OPTICAL AND THERMAL MEASUREMENTS OF BLACK CARBON AEROSOL CONTENT IN DIFFERENT ENVIRONMENTS - VARIATION OF THE SPECIFIC ATTENUATION CROSS-SECTION, SIGMA (SIGMA)

In optical analysers devoted to the analysis of atmospheric black carb on concentration, the specific attenuation cross-section, sigma is the factor used to convert the attenuation of a light beam due to the abs orption of aerosols deposited on a filter into their black carbon cont ent. We have tried to pin further insight for a suitable choice of sig ma value, using both optical analysis (with an aethalometer) and therm al analysis of black carbon aerosols and comparison of the two sets of results. Samples which were investigated originate from varying envir onments, including suburban areas, tropical areas where biomass burnin g was prevalent and from more remote locations. In a given type of atm ospheric 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 externa l mixture) and the aging of the atmospheric particulate phase. Our res ults quote unambiguously the need for a modulated calibration of optic al analysers depending on the type of atmospheric environments which a re studied. They suggest the need to reconsider carefully the black ca rbon data obtained at remote locations.