A CCD spectroradiometer for ultraviolet actinic radiation measurements

A new spectroradiometer for spectral measurements of ultraviolet ( UV) atmospheric radiation ( 290 - 400 nm) using a charge coupled device ( CCD) as a detector is introduced. The instrument development is motivated by the need for measurements with ( a) high accuracy in the UV-B spectral range ( 290 - 315 nm) for photochemistry applications and ( b) high temporal resolution in quickly changing atmospheric conditions such as partial cloud cover. The new CCD instrument is mainly intended for airborne use. It allows fast data collection ( < 300 ms time resolution for each spectrum) with improved sensitivity in the UV spectral range. The instrumental setup and its characterization in terms of stray light, dark current, noise, and detection limits are described and compared to a spectroradiometer with a photodiode array ( PDA) detector. The new CCD spectroradiometer has a one order of magnitude greater sensitivity than the PDA-based spectroradiometer. However, the stray light of the CCD instrument is wavelength dependent, which requires a more complicated data evaluation procedure than the PDA instrument. Comparison with other UV spectroradiometers ( a PDA spectroradiometer and two ground-based double monochromators) shows the advantages of the CCD system for UV-B measurements of actinic flux densities and photolysis frequencies of ozone and nitrogen dioxide, and the improved performance compared to PDA spectroradiometers.