INTERCOMPARISON OF 6 AMBIENT [CH2O] MEASUREMENT TECHNIQUES

From May 29 to June 3, 1995 a blind intercomparison of six ambient for maldehyde measurement techniques took place at a field site near the N ational Center for Atmospheric Research in Boulder, Colorado. The cont inuous measurement methods intercompared were tunable diode laser abso rption spectroscopy, (TDLAS); coil/2,4-dinitrophenylhydrazine, (CDNPH) ; 1,3-cyclohexanedione-diffusion scrubber (CHDDS); and the coil enzyme method (CENZ). In addition, two different cartridge methods were comp ared: silica gel-2,4-dinitrophenylhydrazine (DPNH) systems and a C-18- DNPH system. The intercomparison was conducted with spiked zero air (p art 1) and ambient air (part 2). The CH2O standards for part 1 were ca librated by several independent methods and delivered to participants via a common glass manifold with potential trace gas interferants comm on to ambient air (O-3, SO2, NO2, isoprene, H2O). The TDLAS system was used to confirm the absolute accuracy of the standards and served as a mission reference for part 1. The ambient phase lasted 44 hours with all participants sampling from a common glass tower. Differences betw een the ambient [CH2O] observed by the TDLAS and the other continuous methods were significant in some cases. For matched ambient measuremen t times the average ratios (+/- 1 sigma) [CH2O](measured)/[CH2O](TDLAS ) were: 0.89 +/- 0.12 (CDNPH); 1.30 +/- 0.02 (CHDDS); 0.63 +/- 0.03 (C ENZ), The methods showed similar variations but different absolute val ues and the divergences appeared to result largely from calibration di fferences (no gas phase standards were used by groups other than NCAR) . When the regressions of the participant [CH2O] values versus the TDL AS values, (measured in part 1), were used to normalize all of the res ults to the common gas phase standards of the NCAR group, the average ratios (+/- 1 sigma), [CH2O](corrected)/[CH2O](TDLAS) for the first me asurement period were much closer to unity: 1.04 +/- 0.14 (CDNPH), 1.0 0 +/- 0.11 (CHDDS), and 0.82 +/- 0.08 (CENZ). With the continuous meth ods used here, no unequivocal interferences were seen when SO2, NO2, O -3, and isoprene impurities were added to prepared mixtures or when th ese were present in ambient air. The measurements with the C-18 DNPH ( no O-3 scrubber) and silica gel DNPH cartridges (with O-3 scrubber) sh owed a reasonable correlation with the TDLAS measurements, although th e results from the silica cartridges were about a factor of two below the standards in the spike experiments and about 35% below in the ambi ent measurements. Using the NCAR gas-phase spike data to calibrate the response of the silica gel cartridges in the ambient studies, the res ults are the same within statistical uncertainty. When the same gas ph ase calibration was used with the C-18 cartridges, the results showed a positive bias of about 35%, presumably reflecting a positive ozone i nterference in this case (no ozone scrubber used). The silica DNPH car tridge results from the second participant were highly scattered and s howed no significant correlation with the TDLAS measurements.