[1] NH 3 measurements made using a chemical ionization mass spectrometry (CIMS) method and a molybdenum-oxide, converter-difference (MoO x -CD) approach were compared with those made using the citric acid denuder method. The citric acid denuder, which has fared well in a variety of other evaluations, was used as the benchmark method in the current intercomparison. A very high degree of correlation, r 2 > 0.9, between the three ammonia measurement techniques was found. For ammonia mixing ratios greater than approximately 100 pptv, the denuder method and the CIMS technique were found to agree to within about 20%. However, the MoO x -CD measurement results were larger by approximately 70% than the denuder measurements. For our measurements, we conclude that the MoO x -CD method measured the sum of gas-phase NH 3 collected by the denuder tube and aerosol ammonium (NH 4 + ) that was simultaneously collected on Teflon filters. The methods and the comparison along with the results of the ancillary measurements are described.

Abstract. A chemical ionization mass spectrometer (CIMS)andand CsHs conjugated dienes were identified as potential interferences to this technique. This indicates that the selectivity of the CIMS isoprene measurement must be tested by intercomparison with well-established methods, e.g. gas chromatography techniques.

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Abstract. An informal intercomparison of two isoprene (C 5 H 8 ) measurement techniques was carried out during Fall of 1998 at a field site located approximately 3 km west of Boulder, Colorado, USA. A new chemical ionization mass spectrometric technique (CIMS) was compared to a wellestablished gas chromatographic technique (GC). The CIMS technique utilized benzene cation chemistry to ionize isoprene. The isoprene levels measured by the CIMS were often larger than those obtained with the GC. The results indicate that the CIMS technique suffered from an anthropogenic interference associated with air masses from the Denver, CO metropolitan area as well as an additional interference occurring in clean conditions. However, the CIMS technique is also demonstrated to be sensitive and fast. Especially after introduction of a tandem mass spectrometric technique, it is therefore a candidate for isoprene measurements in remote environments near isoprene sources.