On 4 days during the 1995 Southern Oxidant Study (SOS), air samples were taken in the plume of the Cumberland Power Plant in Tennessee using an instrumented helicopter. On these days a notable difference in excess ozone in the plumes was observed. Excess ozone varied from 20 ppb on July 7, 1995, up to 55 ppb on July 16. While the total amount of non-methane VOC was quite similar, significant differences were observed in the levels of reactive hydrocarbons, mostly isoprene. This study examines the parameters that govern both emission rates of isoprene and its dispersion. These include temperature and wind speed on the surface and aloft, total solar radiation, and the height of the mixed layer. The results revealed and computer model simulations confirmed that although differences were not very large, the combinations of all of these parameters favored lower ambient isoprene levels and, consequently, lower ozone production on July 7 and higher production on the 3 other days. ¿ 2000 American Geophysical Union