Comparison of airborne NO₂ photolysis frequency measurements during PEM-Tropics B

During the NASA Pacific Exploratory Mission - Tropics B (PEM-Tropics B) the NO₂ photolysis frequency was simultaneously measured by colocated National Center for Atmospheric Research scanning actinic flux spectroradiometer (SAFS) and commercial filter radiometer (FR) instruments on the NASA DC-8 and the NASA P-3B research aircraft. The measurements were collected from early March to mid-April 1999 over the Pacific Ocean, spanning the region between 35° N-25° S latitude and 80° W-170° E Longitude. Over the course of the mission the SAFS and FR instruments performed quite well with both systems reporting data for more than 85% of the PEM-Tropics B flight hours. For both the SAFS and FR systems the reported total jNO₂ is the sum of the signals from independent zenith and nadir viewing instruments. The 1-min average SAFS and FR jNO₂ values were quite highly correlated (r² > 0.98) for both the zenith and nadir systems in this complex radiation environment. During PEM-Tropics B these instruments exhibited a difference ranging from 5 to 40% (depending on the instrument pair), with the jNO₂ FR measurements always being higher. In contrast, a consistent 30% disagreement was observed for the SAFS and FR instruments during PEM-Tropics A. Use of newer NO₂ cross-section data would increase the SAFS jNO₂ by 4%, while the larger difference between one of the FR and SAFS instrument pairs could be related to a change in the FR instrument sensitivity.