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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 > (T¡ /|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.
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