How can the pH of an acid rainstorm fall below 3.0?

The pH of an individual rainstorm can drop below 3.0 in areas where industrial emissions are high and emission controls are insufficient to mitigate the release of sulfur dioxide and nitrogen oxides.1

These pollutants, primarily generated by the burning of fossil fuels at power plants and industrial facilities, react with water, oxygen, and other atmospheric chemicals to form sulfuric acid and nitric acid. When these acids mix with cloud moisture, they significantly lower the pH of the resulting precipitation.23

While normal rainwater is slightly acidic—typically having a pH of about 5.6 due to the presence of carbon dioxide—the introduction of these additional acidic gases leads to much lower pH values. In regions with heavy pollution, the concentration of nitrate and sulfate ions in the rain increases, which is strongly correlated with the drop in pH.143