Mass concentration means mass per unit volume and is used in the construction mass_ concentration_ of_ X_ in_ Y, where X is a material constituent of Y. Precipitation is liquid or frozen water that forms in the atmosphere and falls to the earth. It comes in many forms, like rain, snow, and sleet. The compound o,p-DDE and p,p-DDE belongs to the group of persistent organic pollutants (POPs) and can include both o,p-DDE and p,p-DDE.