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With the continuous methods used here, no unequivocal interferences were seen when SO2, NO2, 03, and isoprene impurities were added to prepared mixtures or when these were present in ambient air. The measurements with the C-18 DNPH (no 03 scrubber) and silica gel DNPH cartridges (with 03 scrubber) showed a reasonable correlation with the TDLAS measurements, although the results from the silica cartridges were about a factor of two below the standards in the spike experiments and about 35% below in the ambient measurements. Using the NCAR gas-phase spike data to calibrate the response of the silica gel cartridges in the ambient studies, the results are the same within statistical uncertainty. When the same gas phase calibration was used with the C-18 cartridges, the results showed a positive bias of about 35%, presumably reflecting a positive ozone interference in this case (no ozone scrubber used). The silica DNPH cartridge results from the second participant were highly scattered and showed no significant correlation with the TDLAS measurements.

[1] The Ozarks Isoprene Experiment (OZIE) was conducted in July 1998 in Missouri, Illinois, Indiana, and Oklahoma. OZIE was designed to investigate the presumed strong isoprene emission rates from the Missouri Ozarks, where there is a high density of oak trees that are efficient isoprene emitters. Ground, balloon, and aircraft measurements were taken over a three-week study period; 0-D and 3-D chemical models were subsequently used to better understand the observed isoprene emissions from the Ozarks and to investigate their potential regional-scale impacts. Leaf-level measurements for two oak tree species yielded normalized average isoprene emission capacities of 66 mgC g À1 h À1 , in good agreement with values used in current biogenic emissions models. However, the emission capacities exhibited a temperature dependence that is not captured by commonly used biogenic emission models. Isoprene mixing ratios measured aloft from tethered balloon systems were used to estimate isoprene fluxes. These measurementderived fluxes agreed with BEIS3 estimates within the relatively large uncertainties in the estimates. Ground-level isoprene mixing ratios exhibited substantial spatial heterogeneity, ranging from <1 to 35 ppbv. The agreement between measured isoprene mixing ratios and regional-scale chemical transport model estimates was improved upon averaging the ground-level isoprene data observed at several sites within a representative area. Groundlevel formaldehyde (HCHO) mixing ratios were very high (up to 20 ppbv) and were consistently higher than mixing ratios predicted by a regional chemical transport model. The spatial distribution and magnitude of the elevated HCHO concentrations showed good agreement with GOME satellite column observations of HCHO.

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