Oxygen isotopic signature of CO2 from combustion processes
For a comprehensive understanding of the global carbon cycle precise knowledge of all processes is necessary. Stable isotope (C-13 and O-18) abundances provide information for the qualification and the quantification of the diverse source and sink processes. This study focuses on the delta O-18 signature of CO2 from combustion processes, which are widely present both naturally (wild fires), and human induced (fossil fuel combustion, biomass burning) in the carbon cycle. All these combustion processes use atmospheric oxygen, of which the isotopic signature is assumed to be constant with time throughout the whole atmosphere. The combustion is generally presumed to take place at high temperatures, thus minimizing isotopic fractionation. Therefore it is generally supposed that the O-18 signature of the produced CO2 is equal to that of the atmospheric oxygen. This study, however, reveals that the situation is much more complicated and that important fractionation effects do occur. From laboratory studies fractionation effects on the order of up to 26 parts per thousand became obvious in the derived CO2 from combustion of different kinds of material, a clear differentiation of about 7 parts per thousand was also found in car exhausts which were sampled directly under ambient atmospheric conditions. We investigated a wide range of materials (both different raw materials and similar materials with different inherent O-18 signature), sample geometries (e. g. texture and surface-volume ratios) and combustion circumstances. We found that the main factor influencing the specific isotopic signatures of the combustion-derived CO2 and of the concomitantly released oxygen-containing side products, is the case-specific rate of combustion. This points firmly into the direction of (diffusive) transport of oxygen to the reaction zone as the cause of the isotope fractionation. The original total O-18 signature of the material appeared to have little influence, however, a contribution of specific bio-chemical compounds to individual combustion products released from the involved material became obvious.