  The Earth's atmosphere is an integral part of the detector in ground-based
imaging atmospheric Cherenkov telescope (IACT) experiments and has to be taken
into account in the calibration. Atmospheric and hardware-related deviations
from simulated conditions can result in the mis-reconstruction of primary
particle energies and therefore of source spectra. During the eight years of
observations with the High Energy Stereoscopic System (H.E.S.S.) in Namibia,
the overall yield in Cherenkov photons has varied strongly with time due to
gradual hardware aging, together with adjustments of the hardware components,
and natural, as well as anthropogenic, variations of the atmospheric
transparency. Here we present robust data selection criteria that minimize
these effects over the full data set of the H.E.S.S. experiment and introduce
the Cherenkov transparency coefficient as a new atmospheric monitoring
quantity. The influence of atmospheric transparency, as quantified by this
coefficient, on energy reconstruction and spectral parameters is examined and
its correlation with the aerosol optical depth (AOD) of independent MISR
satellite measurements and local measurements of atmospheric clarity is
investigated.
