20 Oct 2022
20 Oct 2022
Status: this preprint is currently under review for the journal AMT.

Optical receiver characterisations and corrections for ground-based and airborne measurements of spectral actinic flux densities

Birger Bohn1 and Insa Lohse1,2 Birger Bohn and Insa Lohse
  • 1Institut für Energie- und Klimaforschung, IEK-8: Troposphäre, Forschungszentrum Jülich GmbH, 52428 Jülich, Germany
  • 2Deutscher Wetterdienst, Bildungszentrum Langen, 63225 Langen, Germany

Abstract. Solar actinic radiation in the UV/VIS range perpetuates atmospheric photochemistry by inducing photolysis processes which form reactive radical species. Photolysis frequencies are rate constants that quantify the rates of photolysis reactions and therefore constitute important parameters for quantitative analyses. Photolysis frequencies are usually calculated from modelled or measured solar spectral actinic flux densities. Suitable measurements techniques are available but measurement accuracy can suffer from non-ideal 2π or 4π solid angle reception characteristics of the usually employed 2π optical receivers, or receiver combinations. These imperfections, i.e. deviations from an angle-independent response, should be compensated by corrections of the measured data. In this work, the relative angular sensitivities of four commonly used 2π quartz receivers were determined in the laboratory in a range 280–660 nm. Based on this information, the influence of the non-ideal responses on measured spectral actinic flux densities for ground-based and airborne applications was investigated for a wide range of atmospheric conditions. Spectral radiance distributions and contributions of direct, diffuse downward and diffuse upward spectral actinic flux densities were calculated with a radiative transfer model to derive the corrections. The intention was to determine the ranges of possible corrections under realistic measurement conditions and to derive simple parametrizations with reasonable uncertainties. For ground-based 2π measurements of downward spectral actinic flux densities, corrections typically range <10 % dependent on wavelength and solar zenith angle, with 2–8 % uncertainties covering all atmospheric conditions. Corrections for 4π airborne measurements were determined for the platforms Zeppelin NT (New Technology) and HALO (High Altitude and Long Range Research Aircraft) in altitude ranges 0.05–2 km and 0.2–15 km, respectively. Total, downward and upward spectral actinic flux densities were treated separately. In addition to various atmospheric conditions, different ground albedos and small (<5°) aircraft attitude variations were considered in the uncertainties, as well as aircraft headings with respect to the sun in the case of HALO. Corrections for total and downward spectral actinic flux densities again typically range <10 % dependent on wavelength, solar zenith angle and altitude, with 2–10 % uncertainties covering all atmospheric conditions for solar zenith angles below 80°. For upward spectral actinic flux densities corrections were more variable and significantly greater, up to about −50 % at low altitudes and low ground albedos. A parametrization for corrections and uncertainties was derived using uncorrected ratios of upward/downward spectral actinic flux densities as input, applicable independent of atmospheric conditions for a given wavelength, solar zenith angle and altitude. The use was limited to conditions with solar zenith angles <80° when direct sun radiation cannot strike upward and downward looking receivers simultaneously. Examples of research flights with the Zeppelin and HALO are discussed, as well as other approaches described in the literature.

Birger Bohn and Insa Lohse

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2022-288', Anonymous Referee #1, 12 Nov 2022
  • RC2: 'Comment on amt-2022-288', Anonymous Referee #2, 17 Nov 2022

Birger Bohn and Insa Lohse

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Replication data for “Optical receiver characterisations and corrections for ground-based and airborne measurements of spectral actinic flux densities” Bohn, B.

Birger Bohn and Insa Lohse


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Short summary
Optical receivers for solar spectral actinic radiation are designed for angle-independent sensitivities within a hemisphere. Remaining imperfections can be compensated by receiver-specific corrections based on laboratory characterizations and radiative transfer calculations of spectral radiance distributions. The corrections cover a wide range of realistic atmospheric conditions and were applied to ground-based and airborne measurements in a wavelength range 280–660 nm.