  During the last decade, imaging atmospheric Cherenkov telescopes (IACTs) have
proven themselves as astronomical detectors in the very-high-energy (VHE; E>0.1
TeV) regime. The IACT technique observes the VHE photons indirectly, using the
Earth's atmosphere as a calorimeter. Much of the calibration of Cherenkov
telescope experiments is done using Monte Carlo simulations of the air shower
development, Cherenkov radiation and detector, assuming certain models for the
atmospheric conditions. Any deviation of the real conditions during
observations from the assumed atmospheric model will result in a wrong
reconstruction of the primary gamma-ray energy and the resulting source
spectra. During eight years of observations, the High Energy Stereoscopic
System (H.E.S.S.) has experienced periodic natural as well as anthropogenic
variations of the atmospheric transparency due to aerosols created by biomass
burning.
  In order to identify data that have been taken under such long-term
reductions in atmospheric transparency, a new monitoring quantity, the
Cherenkov transparency coefficient, has been developed and will be presented
here. This quantity is independent of hardware changes in the detector and,
therefore, isolates atmospheric factors that can impact the performance of the
instrument, and in particular the spectral results. Its positive correlation
with independent measurements of the atmospheric optical depth (AOD) retrieved
from data of the Multi-angle Imaging SpectroRadiometer (MISR) on board of the
Terra NASA's satellite is also presented here.
