The NEVALPA project: A regional network for fog chemical climatology over the PO Valley basin

This paper describes the NEVALPA network, aimed at the study of fog water chemical composition in the Po Valley (northern Italy), its variability and the contribution of fog droplets to the total chemical deposition in the valley. The NEVALPA network is comprised of seven sites representative of the whole Po river basin. The results of the fog campaign carried out during the fall-winter season 1992–1993 are presented here. Higher fog occurrence was recorded in the central and eastern part of the valley, with January being the month with the overall highest fog frequency. An overall high pollutant loading of fog droplets was measured at all locations of the network with a maximum value of fog water ionic strength of 78,500 μeq −1 at a liquid water content of 45 mg m−3. NH4+, SO42− and NO3− were the major components of fog water, accounting on average for 91 ± 4% of total ionic strength. Overall, the acid content of fog water in the Po Valley was not high, although pH values as low as 3 were sometimes recorded. The median pH value of fog water increased from west to east, with maximum values in the central part of the valley. Median ionic concentration values in the different stations differ within a factor of 2, an indication of the relative homogeneity of pollution throughout the Po Valley during the fall-winter season. No definite seasonal trend of fog water chemical composition was observed at any of the sites.