Distinct oxidative stabilities of char versus soot black carbon: Implications for quantification and environmental recalcitrance

Abstract

Sequestration in sediments of black carbon (BC) from vegetation fires and fuel combustion may constitute a significant sink of otherwise rapidly cycling carbon from the atmosphere‐biosphere cycle. It also has the potential to provide a historical record of atmospheric BC loadings. Previous treatments of BC as one homogeneous entity are being replaced with the growing awareness of a BC combustion continuum, a range spanning from slightly charred biomass to soot and graphite. Here the relative recalcitrance of different BC forms is evaluated, and implications for both BC quantification and environmental stability are considered. The stabilities of four BC reference materials against thermal oxidation in air were quite distinct with T50%BC values (i.e., the temperature where 50% BC remained in the residue) of 444°C (diesel soot–BC), 388°C (n‐hexane soot–BC), 338°C (wood char–BC), and 266°C (grass char–BC). The implications for BC quantification have been illustrated for a thermal oxidation (the CTO‐375) method commonly applied to study BC in sediments. This technique measured BC:TOC ratios of 78.3 ± 1.3% for the diesel soot–BC and 45.3 ± 6.1% for n‐hexane soot–BC, whereas no CTO375‐BC was detected for the two analyzed char‐BC materials. The greater lability of char‐BC compared to soot‐BC likely reflects higher accessibility to internal microporosity in char‐BC, facilitating internal O2 transfer. Decreasing the temperature cutoff below 375°C to also include char‐BC is not possible as thermograms of nonpyrogenic reference materials indicated that such material would then be artifactually quantified as BC. The presence of mineral oxides in the sediment matrix may lead to a catalytically mediated lowering of the activation energy for soot‐BC oxidation but not for char‐BC or nonpyrogenic organic material. Several recent studies combine to challenge the proposition of complete recalcitrance of BC. Particularly, the thermal lability of char‐BC from grassland fires deserves further attention in order to improve the understanding of BC in the global carbon cycle.