Deterioration of Jerusalem limestone from air pollutants; field observations and laboratory simulation

Samples of Jerusalem limestones were exposed to high levels of common air pollutants (SO2, NO) in the presence and absence of hydrocarbon, water vapor and ultra-violet light. After exposure, the outer layers were shaved off and analyzed for sulfate and nitrate. The results revealed that even after one day of exposure significant concentrations of CaSO4 and Ca(NO3)2 could be detected in the external 40 μm layers. Sulfate formation was found to relate very strongly to relative humidity with nearly undetectable production at humidities below 10%. Nitrate formation was found to relate to UV light and to a limited extent to the presence of hydrocarbon but was unchanged at different humidities. Surface samples were taken from different sites of the old city wall and were also analyzed for the same substances. The data showed that the concentrations of especially CaSO4, and to a limited extend Ca(NO3)2, measured at various points along the city walls were higher than the expected values extrapolated from laboratory simulations. The elevated pollution content may be explained in part due to the deposition of transported ions, especially sulfate, onto the stone surface. It was concluded that although air pollution may not cause any structural damage in the foreseeable future it definitely deteriorates the fine details of the ancient monument.