Microplastic particles and fibers generated from the breakdown of mismanaged waste are now so prevalent that they cycle through the Earth in a manner akin to global biogeochemical cycles. In modeling the atmospheric limb of the plastic cycle, we show that most atmospheric plastics are derived from the legacy production of plastics from waste that has continued to build up in the environment. Roads dominated the sources of microplastics to the western U.S., followed by marine, agriculture, and dust emissions generated downwind of population centers. At the current rate of increase of plastic production (~4% per year), understanding the sources and consequences of microplastics in the atmosphere should be a priority.