Laboratory charring conditions affect black carbon properties: A case study from Quebec black spruce forests

Black carbon (BC) is an important residue of wildfires in boreal forests, but its characteristics depend on its formation conditions. The objective of this study was to characterize the chemical and physical properties of BC produced under controlled laboratory conditions, while mimicking a gradient of wildfire severity. We used fuels originating from mosses, ericaceous shrubs and spruce trees, as they constitute the major types available in boreal forests. We varied the maximum temperature (MT) from 75 to 800 °C, the duration of charring from 0.5 to 24 h and the abundance of O2. BC properties were analyzed using elemental analysis and proximate analysis, solid state 13C nuclear magnetic resonance (NMR) spectroscopy, scanning electron microscopy (SEM) and surface area (SA) analysis. MT was the most significant factor affecting both chemical and physical changes. Results from 13C NMR spectroscopy indicated that 350 °C was a threshold temperature, above which spectra became dominated by aromatic structures for all fuels. Charring duration affected BC composition at both low (250 °C for 12 h vs. 24 h) and high (600 °C for 0.5 h vs. 6 h) MT. The presence of O2 influenced BC composition mainly at low MT (250 °C) and resulted in accelerated alkyl C degradation, accompanied with a distinct decrease in yield. Results from the SA analysis and the SEM micrographs showed that fuel type influenced BC physical properties, with moss-derived BC presenting higher surface area and microporosity than BC of woody origin.