Effect of Acid Deposition on Urban Dew Chemistry in Yokohama, Japan

Dews or frosts formed on artificial collectors were collected during the early morning in Yokohama in 1993 and 1994 and analyzed for weak acids as well as major inorganic ions. In 1993 (n=21) the dew pH ranged from 3.23 to 7.12 and averaged 4.60; in 1994 (n=56) it ranged from 4.11 to 7.74 and averaged 5.41. Based on the average, NH₄⁺ and Ca²⁺ were major cations (423 and 178 μequiv L^-1 for NH₄⁺ and Ca²⁺ in 1993, respectively, and 454 and 274 μeuivq L^-1 in 1994, respectively) and SO₄^2- was the dominant anion (344 μeuivq L^-1 in 1993 and 271 μeuivq L^-1 in 1994) in dew water. The dew chemistry was characterized by relatively high concentrations of weak acid anions, particularly hydrogensulfite ion (HSO₃^-). Dew water sometimes had very high acidity (the minimum pH was 3.23 during the period of this research), which might have been due to the aqueous oxidation of S(IV). From a resistance model calculation, the S(IV) concentration in dew water was suggested to be influenced by the dissolution of HCHO from the atmosphere and the subsequent formation of HMSA.