The Precipitation Acidity Trend in Shanghai during 1993-2018

The long-term trend of pH value and conductivity in precipitation at Baoshan and Pudong in Shanghai during 1993-2018 are reported and the main reasons for the formation of the acid rain at both sites are analyzed. The Yangtze River Delta is one of the severe acid rain areas in China, and the multi-year precipitation pH values at Baoshan from 1993 to 2018 and Pudong from 2008 to 2018 were 5.51 and 4.83, respectively. The precipitation pH trends at Baoshan during 1993-1999, 1999-2009, 2009-2018 were different. The annual precipitation pH variability during 1993-1999 at Baoshan was 0.15 a-1, indicating an increased trend of the pH value. While that during 1999-2009 was- 0.08 a-1, indicating a more severe acid rain pollution trend during this period. But that during 2009-2018 was 0.06 and 0.07 a-1 at Baoshan and Pudong respectively, indicating an upward trend of the pH value at both sites which both consistent with the trend in the Yangtze River Delta. In addition, the acid rain frequency showed the same trend of pH value. The electrical conductivity at two sites declined obviously during 2009-2018. The changes on the regional SO2 emissions during 1993-2018 were possible reason for the variation characteristics of acid rain. Generally speaking, precipitation at Pudong showed weaker acidity than Baoshan which may because the different pollutants around the sites. The pH value and precipitation were negatively correlated at Baoshan while positive correlated at Pudong. The atmospheric pollutants and precipitation were important effects on the changes of precipitation pH at Baoshan and Pudong.