Measurement and analysis of atmospheric concentrations of isoprene and itsreaction products in central Texas

A field experiment was conducted in August 1998 to investigate the concentr ations of isoprene and isoprene reaction products in the surface and mixed layers of the atmosphere in Central Texas. Measured near ground-level conce ntrations of isoprene ranged from 0.3 (lower limit of detection - LLD) to 1 0.2 ppbv in rural regions and from 0.3 to 6.0 ppbv in the Austin urban area . Rural ambient formaldehyde levels ranged from 0.4 ppbv (LLD) to 20.0 ppbv for 160 rural samples collected, while the observed range was smaller at A ustin (0.4-3.4ppbv) for a smaller set of samples (37 urban samples collecte d). Methacrolein levels did not vary as widely, with rural measurements fro m 0.1ppbv (LLD) to 3.7 ppbv and urban concentrations varying between 0.2 an d 5.7 ppbv, Isoprene flux measurements, calculated using a simple box model and measured mixed-layer isoprene concentrations, were in reasonable agree ment with emission estimates based on local ground cover data. Ozone format ion attributable to biogenic hydrocarbon oxidation was also calculated The calculations indicated that if the ozone formation occurred at low VOC/NOx ratios, up to 20 ppbv of ozone formed could be attributable to biogenic pho tooxidation. In contrast, if the biogenic hydrocarbon reaction products wer e formed under low NOx conditions, ozone production attributable to biogeni cs oxidation would be as low as 1 ppbv, This variability in ozone formation potentials implies that biogenic emissions in rural areas will not lead to peak ozone levels in the absence of transport of NOx from urban centers or large rural NOx sources. (C) 2001 Elsevier Science Ltd. All rights reserve d.