Airborne Measurements of Spectrally Resolved Actinic Flux and Photolysis Frequencies:Instrumentation description

R. E. Shetter and B. L. Lefer
Atmospheric Chemistry Division
National Center for Atmospheric Research
Boulder, Colorado

Spectrally resolved upwelling and downwelling actinic flux measurements from 280 to 420 nm were provided on the NASA DC-8 aircraft for the TRACE P mission using scanning actinic flux spectroradiometer (SAFS) instruments [Shetter and Muller, 1998].  The technique is based on hemispherical quartz light collectors, double monochromators, low dark current photomultipliers, and instrument control and data acquisition computers. 

The SAFS instrument package on the aircraft will include 2 independent spectroradiometer systems. Since an individual spectroradiometer system collects the 2p steradian hemisphere above or below the aircraft, addition of the actinic fluxes will give the spherically integrated total actinic flux.  The instrument configuration employs 2400 G/mm gratings which produce a 1 nm FWHM spectral resolution.  Similar spectroradiometers have been deployed on the NASA DC-8 for the PEM Tropics A, PEM Tropics B, and SONEX missions.  Performance on these missions was quite good with >90% data return.  The instruments will have response and detection characteristics represented in Table 1.

      The time response of the instrument depends on the number of wavelength intervals sampled. In the instrument configuration for TRACE P included 140 wavelength intervals can be sampled in <10 sec per scan.

            The spectral response of instruments is calibrated in our laboratory using an optical calibration facility equipped with precision radiometric power supplies and multiple NIST traceable 1000W quartz tungsten halogen lamps.  Secondary lamp standards are employed in the field to calibrate the systems before each aircraft flight.  Wavelength stability calibrations are performed every day using a mercury lamp.

            The wavelength dependent actinic fluxes were used to calculate photolysis frequencies of 11 atmospheric molecules important to the local photochemistry. The molecules include O₃, NO₂, CH₂O, HONO, HNO₃, CH₃NO₃, CH₃CH₂NO₃, H₂O₂, CH₃OOH, CH₃COCH₃, and PAN. Total actinic fluxes and photolysis frequencies for the 11 molecules will be reported to the data archive.

Figure 1