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.
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