Air quality and rainwater composition have recently acquired great importance all over the world. The main effects of acidification of meteorological precipitation reported in the literature are: damages to forests, changes in aquatic plant communities, changes in sediment metal concentration, etc.
Atmospheric deposition is variable in time and space, and its quantification can be evaluated by sampling and analysing the physical and chemical characteristics of the wet and dry depositions. Therefore, a useful monitoring network requires a large number of samplers to estimate accurately the spatial wet and dry deposition fields.
In Italian and in some European networks samples for atmospheric deposition are often collected weekly. This sampling method however, though required in data processing, loses its significance when studying the way a rainfall scavenges the atmosphere below the clouds. At the beginning of the rainfall, the raindrops widely scavenge the atmosphere under the clouds, causing a heavy contamination in the first samples. The washing out of the components of marine aerosol appears to be higher in the first fractions with respect to the following ones.
This paper presents the results obtained with a new rainfall-pH-meter created by the Istituto per il Monitoraggio degli Agroecosistemi. Its characteristics make it suitable for use in networks for monitoring atmospheric deposition.
The components include a water-sensing instrument to detect rainfall occurrence, and containers for wet and dry deposition. A mechanical lid is used to cover both the dry deposition collector during rainfall and the wet deposition collector when there is no rainfall. During rainfall, the pH values are determined every 0.5 mm of rainfall.
Depth, duration and intensity of rainfall are determined by counting the number of pH measurements and the elapsed time interval between each sampling.
To verify the system performance, the rainfall-pH-meter and the dry deposition sampler were installed in an urban area in north Sardinia (Sassari-Italy), 12 km from the sea and data were collected for two years. During testing period, continuous data on depth, duration, and intensity of rainfall and pH data trends during rainfall events were collected.
The values of pH for all the rainfall events showed a characteristic trend with higher values of pH at the beginning of the event (pH=6.5-7) followed by a subsequent decrease (pH=4.5-4).
Higher values for pH at the beginning of the event clearly point out a heavy contribution from the components of marine aerosol; furthermore, they show that calcareous material in air dust is the usual neutralising agent of rainwater.
The results reported in the present investigation provide a useful tool to elucidate the relative contribution of wash-out characteristic of every rainfall event.