Summary of measurement intercomparisons during TRACE-P

The NASA DC-8 and P-3B aircraft flew within about a kilometer or less of each other on three occasions during the Transport and Chemical Evolution Over the Pacific (TRACE-P) campaign in order to intercompare similar measurements on the two aircraft. The first and last intercomparisons were in relatively remote marine environments during transits to and from Asia. The first began with a boundary layer measurement followed by an ascent to 3 km. The second set of intercomparisons was at a fixed altitude of about 5.2 km off the coast of Japan, also in relatively clean air. Finally, the third measurement began at 5.3 km and then descended into the boundary layer. A number of measurements were compared with the best agreement observed for the most abundant compounds such as CO₂ and CH₄ and with very good agreement for CO, O₃, and j values. Other comparisons, including measurements of the same compounds on both aircraft and measurements of the same compound by two different instruments on the DC-8, varied over a wide range from quite good for PAN, NO, HNO₃, H₂O; to reasonable agreement for OH, HO₂ CH₂O, acetone, and methylethylketone; and to generally poor for NO₂,SO₂, PPN, acetaldehyde, and methanol. The comparison results, particularly those for the fast l-s CO and O₃ measurements, suggest that credible intercomparisons can be made using two aircraft in close proximity for relatively long lifetime and stable compounds. Much new understanding can also be gained from measurements of more reactive and generally shorter lifetime compounds, but additional improvements are needed to make such studies as meaningful as those of longer lifetime compounds. Comparisons such as these, made as a component of a larger field campaign, have the advantage that they test the actual instrument configuration used during the field study and they require no additional instrument installation and testing.