Absolute radiance calibration in the UV and visible spectral range using atmospheric observations during twilight

We present an improved radiance calibration method based on the calibration method by Wagner et al. (2015). The updated method uses only measurements during the twilight period instead of several hours as for the original method. The calibration is based on the comparison of measurements and simulations of the radiance of zenith-scattered sun light. The main advantage of our method compared to radiance calibration methods in the laboratory is that the calibration can be directly applied in the field. This allows routine radiance calibrations whenever the sky is clear during twilight. The calibration can also be performed retrospectively, and will thus be applicable for the large number of existing data sets. Also, potential changes of the instrument properties during transport from the laboratory to the field are avoided. The new version of the calibration method presented here has two main advantages: First, the required measurement period can be rather short (only a few minutes during twilight for cloud-free conditions). Second, even without knowledge of the aerosol optical depth, the errors of the calibration method are rather small, especially in the UV spectral range where they range from about 4 % at 340 nm to 8 % at 420 nm. If the AOD is known, the uncertainties are even smaller by about a factor of two. For visible wavelengths, good accuracy is only obtained if the AOD is approximately known with uncertainties from about 4 % at 420 nm to 10 % at 700 nm (generally the AOD is nevertheless smaller in the visible than in the UV spectral range). One shortcoming of the method is that it is not possible to determine the AOD exactly at the time of the (twilight) measurements, because AOD observations from sun photometer measurements or the MAX-DOAS measurements are usually not meaningful for such high SZA. But the related uncertainty can be minimised by repeating the radiance calibrations during the twilight periods of several days.