Calibration and evaluation of CCD spectroradiometers for ground-based and airborne measurements of spectral actinic flux densities

摘要

The properties and performance of charge-coupleddevice (CCD) array spectroradiometers for the measurement of atmosphericspectral actinic flux densities (280–650 nm) and photolysis frequencieswere investigated. These instruments are widely used in atmospheric researchand are suitable for aircraft applications because of high time resolutionsand high sensitivities in the UV range. The laboratory characterizationincluded instrument-specific properties like the wavelength accuracy, darksignal, dark noise and signal-to-noise ratio (SNR).Spectral sensitivities were derived from measurements with spectralirradiance standards. The calibration procedure is described in detail, and astraightforward method to minimize the influence of stray light on spectralsensitivities is introduced. From instrument dark noise, minimum detectionlimits  ≈  1  ×  10 cm s nm werederived for spectral actinic flux densities at wavelengths around 300 nm(1 s integration time). As a prerequisite for the determination of straylight under field conditions, atmospheric cutoff wavelengths were definedusing radiative transfer calculations as a function of the solar zenith angle(SZA) and totalozone column (TOC). Therecommended analysis of field data relies on these cutoff wavelengths and isalso described in detail taking data from a research flight on HALO (HighAltitude and Long Range Research Aircraft) as an example. An evaluation offield data was performed by ground-based comparisons with adouble-monochromator-based, highly sensitive reference spectroradiometer.Spectral actinic flux densities were compared as well as photolysisfrequencies (NO) and (OD), representing UV-A and UV-Branges, respectively. The spectra expectedly revealed increased daytimelevels of stray-light-induced signals and noise below atmospheric cutoffwavelengths. The influence of instrument noise and stray-light-induced noisewas found to be insignificant for (NO) and rather limited for(OD), resulting in estimated detection limits of5  ×  10 and 1  ×  10 s, respectively,derived from nighttime measurements on the ground (0.3 s integration time,10 s averages). For (OD) the detection limit could be furtherreduced by setting spectral actinic flux densities to zero below atmosphericcutoff wavelengths. The accuracies of photolysis frequencies were determinedfrom linear regressions with data from the double-monochromator referenceinstrument. The agreement was typically within ±5 %. Becauseoptical-receiver aspects are not specific for the CCD spectroradiometers,they were widely excluded in this work and will be treated in a separatepaper, in particular with regard to airborne applications.