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


  • B. L. Lefer,

  • S. R. Hall,

  • L. Cinquini,

  • R. E. Shetter,

  • J. D. Barrick,

  • J. H. Crawford


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 totaljNO2 is the sum of the signals from independent zenith and nadir viewing instruments. The 1-min average SAFS and FRjNO2 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.