Interactions between Atmospheric Particulate Matter and Stone Surfaces by Means of Laboratory and Field Studies

The PhD work is focused on the interactions between atmospheric particulate matter (PM) and stone surfaces, particularly with reference to the decay phenomena that can happen as a consequence of PM deposition. Laboratory studies have been focused on the relationship between PM’s chemical composition and the decay hazards to which the stone surfaces subjected to PM deposition are exposed. A new evaluation method has been proposed. The method exploits experimental measurements of deliquescence and crystallization relative humidity (DRH and CRH, which depend on the chemical composition) with the goal to compute hazard indicators for exposed stone substrates, like time of wetness (TOW) and number of dissolution crystallization cycles (Ncy). Experimental measurements of DRH and CRH have been carried out in an Aerosol Exposure Chamber with a conductance method. It has been point out that different decay mechanisms can be potentially active, depending on the season, as a result of a synergic effect between climatic characteristics and chemical composition of PM. In Milan, winter is characterized by high time of wetness (89±11%) and low number of cycles (3±3 cycles/month). Conversely, summer is 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. The field activity has been focused on the set up of a new exposure method, named “deposition box”, which can collect dry PM deposits on different kind of materials. Several sampling campaigns have been performed, some with a seasonal timescale, other over a longer period, in two different exposure sites placed in Milan: Torre Sarca and Villa Necchi Campiglio. A novel method, based on scanning electron microscope images, has been developed to carry out the dimensional characterization of deposed particles. A significant presence of fine particles (0.4-5 µm2) has been highlighted. In addition to this, relevant differences in the deposition rates and chemical compositions of the deposits have been detected depending on the season and the exposure site. Ion chromatography analysis of deposed particles highlighted a different behaviour of the exposed substrates depending on the material. This results confirm the vulnerability of the exposed stone material (Carrara Marble) to certain decay phenomena, like sulphatation. With the aim to study more in detail the impact of the PM fine fraction on the decay of stone substrates, a new sampling device has been designed and assembled. Taking advantage of the principle of sampling by impact, this device can separate and collect single dimensional fractions of PM that are directly deposed on any kind of substrates, including stone specimens. Atmospheric aerosol samples collected with the new device, have been studied by measuring the surface conductance during hygrometric variations. Results of this study highlighted how the fine PM fraction can play a fundamental role in the decay of the materials subjected to PM deposition.