AIRBORNE DUST IN A MUSEUM ENVIRONMENT

Anne Lisbeth Schmidt¹, Pernille Bronée¹, Kåre Kemp², and Jes Fenger²

The National Museum of Denmark¹
National Environmental Research Institute, Denmark²

ABSTRACT

Air pollution in Copenhagen
Urban air pollution in Denmark has been systematically monitored since the early 1980es. In most cases The National Environmental Research Institute has carried out the measurements in collaboration with local authorities.
    Like in most of the industrialised world the nature of air pollution has changed radically during the recent decades. Previously, high concentrations of sulphur dioxide and soot due to combustion for space heating dominated the urban atmosphere. The use of cleaner fuels and improved technology (including transition to district heating based on combined heat and power production) has by and large solved this problem. Today the dominant source of urban air pollution is the increasing traffic. In spite of catalytic converters on petrol driven cars the level of NO₂ has not been reduced, since the ozone available for the oxidation of the primary pollutant, NO largely determines it. Furthermore an increasing number of diesel driven vehicles emits organic compounds and small particles.
    This change in composition of the urban air pollution has primarily had an impact on human health. But also the degradation of materials and objects, many of which are placed in museums, is influenced.

The National Museum
Like many other major Danish museums the National Museum in Copenhagen is situated in the city centre, surrounded by trafficked streets. The narrow Stormgade has an average daily traffic of 16,000 vehicles (85% cars, 15% heavy vehicles), and the street canyon effect enhances the impact of pollution. In the immediate vicinity of the museum runs one of the busiest, broader streets in Copenhagen, H.C.Andersens Boulevard, with a daily average traffic of 62,000 vehicles (90% cars, 10% heavy vehicles).
    The National Museum is housed in a large building square, with the oldest part dating back to 1684. In 1743-44 the buildings were reconstructed and until 1780 the so-called Prince's Palace was used as residence for the royal family and court. From the middle of the 19th century the Prince's Palace has housed the National Museum. In 1929-38 the museum was reconstructed and received its present outer appearance.
    As a part of an extensive renovation in 1989-91 the heating system was changed and a stationary air conditioning system was installed in part of the building complex.

The ethnographic collections
For the newly restored air conditioned exhibition areas at the Ethnographic Treasure Rooms about 10,000 objects from the museum's Ethnographic Collections were prepared at the Department of Conservation during 1991-1998.
    Ethnographic objects often consist of many different organic and inorganic materials (fur, feather, textile, plant material, pigments, metals etc.) and thus have very large surfaces where dust easily deposit. Many objects have been exhibited since the last rebuilding of the museum and had not been cleaned during the intervening more than 50 years.
    Such objects are soiled with a greasy greyish or black layer which is difficult to remove by means of dry methods (vacuum cleaning, compressed air, erasing). Methods using a wet media for instance water, ethanol or white spirit, depending on the material to be treated, had often to be applied to get the wanted cleanliness - wanted both for aesthetic reasons and because dust generally is assumed to accelerate material degradation. Painted objects and other fragile surfaces were often impossible to clean and thus had to be left untreated.
    Maybe too late the question arose: were the newly cleaned objects now more susceptible to degradation in the new exhibition or would they have been more protected with the old layer of dirt? Scanning the conservation database it is obvious that the main cause for the treatment of the objects has been to remove secondary soil from the surface of the objects. As secondary soil is understood dust or soil originating from the storage or exhibition at the museum. Primary soiling, on the other hand, from the original use and handling of the object - for example grease, soot or blood - is normally considered as important evidence and is therefore not removed.

Analyses of the dust layer
In 1995 the Department of Conservation initiated an examination of the nature of the old dust layers on various objects. The analyses were based on scanning electron microscopy, transmission microscopy, elementary analysis (EDAX) and pH-measurements. The test material was either collected on a filter connected to a micro vacuum cleaner or at adhesive stubs pressed to the surface of the soiled object (Hersoug et al, 1995). These analyses surprisingly suggested that dust could act as a buffer against the impact of new dust and aggressive air pollutants. The very limited analyses concluded that the dust layers consisted of non-agressive pollutants with a pH of 7.6.

Monitoring of dust
In the winter 1998 a further study of the properties of the particulate pollution, which is deposited on the exhibited objects today, was initiated. Two rooms in the museum were investigated: One room in the newly restored Ethnographic Treasure Rooms has no windows and 80% recirculation with fresh air intake through filters. The other room has windows facing a courtyard and the above-mentioned heavy trafficked street Stormgade and has no ventilation. In this room silver exhibits from the Medieval Collections are on display.
    In both rooms dust were collected by so called streakers in two size fractions above and below 2 mm respectively. The concentration of NO₂ was determined with passive filter samplers placed adjacent to the streakers and outside the window in the room without ventilation.
    In the room, where the Medieval Collections are exhibited, it is necessary to consider the protection of the silver objects. Installation of an effective ventilation system is greatly to be preferred, but hardly likely. Instead other measures must be taken - e.g. lacquering of the metal surfaces or installation of sealed showcases equipped with absorbents. So far the Department of Conservation surveys these exhibitions closely as regards the silver objects.

Measurements
The streaker sampler is a small aerosol collector with low power consumption and a low noise level. That makes it suitable for i.a. indoor measurements. The aerosols are separated in two size fractions by means of an impaction stage. Particles having an aerodynamical diameter > 2 µm are collected on an impaction stage with a greased polycarbonate film as collection surface. The fine fraction (<2µm) is collected on a polycarbonate filter. It is possible to collect the dust on up to 60 discrete spots on each film/filter (Kemp and Møller, 1981).
    During the project period we changed the exposure time in a scheme between one day and one week for each sample. As it was a pilot project we did not know the concentration levels to be expected. With the long exposure time better detection limits could be achieved, while the short times could give information about the changes depending on time.
    The samples were analysed by means of Proton Induced X-ray Emission Spectroscopy (PIXE). It is a multi-element analysing method, which gives determination of all elements having atomic number greater than 13 (aluminum). In the present samples detection limits below 0.1 ng/m³ were achieved for many elements.
    The streaker measurements were conducted in parallel in the two rooms mentioned. Impregnated filters for collection of NO₂ (nitrogen dioxide) were placed close to the aerosol samplers and in addition one was placed just outside the window in the unventilated room. Collection was done in one-week periods during the sampling period.
    The results are compared to data collected by the air quality network in Copenhagen (Thomsen, 1999; Kemp and Palmgren, 2001). One station, at street level, is placed on H.C. Andersens Boulevard about 500 m from the museum. Another station, at roof level, is placed on the H.C.Ørsted Institute about 3 km from the museum.

Results
Twenty elements were found in concentrations above the detection limits. These elements may not as such constitute any risk for the exhibits, but they can be used as indicators for the type of pollution that enters the building and that may be produced within the building. Typically particles produced by combustion are mainly found in the fine fraction, while particles from mechanical processes are predominant in the coarse fraction.
    Elements as e.g. S, V, Ni and Pb in the air are mainly of anthropogenic origin. Whereas Si, Ca, Ti, Fe and Sr originate primarily from windblown dust and dust from buildings. If we look at the average distributions for these elements a marked difference is found between the two rooms (Fig. 1).

Fig. 1. Average results for "anthropogenic" and "soil" elements. a) Average ratios between the mass found in the fine particles fraction and the mass found in both fractions. b) Ratio between the concentrations in the ventilated and unventilated room.

In the unventilated room (with windows) 2/3 of the anthropogenic particles are found in fine fraction, while only 1/3 of the soil particles are in the fine fraction. Almost all of the coarse particles are removed by the ventilation system. It may be unexpected that more than 90% of the anthropogenic particles are removed by the ventilation system while only 2/3 of the soil particles are removed. An explanation may be that there is a "production" of particles that are not efficiently removed by the recirculation in the ventilation system. These productions may be caused by particles raised by the traffic of museum guests and emission from newly painted surfaces. These properties can be illustrated by the measured time series for some "typical" elements (Fig. 2).

Fig. 2. Time series for the total concentration of "anthropogenic" particulate sulphur and the "soil" element calcium.

For sulphur there is an almost perfect match between the three time series with fixed ratios 1:5:75 between the concentrations at the three locations. The difference between the three locations is much more pronounced for Ca. In the general higher concentrations are found in the ventilated room, with an exception on December 7, which was a Monday when the museum is closed.

The ratios between the NO₂ concentrations at the different locations (Fig. 3) were almost the same during the sampling periods. The two outside locations, just outside the window at the museum and the roof station, were almost identical. The concentration was reduced with a factor of two in the windowed room, while it was 1/3 in the ventilated room.

Fig. 3. Nitrogen dioxide measured in the two rooms compared to two outdoor locations.

Conclusions
The outdoor pollution is a main source for fine particles of "anthropogenic" origin in both rooms. The ventilation system removes more than 98% of these particles. The ventilation system is less efficient for locally produced fine particles than the "natural" ventilation in the medieval room. Gases are removed less efficiently than particles. Potential harmful gases (both of local and outdoor origin) and fine particles of local origin may contribute to the soiling and decomposition of the exhibits, even with an efficient ventilation system.

References

Hersoug, L-G, Mørck, H., Larsen B. (1995) Undersøgelse af sedimenteret bystøv på museumsgenstande fra Nationalmuseets Etnografiske Samling, Prinsens Palais. Intern report.

Kemp, K. and F. Palmgren, (2001), The Danish Air Quality Monitoring Programme. Annual Report for 1999. National Environmental Research Insitute, Roskilde, Denmark 74 pp. NERI Technical Report Nr. 357.

Kemp, K. and J.T. Møller (1981), A two stage "Discrete Streaker" compatible with high-volume samplers. Nuclear Instruments and Methods 181 (pp 481-485).

Schmidt, A. L., Bronée, P, Kemp, K and Fenger , J. (1999) Airborne Dust on Museum Exhibits. International Conference Air Quality in Europe: Challenges for the 2000s 19-21 May 1999. Poster.

Schmidt, A. L., Bronée, P, Kemp, K and Fenger , J. (1999) Airborne Dust on Museum Exhibits of Animal Origin. Advanced Study Courses 1999: Methods in the Analysis of the Deterioration of Collagen based Historical Materials in Relation to Conservation and Storage, European Commision, Directorate-Generale XII, Environmental and Climate Programme, pp. 275-278.

Thomsen, B.B. (1999), Luftkvalitet i Hovedstadsregionen (Air quality in the Greater Copenhagen), Agency of Environmental Protection in Copenhagen; Copenhagen. 108 pp.

A. L. Schmidt and P. Bronée
The National Museum of Denmark
Department of Conservation
P.O.Box 260, Brede, DK-2800 Lyngby, Denmark
E-mail: anne.lisbeth.schmidt@natmus.dk
E-mail: pernille.bronee@natmus.dk

K. Kemp and J. Fenger
National Environmental Research Institute
Department of Atmospheric Environment
P.O. Box 358, Frederiksborgvej 399, DK-4000 Roskilde, Denmark
E-mail: kke@dmu.dk
E-mail: jfe@dmu.dk