Comparison of airborne measured and calculated spectral actinic flux and derived photolysis frequencies during the PEM Tropics B mission

Scanning actinic flux spectroradiometer (SAFS) instruments were deployed on the National Aeronautics and Space Administration (NASA) DC-8 and P-3B research aircraft for the Pacific Exploratory Mission in the Tropical Pacific (PEM Tropics B mission). The SAFS instruments measured the wavelength-dependent downwelling and upwelling actinic flux from 282 to 420 nm. From the actinic flux spectra, photolysis frequencies of a number of molecules were calculated. Flight tracks during the PEM Tropics B mission varied in altitude from the surface up to 12 km, with much time spent above, below, and among broken and scattered clouds. Comparisons of the measured photolysis frequencies to the photolysis frequencies calculated with the National Center for Atmospheric Research (NCAR) tropospheric ultraviolet visible (TUV) radiative transfer model, assuming clear skies, demonstrate some expected differences in this complex radiation environment, while in general the agreement between the measurements and the model is within 15%. Differences between the measurements and the model are expected in and around clouds, because the radiation model is not equipped to input broken and scattered cloud effects, except on an individual case-by-case basis. The differences in the composite mean ratio of SAFS measurement to TUV model for total photolysis frequencies vary from 6% to 18% for jNO2 and from 6% to 11% for jO(1D) for both aircraft. The differences between the measurements and the models represent the accuracy that can be expected when using photolysis frequencies from clear-sky radiative transfer modeling in regional and global atmospheric chemistry models for regions with broken and scattered clouds in the troposphere.