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Based on the measurement records of over 10 years, an analysis is made to reveal the seasonal variation pattern, long term trend and formation causes of acid rain in Tianjin. The statistic results show that obvious seasonal variation patterns exist in both the occurrence frequency of acid rain and the average pH value of precipitation. The acid rains with pH value lower than 5.6 appeared mainly in summer and autumn, but the acid rains with lower pH value appeared in autumn and winter. The result of analysis also shows an obvious decreasing trend of the frequency of acid rain in Tianjin in last 13 years. The possible reasons for this longterm trend are discussed. The reduction of the SO2 emission in Tianjin area is considered as an important contribution to the decreasing trend of the frequency of acid rain locally.