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OCR for page 197
Measurement of Local Cl~matolo~cal
and Air Pollution Factors
Affecting Stone Decay
IVAR TOMBACH
The atmosphere is a primary contributor to the decay of stone in historic
buildings. These atmospheric contributors range from the natural conse-
quences of rainfall, wind, frost, and heat to the more complicated chemical
and biological processes resulting from pollution. A list of such factors, though
extensive, can be broken down into groups depending on: the available mois-
ture Rain, fog, humidity); the temperature of the air; the cooling and heating
of surfaces {by wind and radiation) and the evaporation and condensation of
moisture on them; the motion of the air twind); and the presence of air con-
stituents and contaminants "gaseous and aerosol!. The effectiveness of these
factors depends on the time of day and seasons of the year, as well as on large-
scale meteorological phenomena and human activities.
Techniques for measuring parameters within each group have been well
developed in the fields of meteorology, aerodynamics, and air pollution. These
methods can be applied to assist in research on stone preservation and can
also provide data for developing strategies to protect specific structures.
Throughout the ages, stone has been used as a building material be-
cause it lasts longer then wood or other materials. Even the most
permanent stone structures are subject to attack by nature, of course,
but the typical time scale over which damage occurs from natural
Amp rover many Harmon life snans [except when damage is caused
~ v ~ ~ -a rid At- ~
Ivar Tombach is Vice President of Environmental Programs, AeroVironn~ent; Inc., Pas-
adena, Calif.
197
OCR for page 198
198
CONSERVATION OF HISTORIC STONE BUILDINGS
by cataclysmic events). However, human activities in an industrialized
society have inadvertently contributed to a dramatic acceleration in
the rate of decay of historic stone structures, to the point where the
year-by-year decay of stone structures built decades, centuries, or even
millenia ago is now often clearly perceptible.
Both the natural and human causes of such destruction of stone are
becoming better understood, and efforts are being made throughout
the world to preserve structures of particular historic significance. It
is the purpose of this paper to aid in this preservation effort by eval-
uating some of the factors that cause stone decay from the viewpoint
of atmospheric physics. The intent is to discuss ways to better under-
stand the atmospheric conditions that influence the decay of a partic-
ular structure so that the preservationist can develop the best approach
for protecting the structure. Such protective measures can range from
control of external factors say, by eliminating a source of decay-caus-
ing air pollution or by protecting a structure from rain or air pollution
to physical or chemical treatment of the stone itself.
ATMOSPHERIC VARIABLES AFFECTING STONE DECAY
The `decay of stone can be caused by a variety of mechanisms. ~ - 7 These
mechanisms can be classified into categories as shown in Table 1.
Atmospheric factors that participate in these mechanisms are also
shown in the table. An effort has been made to distinguish factors that
contribute directly to a mechanism or to its destructiveness from those
that participate more indirectly; the distinction is often subtle, and
therefore the assignments are not necessarily unique. As an example
of a secondary factor, the presence of atmospheric pollutants or aerosol
is not necessary for changes to take place in the volume of material
within interstices in the stone, but the material whose expansion causes
the damage is often the by-product of an earlier chemical reaction with
an atmospheric pollutant.
The mechanisms of stone decay require, almost universally, the
presence of water (in either gaseous or liquid form), and many of the
mechanisms require the existence of foreign materials in the stone or
on its surface. These impurities are usually introduced to the stone by
wet or dry deposition from the atmosphere, or are the by-products of
chemical reactions with these atmospheric materials. The processes
of wet and dry deposition of gases and particles are the subjects of
another paper in this volume and therefore will not be discussed here.8
Like the decay mechanisms, the deposition mechanisms depend on
atmospheric factors, as summarized in Table 2.
OCR for page 199
Cl~matological and Air Pollutants Affecting Decay
199
The atmospheric factors that affect stone decay directly, and also
affect the deposition rate, can be grouped into categories, as follows:
1. The available moisture (from precipitation, fog, humidity). Al-
most all decay mechanisms require some water, although heavy rain-
fall can wash away or dilute impurities and slow their attack on the
stone. Hygroscopic aerosol particles grow at high humidities (typically,
relative humidities greater than 70 percent) and are then more prone
to gravitational settling or wind-caused impaction onto stone surfaces.
2. The temperature of the air. Damage occurs whenever the freezing
point is crossed. Most chemical reactions proceed more rapidly as the
temperature increases.
3. Solar insolation. Radiative cooling of stone at night can result in
condensation of water on an otherwise dry surface and cooling or
heating of the stone relative to the air affects deposition rates, as do
evaporation and condensation.
4. Wind. The kinetic energy of abrasive particles and the degree of
inertial impaction of particles or droplets onto the stone are dependent
on the wind.
5. Air constituents and contaminants {gaseous and aerosol). Con-
stituents in the air determine the rates of some forms of chemical
attack and are often a necessary precursor of physical or chemical decay
mechanisms. Natural constituents, such as CO2 and sea-salt aerosol,
play a role, as do manufactured pollutants. Obviously, the rate of dep-
osition of a chemical is proportional to its concentration in the air.
To evaluate the significance of each of these factors in a given sit-
uation requires, first, an understanding of which mechanisms are po-
tentially of concern for the type of stone, the construction method,
and the foundation soil chemistry and moisture. By measuring the
relevant atmospheric variables, it is then possible to determine, at least
qualitatively, the potential diurnal and seasonal variability in the strength
of the decay and deposition mechanisms. For example, Fassina has
studied the effects of environmental conditions on the detenoration
of stonework in Venice using daily measurements of meteorological
conditions and of some atmospheric pollutants.6
In a few cases where a theoretical or empirical basis has been de-
veloped to describe a decay mechanism quantitatively, it may even be
possible to predict the behavior and to compare the relative significance
of several mechanisms. Chemical reaction rates are in this latter cat-
egory,9 along with the stresses caused by freezing wateri° and various
OCR for page 200
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OCR for page 203
Cl~matological and Air Pollutants Affecting Decay
203
deposition relations.8 As an example of an empirical quantitative re-
lationship, Hudec has developed regression expressions relating stone
damage to quantifiable physical properties of the stone and to the
degree of saturation of the stone and the freezing of internal water.
MEASUREMENT OF ATMOSPHERIC FACTORS
The factors described above can be measured easily in some cases and
with great difficulty, or not at all, in others. This discussion will briefly
evaluate the availability of suitable measurement methods for these
factors. The focus is on approaches that could be used by the stone
preservationist, usually within the confines of a limited budget and
without the aid of a meteorologist, atmospheric physicist, or air pol-
lution specialist. The emphasis is on methods that can be used in an
operational mode for long-term data gathering; additional techniques
that are more exacting and labor intensive may be appropriate for
specific short-term studies. Because the measurements discussed will
often be unfamiliar to the stone preservationist, factors that should be
considered in their use will also be mentioned.
Obviously, whenever appropriate meteorological or air pollution data
are available from a government, university, or private monitoring
station, the use of those data is the most efficient and least expensive
way to acquire information. As an example, Winkler has studied mE-
teorological effects on the deterioration of the National Bureau of
Standards test wall using meteorological data from the Washington
National Airport. Such data may not always represent the meteor-
ological conditions that are affecting a specific stone structure, how-
ever; some cases will be pointed out below.
A tabulation of measurement methods that might be useful for stone
preservation work appears in Table 3. As a guide for acquiring suitable
instruments, the purchase cost is described as "low" if the instrument
costs less than $500, "moderate" if it costs between $500 and $2,000,
and "high" if more than $2,000 is required. Operating costs are harder
to quantify and depend considerably on the specific location of the
study site. The same terms "low," "moderate," and "high" are used
to describe operating costs, but only in a relative way. Similarly, the
difficulty of using a given method (in tempts of reaming time, technician
expertise, attention to detail, frequency of calibration, and difficulty
of maintenance) is also indicated in relative tens using the same
expressions. The comments below on the measurement methods sup-
plement the material in the table.
OCR for page 204
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OCR for page 205
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OCR for page 206
206
Rainfall
CONSERVATION OF HISTORIC STONE BUILDINGS
For climatological purposes, daily rainfall data will suffice. For detailed
studies, the intensity of rainfall (cm/min or cm/in) is relevant because
more intense rainfall washes more effectively and also may be chem-
ically less reactive. Data from a nearby government weather station
may be sufficient, but rainfall can vary sigruficantly over distances of
a few kilometers. If the wetting of a specific wall is of interest, then
a rain gauge has to be installed next to that wall. A windward wall
will be wetted considerably more than a leeward wall. Architectural
features can protect some portions of a wall.
Samples for pH measurement or chemical analysis can be those
collected by the rain gauge, but it is frequently more practical to use
a sample from a specifically designed collector. The samples are ana-
Tyzed at an analytical laboratory using relatively standard techniques.
Care has to be taken to avoid changing a sample's chemistry during
collection and handling; it should not be kept in the sampler any longer
than necessary, preferably no more than a day.
Fog
The parameter of greatest interest with fog is its liquid water content,
which is measurable only with specialized research-grade samplers.
The presence or absence of fog, and its visibility-impairing effects, can
serve as useful indices of the presence of liquid water for many pur-
poses, however. For qualitative purposes, visibility determinations at
a nearby airport may be usable, but such information should be used
cautiously since the existence of fog at a specific location depends
considerably on the elevation or proximity to a body of water. The
urban heat-island effect generally reduces fog in cities, but pollution
from a city often increases fog downwind.
Humidity
Relative humidity is fairly easy to measure if great accuracy is not
required. However, it is difficult (and expensive) to measure if, say, 1
percent accuracy is desired or when the humidity is near the saturation
point of air. Local airport or weather service data may be adequate for
many situations. In this case one should use dew point, rather than
relative humidity, because dew point is a characteristic of the water
content of the larger-scale air mass, while relative humidity depends
on the local temperature and therefore depends on local factors. Dew
OCR for page 207
Climatological and Air Polls tan ts Affecting Decay
207
point is converted to relative humidity using the temperature mea-
sured at the study site.
Temperature
Proper measurement of air temperature requires that the sensor not
be cooled or heated by radiation and hence that it be installed in a
radiation shield. Expert advice should be sought for selecting the ap-
propriate shield for use near a wall to avoid reflected radiation. Because
of local heating and limited air circulation, the air temperature near
the walls of a building could vary from one side of the building to the
other.
Thermistors can also be embedded in the stone to measure the wall
temperature. The most useful measurement location is probably as
close as possible to the exterior surface. Circuits are available, at rea-
sonable prices, that can compare the temperatures measured by the
two matched sensors in the wall and in the air with an accuracy of
better than 0.1° C.
Solar Insolation
Solar insolation is a guide to how much the sun's radiation contributes
to temperature changes in a wall. Because local shadows affect the
extent of solar insolation, measurements should be made as close as
possible to the portion of the wall that is of interest, unless only a
general characterization of the amount of insolation is needed.
Standard meteorological sensors of solar insolation can be used to
measure insolation on a wall if they are oriented parallel to the wall's
surface. Similarly, net radiometers are available to measure both the
solar radiation incident on the wall and the radiation emitted by the
wall.
~ Because solar insolation measurements for stone preservation re-
search are somewhat unusual, expert advice should be sought on the
appropriate sensor, its installation, and the interpretation of data from
it.
Wind
The low-speed end of the wind spectrum is of interest for diffusion
and the high-speed end for abrasion. Most inexpensive wind sensors
lack a sufficiently low starting threshold to cover the low speeds char-
acteristic of early morning hours.
OCR for page 208
208 CONSERVATION OF HISTORIC STONE BUlEDINGS
The direction of the wind may not be of interest if near-wall mea-
surements are being made; the airflow is necessarily paraDe! to the
wall there (but frequently has a vertical component). For such work,
propeller anemometers are more useful than the more conventional
cup-and-vane sensors.
Because the wind depends so much on local obstructions, weather
service or airport data are useful only as indicators of general direction
and speed of the airflow through a region.
Gaseous Air Pollutants
Concentrations of air pollutants in and near cities vary dramatically
from hour to hour {or even from minute to minute!. Therefore, only
an instrument that can respond to these changes can lead to meaningful
assessments of the effects of pollutant fluxes to material surfaces. The
same sort of response is needed from the sensors that determine whether
there is a deposition flux toward the surface at any given time. Thus,
sensors that integrate pollutant concentrations over long periods are
generally not useful for detailed studies because the average deposition
flux of material to stone surfaces depends not only on the average
concentration, but also on the correlation of the concentrations with
a positive deposition flux. Sulfation plates are an exception. They are
long-te~m sensors that are potentially useful because they directly
measure the deposition of SO2 to the plate.
Ideally, one would like an existing air pollution monitoring station
within a kilometer or two of the study site, with no nearby pollution
sources to cause the concentrations of SO2 or NOx at the study site
to differ from those at the station. Otherwise, air pollutant monitoring
becomes an expensive venture, and expert help win certainly be needed
to calibrate the instruments. Fortunately, commercially available in-
struments (especially those certified as "reference or equivalent meth-
ods" by the U.S. Environmental Protection Agency) are stable and
reliable when properly used. Fully self-contained dry methods exist for
detecting both NOx and SO2; there is no need to deal with the com-
plexity of sensors that require auxiliary compressed gases or perform
analyses by wet chemistry. Although the state of the art of air mon-
itoring is changing rapidly, Stem provides an excellent starting basis
for understanding the science.~3
Aerosols
Automated aerosol analyzers that would be appropriate for stone-pres-
ervation research do not exist (with one exception mentioned below).
OCR for page 209
Climatological and Air Pollutants Affecting Decay
209
The primary interest is the chemical content of the aerosol, especially
the presence of salt (NaCll, sulfates (SO4=l, nitrates (NO3 I, and the
ammonium ion (NH4 ). The usual technique, therefore, is to draw air
through one or more filters and analyze the collected material in the
laboratory. Lundgren et al. provide a useful reference on the state of
the art of aerosol collection and analysis.~4 Stem also covers the subject,
but in a more introductory manner.
The filter material is critical because some particles and gases react
with the filter and form 'filter artifacts" and because the filter has to
be compatible with the analysis procedure. For most purposes, Teflon
or Teflon-coated filters are the most appropriate. Polycarbonate filters
(Nuclepore) are necessary for electron microscope analyses; nylon fil-
ters collect gaseous nitric acid tHNO3) in the air; and prefired quartz
fitters are required if an analysis for carbon or soot is planned.
The filters can be analyzed at a commercial, university, or govem-
ment laboratory that is familiar with the handing of air pollution
samples. X-ray spectroscopy techniques POE (particle-induced X-ray
emission spectroscopy) and XRF (X-ray fluorescence~are inexpensive
and describe much of the composition of the aerosol. Such techniques
directly identify the NaC] content; they also indirectly provide the
SO4= content, and experience has shown that essentially all the sulfur
in the air is in the sulfate form. Wet chemical techniques are needed
to identify NO3 and NH4 and are also appropriate for SO4-.
The two-stage approach sampling followed by analysis—need not
be followed to assess sodium and sulfur-containing particles. In situ
measurement of these elements (and thus, for all practical purposes,
of NaC1 and SO4- ~ in particles has been performed by modifying com-
merciallyavailableflame-photometricairpollutionanalyzers. Pueschelis
describes a sodium particle analyzer, and Coburn et al. and Huntz-
icker et al.~7 describe sulfur particle analyzers. Although these tech-
niques are not available off the shelf, they have sufficient utility in
some research applications to justify the special expertise needed to
use them.
CONCLUSIONS
The atmosphere exerts a significant influence on both natural and
human-related mechanisms of stone decay. Although a complete quan-
titative theory of stone decay is unlikely because of the many site-
specific variables, it is possible to infer relationships between stone
decay and atmospheric conditions. In most cases relatively standard
instruments used by meteorologists and air pollution scientists can be
OCR for page 210
210
CONSERVATION OF HISTORIC -STONE BUILDINGS
applied, with perhaps minor adaptations, to studies of stone decay.
Stone preservationists therefore do not need to develop methods for
atmospheric measurements; they are also able to draw on the expertise
of the meteorological and air pollution scientific communities for as-
sistance in their efforts.
REFERENCES
1. Torraca, G. {1976) Brick, adobe, stone, and architectural ceramics: Deterioration
processes and conservation practices. Preservation and Conservation Principles and
Practices. Preservation Press, National Trust for Historic Preservation in the United
States, Washington, D.C., 143-165. Vittori, O. {1976) Secondary sinks in atmospheric
gas dispersion models. Proceedings Seminar on Air Pollution Modelling, IBM Italy, Venice
Scientific Center, 27-28 November 1975.
2. Keller, W.D. t1977) Progress and problems in rock weathering related to stone
decay. Engineering 'Geology Case Histories Number 11, Geological Soc. of Am., 37-46.
3. Winkler, E.M. {1977J Stone decay in urban atmospheres. Engineering Geology
Case Histories, Number 11. Geological Soc. of Am., 53-58.
4. Hyvert, G. (1977) Weathering and restoration of Borobudur Temple, Indonesia.
Engineering Geology Case Histories Number 11, Geological Soc. of Am., 95-100.
5. Gauri, ILL. {1978) The preservation of stone. Scientific American, June.
6. Fassina, V. {1978} A survey on air pollution and deterioration of stonework in
Venice. A twos. Env. 12, 2705-221 1.
7. Hansen, J. (1980) Ailing treasures. Science 80, 1, 58-110.
8. Hicks, B.B. (1981) Wet and dry surface deposition of air pollutants and their
modeling. National Academy of Sciences Conference on the Conservation of Historic
Stone Buildings and Monuments, Washington, D.C., 2-4 February, 1981.
9. Vittori, O. t1976) Secondary sinks in atmospheric gas dispersion models. Pro-
ceedings Seminar on Air Pollution Modelling, IBM Italy, Venice-Scientific Center, 27-
28 November 1975.
10. Winkler, E.M. {1973} Stone 'Properties Durabi17'tyin Man's Environment. Springer-
Verlag, New York-Vienna, 250.
11. Hudec, P.P. (1977) Rock weathering on the molecular level. Engineering Geology
Case Histories Number 11. Geological Soc. of Am., 47-51.
12. Winkler, E.M. {1981) Problems in the deterioration of stone. National Academy
of Sciences Conference on the Conservation of Historic Stone Buildings and Monuments,
Washington, D.C., 2-4 February, 1981.
13. Stern, A.C., ed. (1976) Air Pollution, Vol. m, 3rd edition. Academic Press, New
York, 797.
14. Lundgren, D.A., F.S. Harris, W.H. Marlow, M. Lippman, W.E. Clark, and M.D.
Durham, eds. {1979) Aerosol Measurement. University Presses of Florida, Gainesville,
716.
15. Pueschel, R.F. jl969) Thermal decomposition of sodium-containing particles in
a flame. [. Co17oid and Interface Sci., 30, 12~127.
1-6. Cobum, W.G., R.B. Husar, and J.D. -Husar {1978) Continuous ill situ monitoring
of ambient particular sulfur using flame photometry and thermal analysis. Atmos. Env.
12, 89-98.
17. Huntzicker, J.J., R.S. Hoffman, and C.S. Ling (1978) Continuous measurement
and speciation of sulfur-containing aerosols by flame photometry. Atmos. Et2v. 12, 8
88.
Representative terms from entire chapter:
air pollutants
