Oxygen isotopic signature of CO₂ from combustion processes

Abstract. For a comprehensive understanding of the global carbon cycle precise knowledge of all processes is necessary. Stable isotope (¹³C and ¹⁸O) abundances provide information for the qualification and the quantification of the diverse source and sink processes. This study focuses on the δ¹⁸O 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 ¹⁸O 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%permil; became obvious in the derived CO₂ from combustion of different kinds of material, a clear differentiation of about 7‰ 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 ¹⁸O 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 ¹⁸O 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.