2 BACKGROUND

Calcareous stones exposed to the atmosphere are vulnerable to attack by several processes that occur naturally. These processes include microbial activity on the stone surface, dissolution by rain, and physical stresses such as freezethaw cycles. Anthropogenic air pollutants are frequently responsible for accelerated deterioration, both directly through physical and chemical attack, and indirectly by providing substrates for microbial growth.

In recent years, considerable attention has been given to the role of biological agents in damage to buildings (e.g., Wilmzig and Bock 1995; Freemantle 1996; Mitchell et al. 1996; Young 1996b). In general, species of fungi, algae, lichens, and bacteria have been found on surfaces of building stones (Bock and Sand 1993). These organisms can accelerate deterioration either by physical processes such as alteration of the normal wetting-drying cycle (Young 1996a) or by chemical processes such as mineral and organic acid production and the secretion of metal-chelating agents (Palmer et al. 1991). It is difficult to estimate the quantities and overall effects of biodeteriogens, in part because the fecundity and productivity of these organisms are strongly dependent on microenvironmental factors. These include insolation, stone type and porosity, surface and air temperatures, availability of a suitable substrate, and availability of water from incident rain, stone pore capillarity, or condensation and evaporation cycles (Bock and Sand 1993). In addition to the expected temporal variability caused by changes in the weather (Tayler and May 1991), there can be considerable spatial variability over short distance scales. Understanding biodeterioration processes is further confounded by a possible correlation between air pollution levels and biodeterioration rates (Young 1996a). For example, Warscheid et al. (1991) have shown that some chemo-organotrophic bacteria isolated from sandstones of historic monuments are able to utilize petroleum derivatives as sources of carbon as well as energy.

Several categories of air pollutants can accelerate the natural deterioration of stone through two primary processes: wet deposition and dry deposition (e.g., Amoroso and Fassina 1983; Sherwood et al. 1990). The former refers to the deposition of a pollutant by a precipitation process such as rain or snow; acid rain is an example. Several authors have considered the effects of acid rain on calcareous stones (Braun and Wilson 1970; Livingston 1992; Hutchinson et al. 1993; Mossotti and Eldeeb 1994; Winkler 1996). Dry deposition includes those processes by which pollutants are transported to the surface in the absence of precipitation and become physically or chemically bound to the surface. Damage to calcareous building stone by dry deposition has been attributed largely to sulfur dioxide gas (SO2). For example, Meierding (1993) found that mean surface recession rates of century-old Vermont marble tombstones in the United States were well correlated with SO2 concentrations. In addition, some authors point out that nitric acid gas (HNO3) may also be sorbed onto a carbonate surface (Fenter et al. 1995; Kirkitsos and Sikiotis 1995).

The removal of SO2 by certain stone types is a well-documented phenomenon (Judeikis et al. 1978). Calcareous stones subjected to high relative humidity develop a moist surface layer where SO2 can readily dissolve (Spedding 1969; Spiker et al. 1995); in general, the rate of dissolution increases at higher relative humidities and wind speeds (Spiker et al. 1995). Dissolved SO2 can then oxidize to form a sulfite (SO32−) or sulfate (SO42−) species. The oxidation process results in the production of acid, which can cause the calcium carbonate (CaCO3) in the stone to dissolve. When calcium ions (Ca2+) combine with SO32− or SO42−, CO32− is effectively displaced from the stone surface. This process, known as sulfation, may also involve gaseous and particulate air pollutants other than sulfur species. Gases such as ozone (O3) (Haneef et al. 1992) and nitrogen dioxide (NO2) (Johansson et al. 1988) have been shown to increase SO2 deposition to limestone. Surface crust analyses of damaged stone have also shown a close relation between deposited anthropogenic particles and the formation of gypsum crystals (Del Monte et al. 1981; Zappia et al. 1993; Sabbioni 1994), suggesting a relationship between sulfation and the presence of airborne particles. However, Hutchinson et al. (1992) have reported that limestone seeded with coal fly ash or transition metal oxide catalysts is not susceptible to elevated SO2 deposition. These authors suggest that seeding stone samples with oxidation catalysts has a negligible effect because natural stones already contain high levels of impurities. In contrast, seeding pure CaCO3 with metal oxide catalysts does increase the rate of sulfation.

Urban air pollution studies have considered effects of buildings on dispersion of vehicle emissions as well as dispersion of individual plumes from stationary sources. In general, dispersion of vehicle emissions in street canyons is a function of the building height divided by the street width, known as the aspect ratio (Lee and Park 1994), as well as the geometric configurations of city blocks, the ambient wind direction, and the movement of motor vehicles (DePaul and Sheih 1986; Dabberdt and Hoydysh 1991; Hoydysh and Dabberdt 1994). Qin and Chan (1993) and Qin and Kot (1993) have reported that significant differences in carbon monoxide and nitrogen oxides (NOx) concentrations exist between the top and bottom of buildings surrounding street canyons in Guangzhou, China. Qin and Kot (1990) have also shown that vehicle traffic near a 31-story (100 m) tower can result in elevated NOx concentrations near the downwind building surface up to a height of 66 m. The effect of a building on the dispersion of a stationary source plume is, in general, dependent on the building geometry, source location, and prevailing wind conditions (e.g., Huber et al. 1991; Lee et al. 1991; Thompson 1993). In some cases, direct measurement of the spatial variability of air pollutant concentrations may be easier than application of theoretical considerations.