Abstract
Abstract. The properties and performance of charge-coupled device (CCD) array spectroradiometers for the measurement of atmospheric spectral actinic flux densities (280–650 nm) and photolysis frequencies were investigated. These instruments are widely used in atmospheric research and are suitable for aircraft applications because of high time resolutions and high sensitivities in the UV range. The laboratory characterization included instrument-specific properties like the wavelength accuracy, dark signal, dark noise and signal-to-noise ratio (SNR). Spectral sensitivities were derived from measurements with spectral irradiance standards. The calibration procedure is described in detail, and a straightforward method to minimize the influence of stray light on spectral sensitivities is introduced. From instrument dark noise, minimum detection limits ≈ 1 × 1010 cm−2 s−1 nm−1 were derived for spectral actinic flux densities at wavelengths around 300 nm (1 s integration time). As a prerequisite for the determination of stray light under field conditions, atmospheric cutoff wavelengths were defined using radiative transfer calculations as a function of the solar zenith angle (SZA) and total ozone column (TOC). The recommended analysis of field data relies on these cutoff wavelengths and is also described in detail taking data from a research flight on HALO (High Altitude and Long Range Research Aircraft) as an example. An evaluation of field data was performed by ground-based comparisons with a double-monochromator-based, highly sensitive reference spectroradiometer. Spectral actinic flux densities were compared as well as photolysis frequencies j(NO2) and j(O1D), representing UV-A and UV-B ranges, respectively. The spectra expectedly revealed increased daytime levels of stray-light-induced signals and noise below atmospheric cutoff wavelengths. The influence of instrument noise and stray-light-induced noise was found to be insignificant for j(NO2) and rather limited for j(O1D), resulting in estimated detection limits of 5 × 10−7 and 1 × 10−7 s−1, respectively, derived from nighttime measurements on the ground (0.3 s integration time, 10 s averages). For j(O1D) the detection limit could be further reduced by setting spectral actinic flux densities to zero below atmospheric cutoff wavelengths. The accuracies of photolysis frequencies were determined from linear regressions with data from the double-monochromator reference instrument. The agreement was typically within ±5 %. Because optical-receiver aspects are not specific for the CCD spectroradiometers, they were widely excluded in this work and will be treated in a separate paper, in particular with regard to airborne applications.
Highlights
Solar actinic radiation is the driving force of atmospheric photochemistry because it produces short-lived reactive radicals in photolysis processes
The major technical difficulties related to the radiometric determination of Fλ in the atmosphere are (i) the quality of optical receivers for actinic radiation and (ii) the accuracy of measurements in the UV-B range that can be affected by low detector sensitivities and non-regularly reflected radiation within monochromators
For the evaluation of field data, we introduce atmospheric cutoff wavelengths from radiative transfer calculations to define safe, conditiondependent wavelength ranges for stray-light determination
Summary
Solar actinic radiation is the driving force of atmospheric photochemistry because it produces short-lived reactive radicals in photolysis processes. A further advantage of spectroradiometry is that the temperature and pressure dependencies of photolysis frequencies are obtained directly by taking into account the respective dependencies of the molecular parameters This is important for aircraft measurements where ambient conditions are most variable. The major technical difficulties related to the radiometric determination of Fλ in the atmosphere are (i) the quality of optical receivers for actinic radiation (ideally with 4π and angle-independent reception characteristics) and (ii) the accuracy of measurements in the UV-B range that can be affected by low detector sensitivities and non-regularly reflected radiation within monochromators (stray light). Example data from a flight on HALO as well as ground-based comparisons with a double-monochromator reference instrument will be shown
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