VERTICAL-DISTRIBUTION OF ISOPRENE IN THE LOWER BOUNDARY-LAYER OF THE RURAL AND URBAN SOUTHERN UNITED-STATES

An analysis is presented of vertical profiles of isoprene concentratio n and meteorological parameters measured in the boundary layer (BL) du ring the daylight hours at a rural site in Alabama and an urban site i n Atlanta, Georgia, during the summer of 1990, as part of the Southern Oxidants Study. 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 tu rbulent processes fostered by horizontal inhomogeneities in the surfac e emissions of isoprene and by the transient appearance of layers of s trong wind shear and/or vertical stability within the BL. A statistica l analysis of the data suggests that the complex features observed in the individual profiles are stochastic in nature and tend to cancel ou t upon averaging over all profiles. Nevertheless, these complex struct ures can confound attempts to infer the BL abundance of a short-lived hydrocarbon like isoprene from a set of measurements at a single heigh t. Our calculations suggest that measurements made at a height of 40 - 100 m above the surface will yield the most reliable measure of avera ge BL concentrations of reactive hydrocarbons.