In the concept of an 'urban plume', the combined air pollutants from several major emission sources in an urban area are considered to be carried, with air masses moving across the city, out over nonurban land or water. A plume may be distinguished from the unpolluted nonurban atmosphere, which is not directly downwind of a city, by its chemical composition and high concentrations of certain pollutants. However, in the eastern U.S., where the most abundant particulate air pollutants in the plume may be sulfate aerosol formed by the oxidation of gaseous sulfur oxides, the urban source areas are spaced closely enough so that the intervening nonurban regions are often under the combined influence of several urban plumes. The resulting generally high nonurban average pollutant concentrations, especially for sulfate, greatly complicate the task of tracing the transport of individual urban plumes, and, of particular importance, of determining the extent of sulfate formation within them.