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

[摘要] Absolute radiometric calibrations are important formeasurements of the atmospheric spectral radiance. Such measurements can beused to determine actinic fluxes, the properties of aerosols and clouds, andthe shortwave energy budget. Conventional calibration methods in thelaboratory are based on calibrated light sources and reflectors and areexpensive, time consuming and subject to relatively large uncertainties.Also, the calibrated instruments might change during transport from thelaboratory to the measurement sites. Here we present a new calibrationmethod for UV–vis instruments that measure the spectrally resolved skyradiance, for example zenith sky differential optical absorptionspectroscopy (DOAS) instruments or multi-axis (MAX)-DOAS instruments. Ourmethod is based on the comparison of the solar zenith angle dependence ofthe measured zenith sky radiance with radiative transfer simulations. Forthe application of our method, clear-sky measurements during periods withalmost constant aerosol optical depth are needed. The radiative transfersimulations have to take polarisation into account. We show that thecalibration results are almost independent from the knowledge of the aerosoloptical properties and surface albedo, which causes a rather smalluncertainty of about < 7 %. For wavelengths below about 330 nmit is essential that the ozone column density during the measurements beconstant and known.

[学科分类] 几何与拓扑