Results from an airborne intercomparison of techniques to measure tropospheric levels of NO are discussed. The intercomparison was part of the National Aeronautics and Space Administrations's Global Tropospheric Experiment and was conducted during the summer of 1986. Instruments intercompared included a two-photon laser-induced fluorescence system and two NO/O3 chemiluminescence detectors. These three instruments also participated in the NO intercomparisons conducted during the Chemical Instrumentation Test and Evaluation (CITE) 1 mission in 1983--1984, and therefore the current results are a revisit to the question of how well NO can be measured from an aircraft in remote tropospheric environments. The intercomparisons were weighed to mixing ratios less than 20 pptv. Results from CITE 2 were similar to those of CITE 1. The most noteworthy observation common to both campaigns is that these low mixing ratios, agreement among the three instruments is almost always within about 15 or 20 pptv for sampling periods of 1--6 min.

Analyses of multipoint data sets suggest that on the average the different between data from any pair of the instruments at the low mixing ratios is about 5--7 pptv. Since this agreement was obtained among instruments using fundamentally different detection principles (chemiluminescent and laser) and the data suggested that all provided equally valid measurments of NO, the observed level of agreement perhaps represents the uncertainity in the current measurement of low mixing ratio ambient NO. The major question which must be addressed by the scientific community is the influence of, for example, a 5-pptv uncertainty in NO at 10 pptv in photochemistry studies, and so on, in the ''clean'' remote troposphere. ¿ American Geophysical Union 1990