Of the 37 isoprene profiles recorded at the sites, 16 exhibited complex vertical structure with local maxima within the BL. This complex vertical structure appears to arise from a variety of turbulent processes fostered by horizontal inhomogeneities in the surface emissions of isoprene and by the transient appearance of layers of strong wind shear and/or vertical stability within the BL. A statistical analysis of the data suggests that the complex features observed in the individual profiles are stochastic in nature and tend to cancel out upon averaging over all profiles. Our calculations suggest that measurements made at a height of 40-100 m above the surface will yield the most reliable measure of average BL concentrations of reactive hydrocarbons. -from Authors