In optical analysers devoted to the analysis of atmospheric black carbon concentration, the specific attenuation cross-section, sigma is the factor used to convert the attenuation of a light beam due to the absorption of aerosols deposited on a filter into their black carbon content.We have tried to pin further insight for a suitable choice of sigma value, using both optical analysis (with an aethalometer) and thermal analysis of black carbon aerosols and comparison of the two sets of results. Samples which were investigated originate from varying environments, including suburban areas, tropical areas where biomass burning was prevalent and from more remote locations. In a given type of atmospheric environment, sigma values are found to be constant. However, sigma displays an important variability (range: 5-20 m2 g-1) which may be related to the variability of the aerosol mix (internal or external mixture) and the aging of the atmospheric particulate phase.Our results quote unambiguously the need for a modulated calibration of optical analysers depending on the type of atmospheric environments which are studied. They suggest the need to reconsider carefully the black carbon data obtained at remote locations.