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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.
