1.9 Protection of Works of Art from Damage Due to Deposition of Airborne Particulate Matter
 

California Institute of Technology
California Department of Health Services
The Getty Conservation Institute

William Nazaroff
Mary Ligocki
Lynn Salmon
H. I. H. Liu
T. Ma
W. John
Frank Preusser
James R. Druzik
Department of Civil Engineering, University of California,
633 Davis Hall, Berkeley, California 94720
Period of Activity: 1/87 to 1/90

Project Abstract
This project was designed to establish the physical and chemical pathways by which airborne particulate matter acts to damage works of art. Emphasis was placed on understanding how to break the chain of events leading to the soiling and corrosion of art objects by depositing particulate matter.

Indoor particulate standards have not been established by the Environmental Protection Agency (EPA) or the California Air Resources Board (ARB) in part due to a lack of information on the character and quantity of these materials. The results of this study between the GCI and Caltech, with the participation of five Los Angeles-based museums, establish baseline values for just such standards.

Primary Publications
Nazaroff, W. W., M. P. Ligocki, L. G. Salmon, G. R. Cass, T. Fall, M. C. Jones, H. I. H. Liu, and T. Ma, Airborne Particles in Museums, Research in Conservation, Nº 6, 1993, 148 pages.

ABSTRACT-The risk to museum collections from airborne particles, whether considered air pollution or soil dusts, has had very limited study in the past. To fill in many of the holes in our understanding, the present study was conducted to learn the character, concentration, and fate of airborne particles inside a small but representative set of Southern California Museums. From these data, a mathematical model was derived which can be used to design and quantify protective strategies. This report is a shortened and less technical version of the final report on the research.

Nazaroff, W. W., M. P. Ligocki, L. G. Salmon, G. R. Cass, T. Fall, M. C. Jones, H Liu, and T. Ma, "Protection of Works of Art from Damage Due to Deposition of Airborne Particulate Matter," Final Report to the Getty Conservation Institute, July 1990.

OBJECTIVES AND APPROACH-The purpose of the research effort reported here is to establish some of the physical and chemical pathways by which airborne particulate matter acts to damage works of art. Emphasis will be placed on understanding how to break the chain of events leading to the soiling of art objects by deposited particulate matter. We seek to answer the following questions:

1. What is the range of chemical composition, concentration, and size distribution of the airborne particulate matter found in the indoor atmosphere of museums?

2. What physical mechanisms govern the transfer of particles from room air to the surface of works of art? How rapidly do these deposition processes proceed?

3. How do the deposited particles alter the optical appearance of the surface?

4. Can particulate matter deposition processes within buildings be modeled on the computer in a way that will aid the design of new museum facilities in advance of their construction?

5. What approaches can be devised that will protect works of art from damage due to airborne particulate matter?

A program of measurement and computer-based data analyses is conducted to address these questions. In Chapter 2 of this study, indoor and outdoor measurements of airborne particle chemistry at five Southern California museums are reported. This work provides baseline data on existing conditions in a variety of museums and helps us to explain significant differences between museums based on the general nature of their construction and ventilation system design. Chapter 3 describes a mathematical model that predicts the concentration, chemical composition, and size distribution of indoor airborne particles as well as particle deposition rates to indoor surfaces. The purpose of this model is to provide a tool that can be used to evaluate particle deposition control methods in advance of the construction of a new museum and to provide a method for examining the degree of improvement that could be obtained by modifying an existing facility. This model is tested against a detailed program of airborne particle concentration measurements and particle deposition measurements made at Southern California museums, as reported in chapters 4, 5, and 6. In Chapter 7, the measured and computed particle fluxes to indoor surfaces in museums are compared to the changes in the optical appearance of white surfaces placed in Southern California museums in order to test the connection between particle deposition and visually observable soiling.

Protection of works of art from soiling due to deposition of airborne particles is discussed in Chapter 8. The completed indoor/outdoor particulate air quality and deposition model is employed to study the effect of alternative control measures that could be employed to retard soiling by airborne particles. These methods include:

a. Increasing the efficiency of airborne particle filtration into a building;

b. Reducing the rate of induction of outdoor particles into a building;

c. Placing objects within display cases or glass frames;

d. Managing the outdoor are a surrounding the building to achieve lowered outdoor aerosol concentrations;

e. Eliminating indoor particle sources; and

f. Reducing the particle deposition velocity (e.g., the particle flux per unit atmospheric particle concentration) onto surfaces of concern.

 

It is seen that the simultaneous use of several of these control measures may be advisable in order to lengthen the characteristic time before soiling due to deposited particles would become noticeable to a human observer.

The major findings of each chapter are summarized in a brief abstract that appears at the start of each chapter.

Ligocki, M. P., H. I. H. Liu, G. R. Cass, and W. John, "Measurements of Particle Deposition Rates Inside Southern California Museums," Aerosol Science and Technology, Vol. 13, 1990, pp. 85-101.

ABSTRACT - Rates of deposition of submicron particles were measured inside five Southern California museums using an automated particle counting technique on a scanning electron microscope scanning equipped with an energy dispersive X-ray detector. Deposition velocities to vertical and upward-facing horizontal surfaces were determined as a function of particle size for each site by analysis of indoor suspended particles sampled onto filters and particles deposited on vertically and horizontally oriented deposition plates. Measured deposition velocities to vertical indoor surfaces were in the range of 10-6-10-5 m/s at all sites but varied from site to site in terms of their dependence upon particle size. Deposition velocities to horizontal surfaces were in the range of 10-6-10-3 m/s and showed the expected increase with particle size due to gravitational settling. The deposition velocities measured by single particle analysis are compared to deposition velocities determined by bulk chemical analysis of deposition plates and indoor ambient filter samples for several chemical species. Advantages and drawbacks of the automated particle counting technique are discussed.

Ligocki, M. P., L. G. Salmon, and G. R. Cass, "Fine Particle Concentrations and Deposition Rates in Southern California Museums," American Chemical Society, Division of Environmental Chemistry, Miami, Florida, September 10-15, 1989.

ABSTRACT-The chemical composition of fine airborne particulate matter was measured inside and outside of five museums in Southern California during both summer and winter seasons. Pollutant measurements made at one site that lacks a conventional air conditioning and air filtration system showed that indoor fine particulate matter concentrations were nearly as high as those outdoors, with the majority of the chemical species present as organic matter, elemental carbon, sulfates, and nitrates. High soiling rates were experienced indoors at this site. In contrast, one museum that possessed an unusually well engineered ventilation and filtration system exhibited indoor fine particle concentrations less than 20% of those outdoors, with very low indoor soiling rates. The factors that contributed to this improved level of indoor pollution control are discussed.

Ligocki, M. P., L. G. Salmon, T. Fall, M. C. Jones, W. W. Nazaroff, and G. R. Cass, "Characteristics of Airborne Particles Inside Southern California Museums, California," Atmospheric Environment, Vol. 27a, Nº 5, 1993, pp. 697-711.

ABSTRACT-The concentration and chemical composition of suspended particulate matter suspended were measured in both the fine and total size modes inside and outside of five Southern California museums. Mass balances and indoor/outdoor ratios were calculated for the major chemical components of the aerosol for both summer and winter sampling periods. The seasonally averaged indoor/outdoor ratios for particulate matter mass concentrations ranged from 0.16 to 0.96 for fine particles and from 0.06 to 0.53 for coarse particles, with the lower values observed for buildings with sophisticated ventilation systems which include filters for particulate matter removal. Fine particles, particularly those composed of organic matter, dominate the aerosol inside these buildings. Analyses of indoor vs. outdoor concentrations of major chemical species indicated that indoor sources of organic matter may exist at all sites, but that none of the other measured species appears to have major indoor sources at the museums studied. Significant fractions of the dark-colored fine elemental (black) carbon and soil dust particles present in outdoor air are able to penetrate to the indoor atmosphere of the museums studied, and may constitute a soiling hazard to works of art displayed in museums.

Nazaroff, W. W., and G. R. Cass, "Particle Deposition from a Natural Convection Flow onto a Vertical Isothermal Flat Plate," Journal of Aerosol Science, Vol. 18, Nº 4, 1987, pp. 445-455.

ABSTRACT-The deposition of particles from a laminar, natural convection boundary layer flow laminar, adjacent to a heated or cooled flat plate occurs due to a combination of thermophoretic drift and Brownian motion. In this paper, scale analysis is used to determine the magnitude of the concentration boundary layer thickness and the normalized particle flux. Scaling arguments are also used to show that thermophoresis dominates Brownian motion in the concentration boundary layer whenever Le1/3 > (Tí /|DT|)1/2. Using a similar transformation, the governing partial differential equations are converted to a system of ordinary differential equations which are solved numerically. Dimensionless particle flux is determined as a function of particle diameter in the range of 0.001 to 3.µm and plate-air temperature differences in the range of -10 to 10 K. The results have application in understanding and preventing the soiling of indoor surfaces, including works of art.

Nazaroff, W. W., L. G. Salmon, and G. R. Cass, "Concentration and Fate of Airborne Particles in Museums," Environmental Science and Technology, Vol. 24, Nº 1, 1990, pp. 66-77.

ABSTRACT-Objects displayed in museums are subject to a soiling hazard due to the deposition of airborne particles. To investigate this hazard, time-resolved measurements were made of the indoor and outdoor aerosol size distribution and chemical composition in three Southern California museums. A mathematical model of indoor aerosol dynamics was applied to the three sites to relate the data on outdoor aerosol characteristics, and building parameters to observed indoor aerosol properties. The predicted indoor aerosol characteristics agree well with the indoor measurements. At all three sites, the fraction of particles entering from outside air deposition surfaces varies strongly with particle size, ranging from a minimum of 0.1-0.5% for particles having a diameter in the vicinity of 0.15 mm to greater than 90% for particles larger than 20 mm in diameter. Deposition calculations indicate that, at the rates determined for the study days, enough elemental carbon (soot) would accumulate on vertical surfaces in the museums to yield perceptible soiling in as little as one year at one site to as long as 10-40 years at the other two sites.

Nazaroff, W. W., and G. R. Cass, "Mathematical Modeling of Indoor Aerosol Dynamics," Environmental Science and Technology, Vol. 23, Nº 2, 1989, pp. 157-166.

ABSTRACT-A general mathematical model is presented for predicting the concentration and fate of particulate matter in indoor air. Using a multi-compartment sectional representation, the model accounts for the effects of ventilation, filtration deposition onto surfaces, direct emission, and co-agulation Model predictions are compared with the evolution over time of the measured aerosol size distribution following combustion of a cigarette in a single room with a low air exchange rate. Reasonable agreement is obtained; however, further experiments are required for full validation of additional capabilities of the present model. Important environmental problems to which the model may be applied include analysis of the soiling of surfaces due to deposition of airborne particles and control of human exposure to environmental tobacco smoke. The model may also serve as a foundation for improving the understanding of the risk of human exposure to radon decay products indoors.

Nazaroff, W. W., and G. R. Cass, "Mass-Transfer Aspects of Pollutant Removal at Indoor Surfaces," Paper presented at the 4th International Conference on Indoor Air Quality and Climate, Berlin, August 17-21, 1987.

ABSTRACT-The loss rate of particles and highly reactive gases to indoor surfaces depends on the diffusivity of the species and on the nature of the air circulation pattern within a room. Analysis of three airflow conditions that may be found adjacent to the walls of an enclosure-homogeneous turbulence; laminar forced convection, and laminar natural convection-reveals that the average pollutant deposition velocity may vary for order 10-4 to 1 cm/s over the range of pollutant diffusivities and particle sizes encountered. Theoretical predictions are in rough agreement with the limited experimental data that have been taken inside actual rooms.

Nazaroff, W. W., and G. R. Cass, "Mass-Transfer Aspects of Pollutant Removal at Indoor Surfaces," Environment International, Vol. 15, 1989, pp. 567-584.

ABSTRACT-The mass-transport-limited rate of pollutant deposition onto indoor surfaces is examined in this paper. Transport of both particles and highly reactive gases through the boundary layer of air adjacent to a surface is analyzed for three model airflow conditions: (1) natural convection natural convection; flow along room surfaces, (2) forced laminar flow parallel to room surfaces, and (3) homogeneous turbulence in the core of the room. Transport mechanisms considered include convective diffusion, thermophoresis, and gravitational sedimentation. The predicted mass-transport-limited deposition velocity, averaged over the surface of a room, varies from 10-6 to 10-2 m s-1 over the range of pollutant diffusivities and particle sizes encountered. Theoretical predictions are in rough agreement with the limited experimental data that have been taken inside rooms. Results show that if buildings were designed and operated such that natural convection or forced laminar flow conditions prevailed with surface temperatures a few degrees K warmer than the room air, soiling of vertical surfaces due to deposition of soot particles in the size range 0.1-1 mm diameter could be greatly reduced.

Nazaroff, W. W., M. P. Ligocki, T. Ma, and G. R. Cass, "Particle Deposition in Museums: Comparison of Modeling and Measurement Results," Aerosol Science and Technology, Vol. 13, 1990, pp. 332-348.

ABSTRACT-Deposition of airborne particles may lead to soiling and/or chemical damage of objects kept indoors, including works of art in museums. Measurements recently were made of the deposition velocity of fine particles (diameter range: 0.05-2.1 mm) onto surfaces in five Southern California museums. In this paper, theoretical predictions of particle deposition velocities onto verticle surfaces are developed for comparison against the experimental results. Deposition velocities are calculated from data on surface-air temperature differences and near-wall air velocity using idealized representations of the air flow field near the wall. For the five sites studied, the wall-air temperature differences were generally in the range of a few tenths to a few degrees Kelvin. Average air velocities measured at 1 cm from the wall were in the range 0.08-0.19 m s-1. Based on the combination of modeling predictions and measurement results, the best estimate values of deposition velocity for the wall studied at each site are obtained. These values are in the range (1.3-20) x 10-6 m s -1 for particles with 0.05-mm diameter and (0.1-3.3) x 10-6 for particles with 1-mm diameter. The range of 15-30 in deposition velocity for a given particle size is due primarily to differences among sites in the near-wall air flow regime, with the low and high values associated with forced laminar flow and homogeneous turbulence in the core of the room, respectively.

Nazaroff, W. W., and G. R. Cass, "Protecting Museum Collections from Soiling due to the Deposition of Airborne Particles," Atmospheric Environment, Vol. 25a, Nº 5/6, 1991, pp. 841-852.

ABSTRACT-Objects in Southern California museums may become perceptibly soiled within periods as short as a year due to the deposition of airborne particles onto their surfaces. Methods for reducing the soiling rate include reducing the building ventilation rate, increasing the effectiveness of particle filtration, reducing the particle deposition velocity onto surfaces of concern, placing objects within display cases or glass frames, managing a site to achieve low outdoor aerosol concentrations, and eliminating indoor particle sources. A mathematical model of indoor aerosol dynamics and experimental data collected at a historical museum in Southern California are combined to illustrate the potential effectiveness of these control techniques. According to model results, the soiling rate can be reduced by at least two orders of magnitude through practical application of these control measures. Combining improved filtration with either a reduced ventilation rate for the entire building or low-air-exchange display cases is a very effective approach to reducing the soiling hazard in museums.

Secondary Publications
Nazaroff, W. W., Mathematical Modeling and Control of Pollutant Dynamics in Indoor Air, Ph.D. Thesis, California Institute of Technology, Pasadena, California, 1989, 296 pages.

ABSTRACT-To assess the total human-health and material-damage associated with air-pollutant exposure, the concentration and fates of pollutants in indoor atmospheres must be understood. Three observations reinforce this point: (1) concentrations of many pollutants are commonly higher in indoor air than in outdoor air; (2) in many countries, people spend more time indoors than outdoors; (3) many of the most precious material possessions of society are kept indoors. In this thesis, mathematical models are developed as tools to improve the understanding of pollutant dynamics in indoor air. These tools are applied to the problem of protecting works of art from damage due to air pollutant exposure, particularly for the purpose of understanding how to control soiling due to airborne particle deposition.

A deterministic mathematical model is first formulated to describe the time-dependent concentrations of chemically reactive gases and airborne particles in indoor air, then implemented as a computer program. Using a flexible, multichamber description of a building, the model accounts for the effects of ventilation, filtration, deposition onto surfaces, and direct emission for all pollutants. In addition, the influence of homogeneous photolytic and thermal chemical reactions homogeneous photolytic and is computed for gases that are present in photochemical smog. The model is capable of determining the chemical composition and size distribution of indoor aerosols, accounting for the effect of coagulation in addition to the processes itemized above. The model computes the fate of pollutants in indoor air, determining the absolute strengths of the sources and sinks for each species.

To permit the simulation of soiling problems, modeling calculations for the deposition of particles and other pollutants onto surfaces are particularly detailed. Equations that predict the rate of pollutant deposition onto indoor surfaces are developed, accounting for the effects of advection, diffusion, and, for particles, gravitational settling and thermophoresis. Three airflow regimes are analyzed: natural convection induced by a temperature difference between the surface and the nearby air, forced laminar flow parallel to the surface, and homogeneous turbulence in the core of the room. The analysis of a vertical isothermal flat plate in natural convection flow shows that, for this flow regime, thermophoresis is an important particle transport process within the boundary layer adjacent to the surface, effectively repelling particles larger than approximately 0.1 µm in diameter if the surface is even a few degrees K warmer than the nearby air.

To test model performance, and to investigate the dynamic behavior of indoor pollutants, the model is applied to several indoor air quality problems. In one case, modeling predictions are made of pollutant concentrations in a museum gallery in Southern California into which photochemical smog is introduced by the ventilation system. Good agreement is obtained between measured and modeled concentrations of NO, NO2, NO3, and N2O5, due to chemical reaction within the museum atmosphere. The aerosol mechanics aspects are tested by applying the model to the problem of predicting the evolution of the aerosol size distribution following combustion of a cigarette in a single room having a low air-exchange rate, and good agreement is found between model predictions and measured values.

The completed indoor air quality model next is used to evaluate the soiling hazard to art objects in museums resulting from the deposition of particles containing elemental carbon (soot) or soildust. Time-resolved measurements of the indoor and outdoor aerosol size distribution in three Southern California museums are reported. Model predictions of indoor aerosol characteristics based on measured outdoor aerosol characteristics and data on building dynamics agree well with measurements. The predictions also show that generally less than 1% of the fine particles (0.05-2 µm in diameter) entering the museum deposit onto the walls. Nevertheless, deposition calculations indicate that, at the rate determined for the study days, elemental carbon (soot) particles would accumulate on vertical surfaces in the museums at a rate sufficient to yield perceptible soiling in characteristic times of 1-40 years, depending on the museum studied. These are very short periods, considering that many art objects are to be preserved indefinitely.

To test the accuracy of the particle deposition calculations, model predictions are made of the annual mean deposition velocity of particles into walls of five Southern California museums, using the results of short-term monitoring of near-wall air velocities and long-term monitoring of surface-air temperature differences. The predictions are compared against the results of measurements in these museums of the deposition velocities of sulfates and fine particles. The modeling and measurement results generally concur, revealing that the deposition velocities for a given particle size vary by a factor of as much as 30 among the sites studied, with the lowest values being associated with laminar forced flow adjacent to the building walls, and highest values found in museums where deposition is driven by turbulence in the core of the room.

Methods for reducing the soiling rate of objects displayed in museums are identified and include the following: (1) reducing the rate of supply of outdoor air into the building; (2) increasing the effectiveness of particle filtration; (3) altering the airflow conditions within the building to reduce the particle deposition velocity onto surfaces of concern; (4) placing objects within display cases or framing objects behind glass; (5) managing the building site to achieve low outdoor concentrations; (6) eliminating indoor particle sources. The mathematical model of indoor aerosol dynamics is combined with experimental data collected at a historic museum in Southern California to determine the potential effectiveness of these control measures. According to model results, with careful design of control measures the soiling rate can be reduced by at least two orders of magnitude, thereby extending to periods of a century or more the time before noticeable soiling will occur.