Rainwater with a pH less than 5.6 is called acid rain. Acid rain is caused by the sulfur oxides or nitrogen oxides produced after burning coal, oil, natural gas and other fossil fuels with high sulfur content, which undergo complex chemical processes in the atmosphere. After the reaction, sulfuric acid or nitric acid aerosols are formed, or are captured and absorbed by clouds, rain, snow, or fog, and fall to the ground as acid rain.
The anions in acid rain are mainly nitrate and sulfate ions. Based on their concentrations in acid rain samples, it can be determined whether the main influencing factor of precipitation is sulfur dioxide or nitrogen oxides. Sulfur dioxide mainly comes from the combustion of fossil fuels (such as coal), and nitrogen oxides mainly come from pollution sources such as vehicle exhaust.