On August 14, 1980, a Lagrangian/Eulerian air pollution sampling mission was conducted near and up to 400 km downwind of Baltimore, Maryland. This mission was an integral part of the U.S. Environmental Protection Agency's Northeast Regional Oxidant Study (NEROS). The mission began with the release of a 6-cu m tetroon from a site 20 km east-northeast of downtown Baltimore at 1045 LDT. Instrumented aircraft were directed to the tetroon location at approximately 3-h intervals to measure the concentrations of NOx, O3, and other pollutants across approximately 100-km long transects of the urban plume and its environs at two to three altitudes. Measurements indicated a well-defined urban plume up to 190 km downwind of Baltimore with O3 concentrations exceeding 160 ppb. However, no definite indications of the plume were found 400 km downwind of Baltimore.