Diurnal variations in nonmethane hydrocarbon (NMHC) mixing ratios were measured in Nashville during the Southern Oxidant Study 1999 experiment at a location near the top of a downtown high-rise, the Polk Building. Measurements made during the period of 3-7 July 1999 are analyzed with trajectory photochemical models and a regional-scale chemical transport model (CMAQ). The focus of this study is on understanding the diurnal variation of NMHCs and the contributions of local sources and long-range transport to the measured variability. The sources for the measured large isoprene mixing ratio during the night of 5-6 July and the possible effect on the following day’s ozone mixing ratio are of particular interest. Measurements of NOx from the same site are also being analyzed with the use of models. Results of model calculations are interpreted to evaluate (a) the origins of the air measured atop the Polk Building during this period (b) isoprene emissions from BEIS2 with a modified algorithm developed at Argonne, as compared with the measured ambient mixing ratios of isoprene; and (c) the contribution of isoprene to the elevated ozone concentrations observed during the episode.