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Experimental Measurements of Particulate Matter Deliquescence and Crystallization Relative Humidity: Application in Heritage Climatology

Category: Aerosol and Atmospheric Chemistry

Volume: 15 | Issue: 2 | Pages: 399-409
DOI: 10.4209/aaqr.2014.11.0289
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Marco Casati , Grazia Rovelli, Luca D’Angelo, Maria Grazia Perrone, Giorgia Sangiorgi, Ezio Bolzacchini, Luca Ferrero

  • Department of Earth and Environmental Sciences (DISAT), University of Milano – Bicocca, Piazza della Scienza 1, 20126 Milano, Italy


Aerosol’s deliquescence and crystallization measurements in the Po Valley.
First application in conservation science of aerosol DRH and CRH.
Evaluation of stone decay hazard combining climatic and pollution characteristics.
Chemical stress induced by wetness and mechanical stress induced by crystallization.


Climate and pollution can lead to materials weathering. In this study, an innovative method is employed to evaluate the hazard for heritage stone substrates induced by the synergic effect of particulate matter (PM) and climate. In this respect, two hazard indicators for stone materials were determined: the time of wetness and the number of dissolution and crystallization cycles. The two indicators were computed by coupling experimental measurements of the PM deliquescence and crystallization relative humidity with climatic data. For the first time, these indicators were estimated based on the PM hygroscopic properties, considering its whole hysteresis loop and its consequent hydration level.

The proposed method was applied to PM samples collected in the polluted Po Valley (Milan): the experimental measurements of both PM deliquescence and crystallization relative humidity were performed in an environmental-controlled chamber using an electrical conductivity method. The time of wetness and the number of dissolution and crystallization cycles were then calculated by coupling the PM deliquescence and crystallization relative humidity with climatic data of Milan over the last decade (2003–2013). Results point out that, depending on the seasons, different hazards were identified. In winter, high time of wetness (89 ± 11%) and low number of cycles (3 ± 3 cycles/month) were found. Conversely, summer was characterized by low time of wetness (20 ± 13%) and high number of cycles (11 ± 5 cycles/month). Interestingly, spring and fall resulted the most dangerous seasons for outdoor-exposed stones, since they presented both high time of wetness and number of cycles. Since the two indicators are calculated considering PM properties and climatic data, their values are site-specific, while the method used for their determination is of general application and it can be used for an efficient hazard assessment in a heritage climatology perspective.


Time of wetness Hygroscopicity DRH CRH Particulate matter Cultural heritage

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