Using data collected on July 11, 1995, from the Tennessee Valley Authority (TVA) instrumented Bell 205 helicopter during the 1995 SOS Nashville/Middle Tennessee Ozone Study, we examine the relative roles of NOx and volatile organic compound (VOC) emissions on ozone photochemical production in the Nashville area. On this day, instruments onboard the helicopter separately sampled air from within a plume emanating from a nearby coal-fired power plant and air from the Nashville urban plume. Using SO2 concentrations and latitude to discriminate between the plumes, we compare the effects on O3 production of only adding NOx to the background air (i.e., the power plant plume) to that of adding both NOx and VOC (i.e., the urban plume). Our analysis indicates that NOx was the prime anthropogenic precursor causing the generation of O3 during the experiment, with anthropogenic VOC perhaps playing a secondary but significant role. The addition of VOC emissions from the urban area had no appreciable effect on peak O3 concentrations, clearly demonstrating that anthropogenic NOx emissions by themselves can produce elevated O3 concentrations in the Nashville area. Anthropogenic VOC emissions, however, may have enhanced the ozone production efficiencies and the rate at which O3 reached its peak values. While not conclusive, the data also suggested that the highest O3 concentrations in the Nashville area on July 11, 1995, appeared within embedded plumes that combined the effects of both urban and power plant precursor emissions. ¿ 1998 American Geophysical Union