Fog occurrence and chemical composition in the Po valley over the last twenty years

Frequency of fog events together with fog water chemical composition, pH, conductivity and liquid water content have systematically been measured from the end of the 1980's at the field station of San Pietro Capofiume, in the eastern Po Valley, Northern Italy. In agreement with what has been observed in other regions in Europe, fog frequency (visibility < 1 km) has decreased over the last three decades. Ionic strength and conductivity of fog samples also decreased over the period indicating a reduction of the ionic load of the droplets. Specifically, the three major inorganic ions (NH4+, SO42−, NO3−), accounting for 86% of the total fog water ionic strength, show a decreasing trend in concentration over the period, which can be linked to the decreasing trend of NH3, SO2 and NOx emissions registered in northern Italy over the same period. Sulphate exhibits the highest relative decrease (76%). Seasonal volume-weighted means of pH show an increasing trend over the observed period. The available data of total water-soluble organic matter concentrations indicate that organic compounds represent a considerable fraction (25% on average) of the total solute mass of fog water. Fog water samples often contain suspended insoluble particles, which were collected by filtering fog water through quartz fibre filters. EC-OC analysis performed on the filters collected over a four-year period, show that the sum of elemental carbon (EC) and water-insoluble organic mass accounts on average for 46%-56% of the total suspended material mass. Insoluble carbonaceous material is composed mainly of organic matter, with EC accounting on average only for 19% of the insoluble carbon.