Analysis of the real-time measurement of the pH of rainfall at Padova, Italy: Seasonal variation and meteorological aspects

For some years, the free acidity in rainfall has been measured. Sampling was carried out on a weekly basis and in real time, in order to determine the effects of precipitation that are masked by dry deposition or post-precipitation equilibrium values. First, three classes of rainfall were established, MA, MB and MN (i.e. mainly acid, mainly alkaline and mainly neutral), based on the mean pH value during rainfall. A second classification, based on the trend of the pH curve at the initial stage of precipitation, was made: IA, IB and IC (i.e. initial stage more acid than the mean value, more alkaline, constant). The second grouping was made because the greatest variation in the pH value during rainfall occurs in the initial stage. Surprisingly, when a seasonal analysis was made, the MA class was much less frequent than expected, and was found to be lowest in winter (when SO2 emission is highest) and highest in autumn. The MB class of rainfall was found most frequently in spring, whereas the MN class was most frequent in summer, when precipitation is mainly in the form of showers or thunderstorms, brought about by local conditions of atmospheric instability. Rainfall is often more acidic initially due to the presence of highly soluble anthropogenic emissions, but as it continues, its pH value may change because of the buffering action of soil particles and Saharan dust, which have slower kinetic reactions. It would seem, therefore, that a short-lasting drizzle would be more damaging, as it is in itself more acidic, and because it activates any already existing dry deposition. When the taking of acidity measurements is delayed, the resulting pH represents the equilibrium value rather than the real degree of rainfall acidity. Consequently, the real effect that precipitation has on an exposed surface is not clear. Analysis of the acidity roses showed that the nearby towns of Treviso and Venice, with its industrial zone, have an unfavourable contribution on a local scale. Under certain, well-defined circumstances, it is possible to forecast the relationship between pH and meteorological conditions, on the basis of three different systems of classifying weather types (i.e. Borghi and Giuliacci; U.K. Meteorological Office; Aerospace Science Division). This approach was useful in demonstrating the relationship between acid rain and physical processes, and meteorological conditions involved in the formation and transport of air masses. This approach also proved particularly useful in determining the alkalizing role of both the Bora (which transports soil particles) and the Scirocco (carrying Saharan dust) winds.