FORMIATE AND ACETATE IN WET DEPOSITION AT PALLANZA (NW ITALY) IN RELATION TO MAJOR ION CONCENTRATIONS

Weekly samples of wet deposition were collected at Pallanza (NW Italy) from January 1987 to December 1988. Their chemistry is characterized by high mineral acidity, with a median pH value of 4.26; only in 2 out of 62 samples was pH higher than 6.0. Sulphate, nitrate and ammonium are the main ions. Formiate and acetate showed a volume weighted average of 7 and 4-mu-eq L-1, respectively; the values for the dissociated forms are 5 and 1-mu-eq L-1, respectively. The contribution of formiate and acetate (dissociated form) to the total ionic concentration is about 2%, while the contribution to free hydrogenion is about 13%, mainly deriving from formic acid. Seasonal variations in concentration and the relationships between organic acid and other ions indicate that the photolysis of isoprenoid compounds released into the atmosphere from vegetation is a significant source for formic acid. In the case of acetic acid there is a contribution from anthropogenic emissions, more marked during the winter period.