The objectives are: to provide a better understanding of the processes related to dry deposition of sulphur dioxide and other pollutants, such as nitrogen oxide, hydrous nitrogen oxide and hydrogen nitrate, on stone surfaces (particularly calcium carbonate), from rate measurements; to compare the results obtained by the participants using different methodologies and experimental arrangements, under varying conditions; and to formulate appropriate damage functions of the damage phenomena.
Laboratory and field experiments have been carried out on deposition rates of acidic and other pollutants on calcium carbonate surfaces of various types.
Kinetic laboratory experiments, at various temperatures, using reversed flow gas chromatography (RF-GC), permitted the simultaneous determination of physico chemical parameters pertaining to pollutant deposition velocities and reaction probabilities on solids under nonsteady state conditions (eg propane and hydrogen sulphide on silica, aluminium, marble or silver foil).
A flat diffusion stripper has been built for studying the reactivity of sulphur dioxide and of mixtures of various gases. The heterogeneous oxidation of sulphur dioxide in the presence of nitrogen dioxide was found to depend on the nature of the sample chosen and on relative humidity and thus it was impossible to formulate a general statement regarding the behaviour of carbonaceous materials of different origin (eg charcoal, soot from propane, acetylene flames).
Acid fog is contributing together to other pollutants to destroy the historic buildings in Venice. The concentrations of anions present in fog was found to be higher than in rain. The high sulphate concentration found in fog water confirmed that urban fog conditions enhance the rate of sulphur dioxide oxidation and consequently the damage on stone monuments.
The deposition of sulphur dioxide on marble as a function of time was studied. The sulphur dioxide uptake on clean marble has a strong time dependence. However, towards the end of a 20 hour experiment the samples reach a steady state with respect to sulphur dioxide deposition. The acceleration of sulphur dioxide deposition by humidity was also obvious. The presence of sodium chloride on the stone surface increased the deposition of sulphur dioxide.
Sulphur dioxide was found to be rapidly adsorbed on the surface of calcite forming at least 2 different species, one being present in both humid and dry conditions, while the other is only detect ed in the presence of water vapour. The study also showed that water vapour is necessary for the formation of surface sulphite. The surface sulphite on calcite is rather stable, no evidence for sulphate formation being visible in the infrared (IR) spectra after 20 hours exposure to humid, sodium oxide containing air.
Additional experiments, using other gases as aliphatic and aromatic hydrocarbons, dimethyl sulphide, etc, have been performed with the RF-GC method. The theoretical model was improved to include interactions between two gaseous pollutants and the synergistic effect of these interactions on the deposition and corrosive parametewrs on cultural surfaces.
The methodology to achieve the above goals consists of both laboratory and field experiments on deposition rates of the above acidic pollutants on various samples. These are selected to be the same for all participants, and include chemically pure calcium carbonate, limestone of different origins, marbles, calcareous sandstones, pieces of monuments and others. The polluting atmosphere will be either synthetic of known composition or a real atmosphere. The same gaseous phases are used by all participants.
The studies described above are expected to permit the laboratory and field evaluation of the different measurements, so that a reference method, or a combination of methods, is established, for assessing the deposition rates and fluxes of acidic species on stones, particularly on calcium carbonate. This will be achieved by the better understanding of the mechanisms and the processes related to dry deposition, as well as by defining the physicochemical parameters involved and affecting the rates of the various steps and the overall deposition rate. This will lead to tentative damage functions, thus improving the scientific basis of protection of historic stone buildings. The term damage function does not only mean an equation relating damage to a particular cause. It should rather be interpreted more generally as meaning the development of new concepts, both experimental and theoretical, concerning the action of atmospheric pollutants on stones, thus establishing experimental and theoretical laws for the ph nomena of damage and corrosion.
Funding SchemeCSC - Cost-sharing contracts
412 96 Goeteborg