The pH of the atmospheric precipitation in Venice, related to both the dynamics of precipitation events and the weathering of monuments

Abstract

The pH of precipitation in Venice during the 1960s appears to have been slightly above the level of uncontaminated rainfall and very high compared with data generally found in the literature for the rest of Italy and Europe in general. On the contrary, from observations of the weathering of marble monuments in Venice, one would expect aggressive precipitation, i.e. that characterized by lower pH levels. However, the pH level is recognized as an inadequate index for the aggressivity of precipitation and the stone deterioration appears to be closely related to the way rainfall wets the surface of the monuments and activates the dry deposition occurring between two precipitation events. Only rarely does it seem to be due to acid rainfall. On the basis of local dynamic climatology, an analysis has been carried out of present and past data and of the possible hypotheses leading to these unusual values. The analysis indicates some interesting situations as being responsible for many precipitation events and also for a high correlation with wind direction. Particularly interesting is the frontal situation originated aloft by a warm, moist wind (i.e. Scirocco) passing over the Alps and a cold wind below from Dalmatia (i.e. Bora). This situation favours heavy rainfall which is relatively uncontaminated by anthropogenic emissions, but is contaminated, on the other hand, by Saharian dust. From laboratory tests this dust was found to be capable of neutralizing the free acidity in rain. Precipitation events associated with NE wind are very frequent, as shown by the precipitation roses. Therefore, in Venice and its hinterland, precipitation dynamics is often associated with the meeting of air masses which are relatively uncontaminated by anthropogenic activity. The presence of buffering agents, both natural (i.e. Saharian dust, soil particles) and anthropogenic (i.e. agricultural fertilizers) are the main causes of precipitation, with high pH levels in the whole of the region subjected to Bora-type inflows, unlike the rest of the territory, as present measurements appear to confirm.