Studies aiming at modelling the transfer of the solar radiation through the atmosphere require as main parameter the solar spectrum incident on the top of the atmosphere, for instance to calculate the surface UV radiation using satellite sensors. Similarly, investigations combining solar radiation measurements with radiative transfer calculations to retrieve atmospheric constituents need an accurate representation of the extraterrestrial solar spectrum.
In recent years satellite experiments have measured the solar extraterrestrial spectrum from space to avoid atmospheric absorption and scattering effects, especially at wavelengths shorter than 300 nm where ozone and oxygen in the atmosphere absorb all incident radiation. While prelaunch calibration and characterisation procedures reach very low uncertainties, once in space the possibilities of verifying or recalibrating such an instrument become very challenging. As recent studies have demonstrated (e.g. Schöll et al., 2016), the solar spectra measured from satellite platforms can differ significantly between each other, due in part to instrument degradation issues arising from the harsh space environment and the difficulties in accounting for possible instrument changes between the pre-flight calibration and their operation in space. In a strict metrological sense, such measurements cannot be considered traceable to SI since metrological traceability inherently requires the repeated demonstration of the uninterrupted traceability to primary standards which currently is not available to instruments located in space.
In contrast, while surface-based measurements of the solar irradiance have the disadvantage of needing to account for changing atmospheric conditions, the considerable advantage over space-based instruments is the possibility of recalibrating ground-based instruments and thereby validating and confirming their traceability to SI.
In this study we present ground-based direct spectral solar irradiance measurements obtained with the transportable reference double monochromator spectroradiometer QASUME and a high resolution Fourier Transform Spectrometer (FTS) over the wavelength range 300 nm to 500 nm. A high resolution absolute extraterrestrial solar spectrum is then derived by applying the Langley-plot technique to the measurements of each instrument before combining them to a single solar spectrum.
Gröbner, J., Kröger, I., Egli, L. and Hülsen, G., Determining the solar extraterrestrial irradiance spectrum from the surface. Editorial _ 3 EMRP ENV59 Traceability for atmospheric total column ozone _ 4, p.25.