A field experiment was conducted in August 1998 to investigate the concentrations of isoprene and isoprene reaction products in the surface and mixed layers of the atmosphere in Central Texas. Measured near ground-level concentrations of isoprene ranged from 0.3 (lower limit of detection - LLD) to 10.2ppbv in rural regions and from 0.3 to 6.0ppbv in the Austin urban area. Rural ambient formaldehyde levels ranged from 0.4ppbv (LLD) to 20.0ppbv for 160 rural samples collected, while the observed range was smaller at Austin (0.4-3.4ppbv) for a smaller set of samples (37 urban samples collected). Methacrolein levels did not vary as widely, with rural measurements from 0.1ppbv (LLD) to 3.7ppbv and urban concentrations varying between 0.2 and 5.7ppbv. Isoprene flux measurements, calculated using a simple box model and measured mixed-layer isoprene concentrations, were in reasonable agreement with emission estimates based on local ground cover data. Ozone formation attributable to biogenic hydrocarbon oxidation was also calculated. The calculations indicated that if the ozone formation occurred at low VOC/NO(x) ratios, up to 20ppbv of ozone formed could be attributable to biogenic photooxidation. In contrast, if the biogenic hydrocarbon reaction products were formed under low NO(x) conditions, ozone production attributable to biogenics oxidation would be as low as 1ppbv. 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 NO(x) from urban centers or large rural NO(x) sources. Copyright (C) 2001 Elsevier Science Ltd.
