Deposition of Airborne Organic Pollutants on Historic Buildings

-The black crusts coating the surfaces of building materials located in urban (polluted) environments contain all kinds of organic compounds present in aerosols and particulate matter. Wet and dry deposition processes combined with gypsum crystal growth result in dirty, grey-to-black crust formation, in which aerosols, spores, pollen, dust and every class of particulate matter are entrapped in the mineral matrix. Analysis of the organic compounds extracted from black crusts demonstrate them to be mainly composed of molecular markers that are characteristic of petroleum derivatives. The composition of each crust is governed by the composition of the particular airborne pollutants in the area. Key word index: Black crust, gypsum, petroleum biomarkers, fungi, pyrolysis. I N T R O D U C T I O N The Northern Hemisphere is experiencing a series of severe ecological problems with forest decline, acidification of lakes, and accelerated corrosion of monuments, buildings and metallic structures. Thousands of chemicals are emitted directly or indirectly to the atmosphere because of human activities. These pollutants are distributed in the environment and the atmosphere is recognized as a major route for worldwide dispersion. The interaction of materials with the atmosphere has recently received increased attention as a result of concerns regarding the effects of acid deposition. In fact, most pollutants emitted into the atmosphere are eventually removed through naturally occurring cleansing mechanisms. These removal and deposition processes represent the final stages of a complex sequence of atmospheric phenomena. Once released into the atmosphere, organic and inorganic airborne pollutants may undergo a variety of complex interactions determined by physical, chemical and photochemical processes occurring during their residence time. These processes influence the nature of the capture of the pollutants in a sink or reservoir, where they are transformed, immobilized or encapsulated (Schroeder and Lane, 1988). The two mechanisms for transferring pollutants to material surfaces are defined as dry and wet deposition. Dry deposition proceeds without the aid of precipitation and denotes the direct transfer of gases and particulates to the Earth's surface. Wet deposition, on the other hand, encompasses all processes by which airborne pollutants are transferred to the Earth's surface in an aqueous form (i.e. rain, snow or fog). In this case, pollutants can become incorporated by two different mechanisms, in-cloud scavenging (rain-out), which is the main route for wet deposition 77 and occurs when pollutants are included in the droplets developing within a cloud, and below-cloud scavenging (wash-out), which involves the take-up of pollution by precipitation as it falls from the cloud. This is a much less efficient removal process than in-cloud scavenging (Department of Environment, 1989). Sulphur dioxide has long been recognized as the primary gaseous component of air pollution and a correlation exists between sulphur dioxide concentration and sulphatation of carbonatic stones, because oxidation of sulphur dioxide results in sulphuric acid production. In this process, carbonaceous particles are capable of greatly accelerating the destructive action, and behave as catalysts--they contain V, Ti, Fe, Mn, Cu--for the oxidation of sulphur dioxide. Aerosols in the form of sulphuric acid droplets also attack building limestone, the reaction ultimately resulting in gypsum formation. Buildings and outdoor-exposed statues and monuments act as repositories of airborne organic pollutants, which accumulate at the surfaces in zones frequently soaked by rain water but are not washed out. Wet and dry deposition processes combined with gypsum crystal growth result in dirty, grey-to-black crust formation, in which aerosols, spores, po i lu , dust and every class of particulate matter are entrapped in the mineral matrix. The industrialized society of the 20th century has thus caused a radical change in the conditions of preservation and conservation of stone, and the atmospheric pollution associated with industrialization is currently threatening extinction for both cultural heritage and nature itself. C O M P O S I T I O N O F BLACK S U L P H A T E D C R U S T S Generally, black and white areas can be observed on the exposed surfaces of any building. The black