Abstract:Based on the acid rain records over 20 years in Tianjin, characteristics and long term trend of acid rain were analyzed. The results show that the precipitation pH varies in the range of 3.30-8.80, with a large averaged seasonal variation from 1992 to 2012. Totally, there are 182 acid rain events (pH below 5.6), and the frequency is 21.2%. The annual mean precipitation pH shows different variation patterns in the periods of 1992-2002 and 2003-2012. In the former period, it is a steady slow increasing trend and the averaged annual rate of precipitation pH is 0.13 a-1, in the later period, the increasing trend slows down and the averaged annual rate of precipitation pH is 0.11 a-1. The seasonal variations of the pH values are different in recent 20 years. There is no obvious changes in spring, but significant increasing trend in summer and autumn. Precipitation pH value in summer and autumn is the most important factor that affects the pH values of long term trend in Tianjin. The analysis of correlation indicates that the precipitation pH value is negatively correlated with concentrations of SO2 and NO2. Rainfall is acidic due to the influence of local air pollution in addition to the influence of the pollution source around Tianjin.