 In this diagram, we are seeing how the acid deposition occurs. When the sources emit pollutants such as SO2, NOx, mercury, and volatile organic compounds, primarily SO2 and NOx, which are acidic gases, are deposited in two ways. One is dry deposition, the other one is wet deposition. The SO2 and NOx, when they deposit back either gaseous pollutants or as particulates, it's called dry deposition. When these pollutants dissolve in water, cloud water, and then deposit, it's called wet precipitation, or that is what we call acid rain. The dry gaseous pollutants or particulate matter can sometimes get dissolved in water and come down again as wet precipitation. Receptors are the species that receive this acid rain and get affected. These receptors can be materials that we care about, or aquatic life, human beings, or lakes and streams.