Response of surface water acidification in upper Yangtze River to SO2 emissions abatement in China.

Surface waters in Europe and North America are slowly recovering from acidification following major reductions in emissions of sulfur dioxide (SO(2)) since the 1980s. In contrast, regions affected by acid rain have been reported to be growing in China. Here we show that the rapid change in surface water chemistry in the 1990s in large areas in Southwestern China, specifically the tributary rivers of the Upper Yangtze River, caused by increasing SO(2) emissions, has leveled off. During the 1990s the sulfate (SO(4)(2-)) concentrations in river water increased rapidly and, on average, doubled in only eight years. Simultaneously, calcium (Ca(2+)) concentrations increased, while pH values decreased. In the following decade (2000s), SO(2) emissions stabilized, causing a subsequent stop to the increasing SO(4)(2-) concentrations and pH decline in river water. Although a rapid response to future reduction in SO(2) emissions can be expected, a rapid increase of nitrogen (N) emissions could lead to increases in N leaching and delay recovery.