Relationship between OH measurements on two different NASA aircraft during PEM Tropics B

OH measurements were performed on both the P3-B and the DC-8 aircraft throughout Pacific Exploratory Mission (PEM) Tropics B, using two very different measurement techniques. While no direct comparison was possible because of the difference in flight paths of the two aircraft, two brief measurement periods in close proximity allow at least a glimpse of how the two measurements compare. The first comparison took place in the marine boundary layer and showed exceptionally good agreement between the two aircraft OH measurements. The second set of close proximity flights at 5.5-km altitude resulted in an average concentration difference approximately equal to the uncertainty limit associated with either individual instrument but much less than the combined uncertainties from all the contributing measurements. In both cases the comparisons were made by normalizing the data with a photostationary state model. In addition, a comparison of the OH concentrations measured by each aircraft from 25°N latitude to 25°S latitude over the entire longitude and altitude range of the mission resulted in agreement within ∼10% but with a trend of higher DC-8/P-3B OH ratios at higher altitudes. While a definitive comparison of these two OH instruments must await a far more rigorous future test, the present results show that there were no clear OH measurement discrepancies observed. The dramatic differences in these two measurement techniques also minimizes the chance of common interferences or calibration errors.