Articles | Volume 8, issue 10
Research article
14 Oct 2015
Research article |  | 14 Oct 2015

A new method for the absolute radiance calibration for UV–vis measurements of scattered sunlight

T. Wagner, S. Beirle, S. Dörner, M. Penning de Vries, J. Remmers, A. Rozanov, and R. Shaiganfar

Abstract. Absolute radiometric calibrations are important for measurements of the atmospheric spectral radiance. Such measurements can be used to determine actinic fluxes, the properties of aerosols and clouds, and the shortwave energy budget. Conventional calibration methods in the laboratory are based on calibrated light sources and reflectors and are expensive, time consuming and subject to relatively large uncertainties. Also, the calibrated instruments might change during transport from the laboratory to the measurement sites. Here we present a new calibration method for UV–vis instruments that measure the spectrally resolved sky radiance, for example zenith sky differential optical absorption spectroscopy (DOAS) instruments or multi-axis (MAX)-DOAS instruments. Our method is based on the comparison of the solar zenith angle dependence of the measured zenith sky radiance with radiative transfer simulations. For the application of our method, clear-sky measurements during periods with almost constant aerosol optical depth are needed. The radiative transfer simulations have to take polarisation into account. We show that the calibration results are almost independent from the knowledge of the aerosol optical properties and surface albedo, which causes a rather small uncertainty of about < 7 %. For wavelengths below about 330 nm it is essential that the ozone column density during the measurements be constant and known.

Short summary
We present a new method for the absolute calibration of atmospheric radiance measurements. Existing methods are based on laboratory measurements, but our method uses the atmospheric radiance measurements themselves. For selected sky conditions these measurements are compared to radiative transfer simulations. The method is very accurate (better than 7%) and might be used for a variety of scientific applications, as well as for the determination of the energy yield of photovoltaic cells.