METHOD FOR DETERMINING BLACK CARBON IN RESIDUES OF VEGETATION FIRES

We present here a simple versatile method for the direct determination of black carbon in residues of vegetation fires. Differentiation of black carbon from organic carbon is based on solubility, vapor pressure, and oxidative stability. Charring,the formation of black carbon due to the analytical method, is significantly reduced, achieved by a solvent extraction prior to a thermal treatment. It is shown that the final step for the differentiation, the thermal treatment, can be looked at as a fractionation of carbon compounds by their molar H/C ratio. The molar H/C ratio determined for black carbon in ash was 0.19 +/- 0.05 but has to be corrected for some hydrogen bonded to elements other than carbon, most likely silicate. Thus, black carbon appears to have high aromaticity and a low organic carbon content. Black carbon comprised 16.0 +/- 2.0% of the total carbon in the ash residues. Due to a favorable intercomparison to another method determining black carbon in aerosols and to the possibility to adjust the solvent extraction to different matrices, we believe that the method presented here is satisfactory and can be used to derive information on the global cycling of black carbon.