Development of an early warning sensor for assessing deterioration of organic materials indoor in
museums, historic buildings and archives
Elin Dahlin* and Terje Grøntoft*
Norwegian Institute for Air Research
P O Box 100
NO-2027 Kjeller
Norway
Tel.: + 47 63898000
Fax: +47 63898050
E-mail: elin.dahlin@nilu.no; terje.grontoft@nilu.no
Web-site: www.nilu.no
Sara Rentmeister
Albert-Ludwigs Universität Freiburg
Freiburger Materialforschungszentrum der Albert-Ludwigs-Universität
Stefan-Meier Strasse 21
DE-79104 Freiburg
Germany
E-mail: sara.rentmeister@fmf.uni-freiburg.de
Christopher Calnan
The National Trust
Ickworth Regional Office
Ickworth Estate
IP29 5QE
Suffolk
United Kingdom
E-mail: christopher.calnan@nationaltrust.org.uk
Janusz Czop
National Museum in Krakow
Al. 3 Maja 1
PL-30-062 Krakow
Poland
E-mail: janusz_czop@muz-nar.krakow.pl
Kathryn Hallett and David Howell
Historic Royal Palaces
Conservation and Collections Care
Hampton Court Palace
Surrey KT8 9AU
United Kingdom
E-mail: kathryn.hallett@hrp.org.uk
Christoph Pitzen
The Consulting and Support Centre for the Museums of Baden-Württemberg
Schloßstrasse 96
DE-70176 Stuttgart
Germany
E-mail: pitzen@landesstelle.de
Anne Sommer Larsen
Trøndelag Folk Museum
Riiser Larsens vei 16
NO-7020 Trondheim
Norway
E-mail: anne.sommer-larsen@sverresborg.no
*Authors to whom correspondence should be addressed
Abstract
Results from laboratory and field testing of an early warning (EWO) sensor developed in the EUfunded MASTER project (EVK-CT-2002- 00093) are discussed. Laboratory testing shows a clear
response to low concentrations of the single contaminant gases NO2, O3 and SO2. In a
comprehensive European field test programme a corresponding effect to that seen in the
laboratory was observed for indoor NO2. The main aim of the MASTER project is to develop a
preventive conservation strategy including an early warning system, with an EWO sensor, for
protection of organic objects in museums, historic buildings and archives.
Keywords
MASTER, preventive conservation, early warning sensor, museums, organic objects, nitrogen
dioxide
Introduction
In museums, historic buildings and archives all over the world, organic objects such as fibre
materials are being affected either by display or by storage conditions. Unsuitable environmental
conditions are a serious cause of decay. The key to the survival of these objects is achieving an
acceptable indoor environment. Vital to this is a sustainable management of the cultural property
including better preventive conservation strategies (Dahlin 2003). On this background an EUfunded project with the acronym MASTER started in February 2003. The project will last for 36
months. The MASTER project aims to provide conservators in museums, historic buildings and
archives with a new preventive conservation strategy for the protection of cultural property,
based on an early warning system that assesses the environmental impact on organic objects. An
important part of the early warning system is the development of an effect sensor for organic
materials (EWO sensor). A novel polymer is being researched as a potential generic EWO sensor,
namely a sensor that will respond to the range of environmental parameters implicated in the
chemical deterioration of organic materials. Certain polymer films are known to deteriorate
chemically and photochemically owing to environmental stress and light. They do so at a rate
appropriate for a sensor. Measurable deterioration appears after a few weeks’ exposure in typical
environments. Three months were chosen as the exposure time. This is sufficiently long to give a
good estimate of the quality of the long-term environment objects are exposed to. The chemical
deterioration of certain polymers has been shown to be detectable by simple techniques such as
uv-visible spectrometry. The EWO sensor prototype is manufactured by spin coating the polymer
onto a glass substrate. The three pollutant gases investigated in this work, SO2, O3 and NO2, all
have known deteriorating effects on organic materials (Blades et al. 2000). It is therefore
important to monitor their concentration levels. However, the real effect of the gases depends on
their actual rate of deposition, and consequently on the accumulated mass of deposited gas, on the
art object surface over time. The deposition rate depends on the reactivity of the material surface
towards the contaminant, which again is influenced by air humidity, temperature and duration of
exposure. To assess the real effect of the pollutants on art objects it is then not sufficient to
measure their concentrations in the air. Rather, one needs some kind of measure of the actual
reaction between the art object and the contaminant. Sensors that emulate the reaction between
art objects and the contaminants and which give a measurable effect response can be used for this
purpose. The EWO sensor, which is now being tested in the MASTER project, has been
developed to give such a response.
Theoretical
Indoor concentrations of gases from outdoor sources are generally lower than outdoor
concentrations because of dry deposition to surfaces (see, for example, Liu and Nazaroff 2001,
Grøntoft and Raychauduhri 2004, Weschler 2000, Brimblecombe 1990) and homogeneous
reactions (Seinfeld and Pandis 1998, Weschler and Shields 1997). The reactions involving NO2
and O3 generally decrease the indoor concentrations of these gases except in conditions with light
outdoor conditions and considerable amounts of NO present. This would typically be the case in
the summer in towns with much traffic and NO emissions. The balance for the reaction,
then moves more towards the left side indoors in relative darkness compared with the outdoor
situation with more NO and O3 present. As a result more NO2 could be present indoors than
outdoors despite indoor deposition of NO2.
Methods of study: experimental
Laboratory test
To calibrate the EWO sensors a test system for realistic indoor pollutant gas concentrations was
set up in a climate chamber. To stimulate real indoor conditions the sensors are exposed to
different concentrations of the gases O3, NO2 and SO2, and to various humidity levels, with a
constant temperature of 22 °C, for periods of 4 weeks. This exposure programme is continuing.
Field test
On 1 March 2004 exposure of the EWO-sensors started in a field test programme in 10 different
European museums and historical buildings (Table 1).
Table 1. The ten museums and historic buildings participating in the field test programme
The field test programme will run for one year. To obtain a classification system for the risk of
damage on cultural objects made of organic materials, the field test sites selected have different
environments from low to severe aggressiveness. In each of the five regions one more rural site
with low and one more urban site with higher pollutant levels were selected to obtain the
variations needed. For each museum, one location inside a showcase with paralel measurements
outside the showcase in the exhibition area and a third location outdoor were selected. For all
three locations the gases O3, NO2, and SO2 are monitored by use of passive gas samplers and in
addition organic acids are monitored inside the showcases. Light is the only environmental factor
controlled for in the field test, as it was known from preliminary tests that the sensors had a
comparable light sensitivity as that to the contaminant gases. Parallel samples are exposed
shielded from light and fully exposed to the light on each test location. This makes it possible to
study the light effect separately. In both the laboratory test and in the field test the effect on the
sensor polymer was measured with a spectrophotometer as increased light absorption after
exposure to the environmental contaminants.
Results
Concentration measurements
The main results from the SO2, O3 and NO2 concentration measurements for the outdoor and two
indoor locations for the months March to May 2004 are shown. Figure 1 shows the SO2
concentration measurement results from Krakow (urban) and Zakopane (rural) in the Polish
region as a mean of two paralel samplers for the three months. The negative result seen for the
indoor location in Krakow in March is due to subtraction of a mean blanc measurement of 0.3
μg/m3. In general, over all the sites, indoor concentrations were slightly above the detection limit
where as concentrations in the showcases were on the detection limit. Figure 2 shows typical O3
concentrations indoor and outdoor for the rural (r) and urban (u) sites in four regions, Norway,
England, Germany and Poland for May 2004. Figure 3 shows the measured outdoor and indoor
NO2 concentrations for the three months on the sites in the two regions, Germany and England,
with the highest outdoor NO2 concentrations.
Figure1. Outdoor concentrations divided by 10, indoor and showcase SO2 concentrations
measured from March to May 2004 at one urban location, Krakow, and one rural location,
Zakopane, in the Polish region
Figure 2. Outdoor, indoor and showcase O3 concentrations measured during May 2004 for rural
and urban museum sites in four regions: Norway, England, Germany and Poland
Figure 3. Outdoor, indoor and showcase NO2 concentrations from March to May 2004 for the
museum sites in the two regions, Germany and England, with the highest measured outdoor NO2
concentrations
Figure 4. Effect measured on EWO sensor after laboratory exposure to contaminant gases for 4
weeks under controlled conditions at T = 22 °C and RH = 45 per cent. NO2 and O3 were mixed
in equal concentrations
Effects on the EWO sensor
Laboratory tests
The organic polymer used shows a response to the three gases, in the concentration range 0–100
parts per billion (ppb) ordinary found in indoor air. Figures 4 and 5 show the experimental
change in light absorption of the sensor film after exposure to NO2, O3 and SO2 separately and to
equal concentrations of NO2 and O3 combined at air humidity levels of 45 per cent and 70 per
cent.
Field test
Negative inter-correlations between the independent parameters could hide real effects in simple
correlations of single parameters with sensor response. The only correlation observed between
the measured gas concentrations was a negative one between NO2 and organic acids, acetic acid
plus formic acid, in the showcases. In the outdoor location with climate exposure and strong
light, no statistical correlations were observed between gas concentrations and sensor response.
The climate conditions in the laboratory closely resembled those indoors in the museums. The
mean indoor air humidity in the museums for March, April and May 2004 was 47 per cent,
varying from 43 to 51 per cent. The mean indoor temperature in the museums for the three
months was 18.8 °C, varying from 17.4 to 20.3 °C. Light levels indoors in the museums were
low.
Figure 5. Effect measured on EWO sensor after laboratory exposure to contaminant gases for 4
weeks under controlled conditions at T = 22 °C and RH = 75 per cent
Figure 6. Correlation between indoor NO2 concentrations and EWO sensor response for the 3
months, March to May 2004, for all ten test sites with a linear regression trend line and its data.
The figure also shows the experimental laboratory response
No correlation was observed between the low indoor SO2 concentrations and sensor response.
Plots of O3 concentration (0–20 μg/m3) against sensor response showed a large spread but no
correlation. Both in the museum exhibition rooms and in the showcases a clear positive close to
linear correlation was observed between NO2 concentration (0–40 μg/m3) and sensor response.
Figure 6 shows the correlation between NO2 and sensor response for March, April and May 2004,
with a linear regression trend line and its data. The figure also shows the experimental laboratory
results transferred from Figures 4 and 5.
Time response
The sensors showed a reduced reaction rate with time both in the laboratory and in the field.
After three months field exposure this reduction was considerable for the site with the highest
NO2 concentrations but small for the other sites.
Discussion
Concentration measurements
The SO2 measurements showed an outdoor concentration maximum in March for Krakow
(Figure 1), the most polluted urban site in the five regions. Generally the urban sites showed
higher SO2 concentrations than the rural sites in each region and the outdoor concentrations
decreased from March to April to May. Indoor SO2 concentrations were much lower than outdoor
concentrations. O3 showed a large concentration decrease from outdoors to indoors (Figure 2).
Generally higher concentrations were measured in the rural sites and there was little change in
concentrations from March to May. The NO2 measurements showed a concentration maximum in
May for Stuttgart (Figure 3). NO2 showed a general concentration decrease from outdoor to
indoor, but it was much smaller than for SO2 and O3. The most polluted urban locations showed a
considerable effect of Equation (1). This effect increased from March to May with increased light
intensity, which outdoor moves Equation (1) towards the right and indoor towards the left side.
The negative correlation observed between NO2, infiltrating into the showcase, and organic acid,
with its’ source in the showcase, was probably caused by their opposite dependence on the air
exchange rate in the showcases.
Laboratory- and field-tests
After exposure for 3 months the field test showed one clear simple statistical correlation between
single independent environmental parameters and sensor effect. This was the indoor effect of
NO2, of similar magnitude as that observed in the laboratory (Figure 6). The relative indoor air
humidity (43–51 per cent) and concentrations of SO2 were so low that any effect of SO2 would be
difficult to detect. No effect was observed for O3 even if a clear effect similar to that observed for
NO2 was seen in the laboratory (Figures 4 and 5). In the field there was a mixture of NO2 (0–40
μg/m3) and O3 (0–20 μg/m3). In Figure 6 the effect in the field is seen to be very close to the
effect of pure NO2 at RH = 45 per cent in the laboratory. The laboratory effect of NO2 at RH = 70
per cent was somewhat higher while the combined effect of NO2 + O3 was somewhat lower. To
ascertain this decrease in the reaction rate with mixed gases (Figures 4 and 5), rather than any
synergism, one would, however, need more parallel measurements. In the field test the NO2
concentration seemed to determine the effect on the sensor and overrule any effect of O3. The
surface deposition reactions of NO2 without and in the presence of O3 are probably different.
Without O3 present NO2 is known to react with water on surfaces to give HNO2 and HNO3
according to (Weschler and Shields 2000, Seinfeld 1998):
With O3 present NO2 is known to give nitrate radicals that in relative darkness indoor could go
on to give HNO3 according to (Weschler and Shields 2000):
where S is the material surface. It may be that O3 and the reaction products of the two reactions
(2) and (3)–(4) have a similar ability to oxidize the sensorpolymer. If so, O3 could be consumed
in reaction (3)-(4) before it could oxidize the polymer and the oxidation rate of the polymer by
NO2 would be similar independently of the path, Equations (2) or (3)–(4), of the reaction. From
the laboratory results one would expect an effect of O3 on the sensors when NO2 is low compared
with the O3 concentration. However, because of larger deposition velocities indoor O3 are often
lower than NO2 concentrations, even if outdoor O3 concentrations are considerably higher
(Figures 2 and 3). More experiments would be needed to understand the combined effect and
total reaction mechanism of mixtures of NO2 and O3. The aim with the EWO sensor is that it
should give a response indicative of the response on the organic art objects in atmospheres of
different aggressiveness. Thus one will have to calibrate the sensor response against the effect of
the air environment observed on real museum objects. Some studies (Brimblecombe 1996,
Blades 2003) that could be used in calibration have been performed. By comparison with effects
described in such studies the sensor could be used to evaluate the aggressiveness of the air
environment against single materials. Levels of sensor response would then determine different
expected states of material conservation (or deterioration) with time. As work on understanding
material deterioration goes on in the field the new knowledge could be incorporated as an
extended list of materials that could be compared with the sensor response. Museums often
contain lots of different materials in integrated exhibits. An alternative to the material specific
approach could be to determine levels of sensor response that would generally be acceptable
(unacceptable) to collections including organic objects. This approach is not contrary to the first
approach. It may, however, be more immediately applicable for collections at large. One could
then include more general knowledge and standards when determining threshold levels. In this
regard it is interesting to see the determining influence of the NO2 concentration on the sensor
response. Assuming that the effect of a mixture of NO2 and O3 on the EWO sensor is that seen in
the laboratory and discussed in relation to Equations (2) and (3)–(4), and that indoor NO2 in most
instances are higher or equal to O3 concentrations one could, at least as a starting point, calibrate
the sensor in relation to the observed NO2 effect, using NO2 as an indicator for the total
environment. One would then assume that the general effect of SO2 and the organic acids on the
EWO sensor compared with that on the real organic objects is similar as that of NO2. This
assumption needs further investigation. One may well find that there is not a one-to-one
relationship between the NO2 and the acid effect. Deviations for SO2 may be less important as
long as concentration levels are low (Figure 1). Another concern is high concentrations of organic
acids in enclosed spaces such as showcases with emitting sources. Investigations will be
performed in the MASTER project to study the effect of the organic acids on the EWO sensor.
As one would not be able to determine the separate effects of NO2 and the acids from the
response of the generic EWO sensor it may be wise to use the component with the overall most
severe effect to determine ‘worst case thresholds’. One would then be on the conservative side in
not underestimating the risk. If a severe risk was discovered using the sensor more
comprehensive studies could be performed to determine the separate deteriorating influences in
the environment.
Conclusion
The EWO sensors developed in the EU MASTER project show a clear response to low
concentrations of the single indoor contaminant gases, NO2, O3 and SO2, in the laboratory. In an
ongoing comprehensive European field test programme a corresponding effect to that seen in the
laboratory was observed for NO2. Real indoor SO2 concentrations are so low that effects were
difficult to study. With more or equal amounts of indoor NO2 compared with O3, which was the
situation observed indoors in most cases, the effect on the sensor depended on NO2 alone. This
opens the possibility to calibrate the generic effect on the sensor against the effect on real organic
museum objects using the effect of NO2 as an indicator of the effect of total environment. One
should then do more research on the magnitude of the relative effect of NO2, O3 in mixtures with
NO2, SO2 and especially organic acids on the sensor compared with that on real museum objects.
Acknowledgements
We thank the museums and especially the technicians and the conservators who performed the
practical work with the laboratory and field tests. This work was funded by the European
Commission as a part of the 5th FP, Key Action: Cultural Heritage and City of Tomorrow and
also by a substantial grant from the Norwegian Archive, Library and Museum Authority.
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