Abstract
Globally landscape fires produce about 256 Tg of pyrogenic carbon or charcoal each year. The role of charcoal as a source of environmentally persistent free radicals, which are precursors of potentially harmful reactive oxygen species, is poorly constrained. Here, we analyse 60 charcoal samples collected from 10 wildfires, that include crown as well as surface fires in forest, shrubland and grassland spanning different boreal, temperate, subtropical and tropical climate. Using electron spin resonance spectroscopy, we measure high concentrations of environmentally persistent free radicals in charcoal samples, much higher than those found in soils. Concentrations increased with degree of carbonization and woody fuels favoured higher concentrations. Moreover, environmentally persistent free radicals remained stable for an unexpectedly long time of at least 5 years. We suggest that wildfire charcoal is an important global source of environmentally persistent free radicals, and therefore potentially of harmful reactive oxygen species.
Highlights
Landscape fires produce about 256 Tg of pyrogenic carbon or charcoal each year
No measurements of environmentally persistent free radicals (EPFRs) have been reported from wildfire charcoals to date, but high EPFR concentrations have previously been reported in human-derived pyrogenic products; for example, in particulate matter (PM) from fossil fuel combustion[23] and biochar[14], a man-made product of pyrolysis with some similarities to charcoal but produced under industrial conditions[24,25]
We found that all charcoals analysed via electron spin resonance (ESR) spectroscopy have high concentrations of EPFRs
Summary
Landscape fires produce about 256 Tg of pyrogenic carbon or charcoal each year. To assess the stability of charcoal EPFRs in natural environments, we included samples collected at different times after fire (aged 0–5 years) (Tables 1 and S1). EPFR concentrations were related to charcoal carbonization degree, which in turn is determined largely by the type of fuel material (feedstock) and charring conditions.
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