has participant has participant acidic water water-based rainfall water flow process acid rainfall A water-based rainfall process during which water droplets with increased acidity fall towards a planetary surface. https://orcid.org/0000-0002-3884-3420 http://orcid.org/0000-0002-3410-4655 acid rain On Earth, the acidity of raindrops is typically increased through interactions of water molecules with sulfur dioxide or nitrogen oxide, released into the atmosphere by natural or anthropogenic processes.