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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
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.
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
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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.
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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
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.
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).
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 analysisneed 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
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.
