Abstract

Black carbon (BC) is a highly refractory class of compounds produced by biomass burning and metamorphism of sedimentary rocks. It comprises a continuum of compounds ranging from char and coal, which partly retain the structure of the original source fuel, to soot and petrogenic graphitic carbon, which are products formed through condensation in the gas phase and during metamorphism, respectively. Soot and graphitic carbon, on which this project focuses, are carbon-rich polyaromatic structures with particularly long environmental lifetimes, potentially making them significant contributors to the slowly-cycling carbon cycle. However, recent environmental mass balance studies suggest that soot and graphitic carbon might be more reactive than previously thought. The main focus of this project is to assess the oxidizability of soot and graphitic carbon in natural environments. Sequential long-term UV and ozone exposure experiments of n-hexane soot, graphite and residual graphitic carbon isolated from sediment resulted in carbon losses of 27.1, 5.3 and 79.2%, respectively, suggesting ozonolysis-driven degradation of these samples. In parallel, a micro-scale heavy-liquid fractionation method using sodium polytungstate was developed to separate soot and graphitic carbon in natural samples. This method shows promising results with recoveries >95% for n -hexane soot and graphite, and could therefore allow further studies on the reactivity of each phase separately. Our results emphasize the importance of reassessing the long-term preservation potential of the slowly-cycling carbon pool through studies on the oxidation mechanisms involved in their degradation.