Abstract:
According to monitored data of acid rain in Xi′an City from 1987 to 2000, the time variation and its chemical composition was analyzed. The results are following: monthly changes of rainfall pH and acid rain form a two-hump mold. Annually，from January to March and from October to November the rainfall pH is low and frequency of acid rain is high, while from April to June frequency of acid rain is lower and from July to September the acid rain pH is higher. During the fourteen years, annual change of the rainfall pH and the acid rain can be divided into two stages: before 1994, the rainfall pH is higher, annual change of the acid rain is inconspicuous, after 1994, the rainfall pH declines, annual change of frequency of acid rain is obvious; Rainfall pH and acid rain are decided by contrast of the total acid-alkali ion, when the consistency of acid ion is more than that of alkaline ion, rainfall pH declines and the damage of acid rain increases, otherwise, rainfall pH rises and the damage of acid rain decreases.
