SO42- is the predominant acid anion but the acidic effect of NO3- is more and more important. NH4+ and Ca2+ act as acid neutralizers of the rain acidity.

• SO42-是北京降水中的主要致酸物质,NO3-的致酸作用有所上升,Ca2+、NH4+中和缓冲降水中的酸性物质;