk factors (pollutants) for the different risks, also based on observations from previous chapters. In the following, materials are discussed individually.

When there is a risk for limestone (Table 9) both corrosion and soiling can be an issue (the Würzburg Residence, the Gatehouse of Lorsch Abbey, the Tower of Pisa, Palazzo Madama and the Royal Palace of Caserta). There are also some cases where only soiling can be an issue (Cathedral of Saint Domnius, the Colosseum, Ghirlandina Tower and the wall of the Hanseatic Town of Visby) but no cases where only limestone corrosion can be an issue. PM10 is important both for corrosion and soiling while HNO3 is important only for corrosion (Table 10). However, when looking at individual pollution levels of HNO3 and PM10 (Table 3) there is a strong correlation except for some sites with high levels of PM10 without having correspondingly high levels of HNO3, the extreme being the 3 3 Wall of the Hanseatic Town of Visby with PM10 almost 40 µg/m and HNO3 about 0,5 µg/m . On the other hand, there are no sites, in this selection, with low levels of PM10 and high levels of HNO3, which could explain why there is no risk of corrosion without simultaneous risk of soiling.

Sandstone is not included in Table 9 since the calculated values from the dose-response functions all were below the target level. However, the only available dose-response function was from the SO2 dominating situation and therefore the effect of HNO3 and or PM10, the two main effects for limestone, could not be quantified (Table 10). Previous parallel exposures of limestone and sandstone indicate that the surface recessions were correlated; with high sandstone recession coinciding with high limestone recession. This was one of the reasons limestone (and not sandstone) was kept as a material for exposures in later stages of the ICP Materials programme. Therefore, it is not possible to exclude risks to sandstone monuments even if the risk could not be quantified with the present methodology.

Copper corrosion could be an issue for some monuments (Speyer Cathedral, Porta Nigra, Town Hall of Bremen, Hercules Monument, Nederluleå church and Towers of the cathedral of the Abbey of St. Gall). For these, the main risk factor is the combined effect of SO2 and O3 (Table 10). However, development of corrosion products is quite different in areas with high SO2 / low O3 compared to areas with low SO2 /high O3. Therefore it is important to investigate the protective properties of the patina in order to assess if the short term risk of corrosion also is a risk for prolonged exposure of these monuments to high levels of O3 combined with low levels of SO2 or if the formation of dense layers of cuprite (for example) will result in a significant decrease of corrosion rate with time.

Bronze is not included in Table 9 since the calculated values from the dose-response functions all were below the target level. As for carbon steel, bronze is quite sensitive to SO2 and the decreasing levels of SO2 during the last decades has caused a substantial decrease in bronze corrosion.

Glass soiling is a potential issue at almost all locations in Europe (Table 9) and is related to PM10, NO2 and SO2 (Table 10). Therefore, further evaluation of risks should always have this effect in mind, if glass is present in sufficient amounts.

There are other materials at risk besides those given in Table 9 and 10 but where it has not been possible to quantify the risk due to lack of dose-response functions. However, these risks, even if

they in some cases could be related to environmental effects are often not related to air pollution. This is for example the case with concrete and brick where frost and wetting/drying damage could be a much higher concern and mortar where increased precipitation could an important factor. For example, for the Wall of the Hanseatic Town of Visby, the main reason for wall collapses is not degradation of the limestone but instead degradation of the joints consisting of render/mortar/plaster, which needs to be taken into account when doing damage estimations based on environmental data. For this site it is instead amount of particulates in the air causing soiling that is the only air pollution concern.

In summary, Table 9 and 10 provides a good overview of the identified risks and shows the most important pollution risk factors in Europe today but when making further risk assessment for an individual monument it is necessary to go deeper and take into account interaction between pollution and other risk factors on the local level. This are planned to be done in future studies (Part IV on the relationship between the environment and the artefact) for a selection of monuments. The selection will be based on the potential risk but also the economic impact, which is the next step in the analysis (Part III Economic Evaluation).

Table 9. Risks due to air pollution for the materials constituting the artifacts

Name of the cultural object Materials Limestone Risk Copper Risk Glass Risk (m2) (m2) (m2) Cathedral of Saint Domnius 1385 Soiling - 13 Soiling Aachen Cathedral 3287 - 1557 Soiling Speyer Cathedral - 7800 Corrosion 1040 Soiling Würzburg Residence 1027 Corrosion - 3493 Soiling and soiling Porta Nigra - 660 Corrosion - Town Hall of Bremen - 1218 Corrosion 244 Soiling Wartburg Castle (palace and keep) 201 765 120 Soiling Hercules Monument - 151 Corrosion - The Gatehouse of Lorsch Abbey Corrosion - Yes Soiling 17 and soiling The Colosseum 19450 Soiling - - The Tower of Pisa Corrosion - - 7735 and soiling Palazzo Madama Corrosion - 200 Soiling 2700 and soiling Ghirlandina Tower 2623 Soiling - - Royal Palace of Caserta 54700 Corrosion - 17400 Soiling and soiling Hydroparken - - 2820 Nidarosdomen 60 1165 94 Soiling Drottningholm Palace Theatre - - 288 Soiling Nederluleå church - 2 Corrosion 175 Soiling Wall of the Hanseatic Town of Visby 58900 Soiling - - Towers of the cathedral of the Abbey 158 75 Corrosion - of St. Gall Bern Minster 240 - Yes Soiling

Table 10. Risk factors (pollutants) for different risks to materials constituting the artifacts (+ low impact; ++ medium impact; +++ high impact). An empty square indicate that the specific risk / pollutant combinations was not included in the used dose-response function. Therefore the impact level could not be estimated.

Risk SO2 NO2 HNO3 SO2*O3 PM10 pH Limestone corrosion +/++ ++/+++ ++ + Sandstone corrosion +/++ + Copper corrosion ++/+++ + Bronze corrosion + ++ + Limestone soiling +++ Glass soiling +/++ ++/+++ ++/+++

6. Comparison of predicted degradation rates: local data vs. EMEP model data.

The main weakness of the approach in this report is that corrosion rates are estimated with dose- response functions, where the estimation can differ substantially when comparing to the measured corrosion rate determined by placing materials on a rack for one or several years.

Another weakness of the approach, which is discussed in the following, is the representativeness of the measuring stations with respect to the air quality in the immediate surroundings of the cultural objects selected in this study and which directly impacts on the materials from which they are made.

The results reported in the previous sections were obtained using the meteorological data and concentrations of air pollutants provided by the participants in the Call. This information was obtained from the meteorological stations and monitoring stations for assessing air quality closest to the cultural object under study. The distances between these stations and the sites under consideration vary from a few hundred meters to several kilometers. In a few cases this distance approximates to tens of kilometers.

While the representativeness of meteorological data (the spatial area for which the value measured at the station can be accepted as meaningful) can be considered satisfactory, the levels of atmospheric pollutant concentrations are more critical. Many cultural and historical monuments are located in cities and at the micro-scale level the concentrations of pollutants can vary according to the distance from the dominant sources of pollutants (i.e. distance from the kerbside) and the topography of the city (i.e. street configuration).

Another difficulty in using information from the local air quality network is that nowadays SO2 is measured only at a limited number of monitoring stations because SO2 has been stable for years at a low level and its importance for the assessment of air quality is no longer as important as it used to be. Furthermore, at least 50% of the ozone monitoring stations in air quality monitoring networks are located in suburban areas. Therefore, for many monuments located in urban areas data on SO2 and O3 concentrations in nearby sites are not available.

It was not possible to obtain an accurate representation of air quality in the immediate surroundings of the studied cultural objects due to the available resources. This also limits the accuracy of the estimated pollutant exposure to the materials.

A comparison was made between the degradation rates predicted on the basis of the data collected during the Call and those calculated on the basis of the EMEP/MSC-W model for the year 2015 calculated at the new higher resolution of 0.1º x 0.1º longitude-latitude. This resolution corresponds to grid cell of 6 x 11 km at 60°N and 9 x 11 km at 40°N. The objective of this comparison was to ascertain whether the output data of the EMEP/MSC-W model at this resolution can be used to obtain estimates of degradation rate of materials comparable to those obtained from the local data of the monitoring stations.

Table 10 shows the yearly gridded data of the average annual concentrations of SO2, NO2, HNO3, O3, PM10, the annual averages of temperature and relative humidity as well as the total annual precipitation for the cells of the EMEP/MSC-W model that host the cultural objects of the UNESCO

sites considered in this study. These data were used as input for calculating degradation rates according to the dose-response functions.

The comparison between the limestone and sandstone recession rates and the copper and bronze corrosion rates, calculated using both local data and EMEP model data, is reported in Figures 36-39. Figures 40 and 41 compare the soiling of limestone and glass, respectively, calculated using local data and modeled data, expressed in terms of "tolerable time between cleaning".

Table 10. Average annual concentrations of air pollutants and climatic parameters where the cultural objects are located. Values are from the EMEP/MSC-W model with cells at a resolution of 0.1º x 0.1º longitude-latitude. Note that the number of digits does not represent the precision of the values, which are given as represented in the EMEP database.

Name of the UNESCO cultural Cell EMEP domain SO2 NO2 O3 PM10 HNO3 T RH Rain object (Latitude, Longitude) (µg/m3) (µg/m3) (µg/m3) (µg/m3) (µg/m3) (°C) (%) (mm) Cathedral of Saint Domnius 43.55, 16.45 1.5329 13.5985 66.336 10.8039 0.8661 15.2632 69.6554 1142.9177 Aachen Cathedral 50.75, 6.05 1.4776 10.2994 64.6405 13.5262 0.5975 10.3502 76.8569 870.8955 Speyer Cathedral 49.35, 8.45 3.0622 19.9966 51.5283 15.1512 1.2209 12.0123 71.1561 549.1465 Würzburg Residence 49.75, 9.95 0.9176 11.112 62.123 13.284 0.8132 10.7467 73.1725 611.6462 Porta Nigra 49.75, 6.65 1.0554 10.1835 63.6424 12.4389 0.7843 10.1528 77.3296 735.3819 Town Hall of Bremen 53.05, 8.85 1.6136 13.5525 56.4791 16.9894 0.4409 10.3595 78.4676 821.1638 Wartburg Castle (palace and keep) 50.95, 10.35 0.5338 5.2084 67.667 11.0146 0.6537 9.46 76.7601 683.5729 Hercules Monument 51.35, 9.35 0.6764 6.697 64.1402 12.8312 0.6925 9.5144 77.6154 720.5427 The Gatehouse of Lorsch Abbey 49.65, 8.55 1.3970 13.6286 58.9194 13.4776 1.0698 11.642 71.6687 615.5967 The Colosseum 41.85, 12.45 2.2613 30.7076 50.3264 19.0121 1.4004 15.8845 75.8781 1044.3551 The Tower of Pisa 43.75, 10.35 1.4388 14.3708 62.1939 20.1819 1.4464 15.4695 76.9462 973.6195 Palazzo Madama 45.35, 7.75 0.7478 12.4911 77.8927 21.7638 1.105 12.4246 69.5341 937.3739 Ghirlandina Tower 44.65, 10.95 0.7599 18.233 54.5551 23.8132 1.0311 14.0986 76.6894 781.8159 Royal Palace of Caserta 41.15, 14.35 1.8137 18.9931 58.7338 21.1361 1.7743 15.304 77.1094 1267.386 Hydroparken 59.85, 8.55 0.0550 0.5377 72.4081 1.9492 0.1346 2.8676 79.337 1318.1716 Nidarosdomen 63.45, 10.35 0.4605 5.8182 62.9392 2.872 0.1383 6.5052 77.7413 1170.7566 Drottningholm Palace Theatre 59.35, 17.85 0.2797 8.4943 60.4095 9.0455 0.2768 7.6941 79.8873 665.3147 Nederluleå church 65.65, 22.05 0.1458 0.7401 60.1243 2.4039 0.0771 4.2064 80.751 820.4279 Wall of the Hanseatic Town of Visby 57.65, 18.25 0.2614 1.6423 74.634 6.1906 0.5752 9.025 77.3656 627.7687 Towers of the Cathedral of the Abbey 47.45, 9.35 0.7033 9.4954 71.3361 12.3724 0.41 9.114 76.3158 1367.1191 of St. Gall Bern Minster 46.95, 7.45 1.5413 17.6601 57.9808 14.7374 0.4355 9.0127 77.1157 1056.9106

Figure 36. Comparison of predicted recession rates for limestone: local data and EMEP model data.

Figure 37. Comparison of predicted recession rates for sandstone: local data and EMEP model data. 51

Figure 38. Comparison of predicted corrosion rates for copper: local data and EMEP model data.

Figure 39. Comparison of predicted corrosion rates for bronze: local data and EMEP model data. 52

Figure 40. Comparison of predicted tolerable time before action for soiling of limestone: local data and EMEP model data.

Figure 41. Comparison of predicted tolerable time before action for soiling of glass: local data and EMEP model data (* With EMEP data, a haze value of 1% is never reached).

Overall, a fairly good agreement was observed between the two sets of data for the predicted recession rates of stone materials (limestone and sandstone) and for the corrosion rates of metallic materials (copper and bronze). The differences are generally within the uncertainties associated with the use of dose-response functions, suggesting that the EMEP model data at this resolution level could reasonably be used in the future for similar risk assessments.

In general, the corrosion rates for limestone, sandstone and bronze calculated with local data appear, on average, slightly higher than the corresponding corrosion rates predicted with the use of the data of the EMEP model. On the contrary, the corrosion rates for copper objects appear, on average, higher if the data of the EMEP model are used as input to the dose-response functions. This effect may reflect the differences in the concentrations of air pollutants between urban areas and the outskirts.

Many of the air quality monitoring stations used in this study are located in urban areas, where concentrations of pollutants such as NO2 and PM10 are higher. On the contrary, ozone levels are generally higher downwind of ozone precursor sources, so ozone concentrations in rural areas can be higher than in urban areas. Higher values of pollutant concentrations translate into higher corrosion rates if the dose-response functions include these pollutants. The EMEP data are averaged over a large area (6 x 11 km at 60°N and 9 x 11 km at 40°N), therefore in a cell of the EMEP grid the concentrations of PM10 and NO2 are expected be lower while the concentrations of O3 are expected to be higher than for a measurement point located in the heart of a city. This may partly explain the differences between the two sets of corrosion rates and in particular the difference observed for copper, since only for this material the dose-response function directly includes the ozone concentration.

An underestimation of the soiling of materials (limestone and glass) is also found with the use of the EMEP model data: the "tolerable time between cleaning" is always higher if EMEP data are used as input to the dose-response functions. The dose-response function for the soiling of limestone is based only on PM10 concentrations. PM10 concentrations measured by the air quality monitoring stations are higher than the output values from the EMEP model for the cell where the cultural object is located. This implies that a higher rate of soiling is calculated with local data than with the EMEP model data and therefore that the cleaning intervals are significantly lower when the local data are used. In some cases (Nidarosdomen in Norway and the Wall of the Hanseatic Town of Visby in Sweden) the differences in the magnitude of the input data are very marked, resulting in high cleaning intervals (63.2 and 29.3 years, respectively) when using the EMEP data as input of the dose- response function.

The prediction of the soiling of the glass is strongly influenced by the concentrations of NO2 and PM10. The concentrations of these pollutants in the EMEP grid are lower than the local data, resulting in "tolerable time between cleaning" up to 1.5-2.0 times higher than those calculated using the data from the monitoring stations. This effect is very pronounced for Nidarosdomen in Norway where, due to very low values of the modelled concentrations, it seems that glass surfaces can remain without cleaning for about 2.5 years. With regard to Hydroparken in Norway and Nederluleå church in Sweden, the modelled values of the pollutant concentrations are so low that, due to asymptotic behaviour of the dose-response function, a haze value of 1% is never reached.

7. Conclusions

This report presents an assessment of the risk of corrosion and soiling due to air pollution for twenty- one unique monuments that are part of UNESCO world cultural heritage sites located in six countries in Europe: Croatia, Germany, Italy, Norway, Sweden and Switzerland. The assessment was made on the basis of the information on environmental parameters collected during the Call for Data and by using dose-response functions established by ICP Materials. The estimated degradation rates were compared with the target (tolerable corrosion rate) suggested by ICP Materials for protecting cultural heritage monuments in 2050. This comparison allowed the identification of potential risks for different materials and corresponding air pollution risk factors.

Corrosion of limestone, sandstone, copper and bronze as well as soiling of limestone and glass were investigated. Limestone corrosion was a risk factor at five monuments, limestone soiling at nine monuments, copper corrosion at six monuments and glass soiling at thirteen of the twenty-one monuments included in the study. The required maintenance interval to maintain a tolerable soiling of limestone was about 4-6 years for most of the monuments in Italy and for the Wall of the Hanseatic Town of Visby in Sweden. In the most unfavourable case, Palazzo Madama in Italy, the predicted soiling of glass was so fast that the tolerable level was reached in less than about three months.

PM10 was identified as a risk factor both for corrosion and soiling of limestone while HNO3 was identified only for corrosion. The combined effect of SO2 and O3 was identified as a risk factor for copper and PM10, NO2 and SO2 were identified as risk factors for soiling of glass where PM10 and NO2 were relatively more important at most sites. Sandstone and bronze were not identified as at risk for any of the monuments. The effect of HNO3 and or PM10, the two main effects for limestone, could not be assessed for sandstone due to lack of dose-response functions. Therefore, it is not possible to exclude risks to sandstone monuments even if the risk could not be quantified with the present methodology. Bronze is quite sensitive to SO2 and the decreasing levels of SO2 during the last decades have caused a substantial decrease in bronze corrosion below the tolerable level. SO2 is still an important deterioration agent for some materials used in cultural heritage but no longer the dominant factor. Also the acidity of precipitations seems to have a small impact on the degradation of materials in the current situation.

Corrosion and soiling of the materials of the individual cultural objects, calculated with the environmental parameters collected within the Call for Data, were compared with those obtained by using the data available as output of the EMEP/MSC-W model for the year 2015 at the new resolution of 0.1° x 0.1°. Overall, a relatively good agreement was observed between the two sets of data and within the uncertainties associated with the use of dose-response functions, suggesting that the EMEP model data at this resolution level could reasonably be used in future for similar risk assessments.

The number of cultural heritage objects taken into account in this Call for Data is small compared to the number of objects in UNESCO world cultural heritage sites located in countries which are Parties to the Convention on Long-range Transboundary Air Pollution. Nevertheless, the quantity and quality of the collected data, as well as the wide range of materials and environmental conditions that they cover is substantial. Therefore a sufficiently clear picture is presented of the risk of air pollution damage to outdoor cultural heritage materials.

8. References

CLRTAP, 2014. Mapping of Effects on Materials, Chapter IV of Manual on methodologies and criteria for modelling and mapping critical loads and levels and air pollution effects, risks and trends. UNECE Convention on Long-range Transboundary Air Pollution; accessed on 06 June 2018 on Web at www.icpmapping.org

ICP Materials, 2017. Call for Data “Inventory and condition of stock of materials at UNESCO World Cultural Heritage sites, 2015-2017”. Status Report. ICP Materials Report 80.September 2017.

Irwin J., Tidblad J., and Kucera V., 2009. Air Quality Policy. In: The effects of air pollution on cultural heritage. Watt, J., Tidblad, J., Kucera, V., & Hamilton, R. (Eds.). Springer, New York, USA.

Lombardo T., Ionescu, Chabas A., Lefèvre, R.-A., Ausset P., Candau Y., 2010. Dose–response function for the soiling of silica–soda–lime glass due to dry deposition. Science of the Total Environment 408: 976–984.

MULTI-ASSESS, 2005. “Model for multi-pollutant impact and assessment of threshold levels for cultural heritage”. Final report. http://www.corrinstitute.se/MULTI-ASSESS/

Watt J., Jarrett D., Hamilton R., 2008. Dose–response functions for the soiling of heritage materials due to air pollution exposure. Science of the Total Environment 400: 415–424.

WGE, 2009. “Review of air pollution effects. Indicators and targets for air pollution effects”. Report to the 28th session of the Working Group on Effects, UN-ECE Convention on Long range Transboundary Air Pollution, Working Group on Effects, Geneva, Switzerland, 2009. ECE/EB.AIR/WG.1/2009/16. 12 p. http://www.UNECE.org/fileadmin/DAM/env/docume nts/2009/EB/wge/ece.eb.air.wg.1.2009.16.e.pdf.