| Instrument: | Airborne Measurements of Spectrally Resolved Actinic Flux and |
| Photolysis Frequencies: | Scanning Actinic Flux Spectroradiometer (SAFS) |
| Principal Investigator: | R. E. Shetter |
| Co-Investigators: | B. Lefer, S. Hall, T. Campos, L. Cinquini, and C. A. Cantrel |
| Organization: | Atmospheric Chemistry Division
National Center for Atmospheric Research 1850 Table Mesa Drive Boulder, CO 80303 |
Measurement Description: Spectrally resolved total actinic flux measurements from 280 to 420 nm will be provided on the NASA DC-8 aircraft for the SOLVE mission using actinic flux spectroradiometry [Shetter and Muller, 1998]. Photolysis frequencies of 11 molecules including O3, NO2, HONO, CH2O, H2O2, CH3OOH, HNO3, PAN, CH3NO3, CH3CH2NO3, and CH3COCH3 will be calculated using the most current molecular data. Additional photolysis frequencies of molecules of interest may be added to by sampling some longer wavelengths.
The technique is based on hemispherical quartz light collectors, double monochromators, and low dark current photomultipliers. A single instrument is represented schematically in Figure 1.
Figure 1.The instrument package on the aircraft will include 2 independent spectroradiometer systems. Since an individual spectroradiometer system collects the 2 steradian hemisphere above or below the aircraft, addition of the actinic fluxes will give the spherically integrated actinic flux. The current instrument configuration employs 2400 G/mm gratings which produces 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.
| Measurement | Detection Limit | Time Response | Accuracy | Precision |
|---|---|---|---|---|
| Actinic Flux 280-420nm | <0.05 W/cm2/nm | <10 sec | ~10% | ~5% |
Detection limits for individual photolysis frequencies vary with wavelength range of the absorption cross section/quantum yield combination. The time response of the instrument will depend on the number of wavelength intervals sampled and averaging time needed at each wavelength. In the instrument's current operational configuration 95 wavelength intervals can be sampled in <10 sec per scan using an average of 800 readings at each wavelength interval.
The spectral response of instruments is calibrated in our laboratory using a 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 lines from a mercury lamp. 1