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During the last decade, imaging atmospheric Cherenkov telescopes (IACTs) haveproven themselves as astronomical detectors in the very-high-energy (VHE; E>0.1TeV) regime. The IACT technique observes the VHE photons indirectly, using theEarth's atmosphere as a calorimeter. Much of the calibration of Cherenkovtelescope experiments is done using Monte Carlo simulations of the air showerdevelopment, Cherenkov radiation and detector, assuming certain models for theatmospheric conditions. Any deviation of the real conditions duringobservations from the assumed atmospheric model will result in a wrongreconstruction of the primary gamma-ray energy and the resulting sourcespectra. During eight years of observations, the High Energy StereoscopicSystem (H.E.S.S.) has experienced periodic natural as well as anthropogenicvariations of the atmospheric transparency due to aerosols created by biomassburning.  In order to identify data that have been taken under such long-termreductions in atmospheric transparency, a new monitoring quantity, theCherenkov transparency coefficient, has been developed and will be presentedhere. This quantity is independent of hardware changes in the detector and,therefore, isolates atmospheric factors that can impact the performance of theinstrument, and in particular the spectral results. Its positive correlationwith independent measurements of the atmospheric optical depth (AOD) retrievedfrom data of the Multi-angle Imaging SpectroRadiometer (MISR) on board of theTerra NASA's satellite is also presented here.

Influence of aerosols from biomass burning on the spectral analysis of Cherenkov telescopes