Toward a global estimate of black carbon in residues of vegetation fires representing a sink of atmospheric CO2and a source of O2

Abstract

Vegetation fire residues of various kinds of plant materials were analyzed for black carbon (BC) and the corresponding hydrogen. These data clearly show that BC can be defined as the fire produced carbon fraction with a molar H/C ratio of ≤ 0.2 which is resistant to heating to 340°C in pure oxygen. Relationships of black carbon production to fire characteristics such as fuel type, gaseous emissions, etc., were studied demonstrating that black carbon is mainly a product of the hot flaming combustion and that more than 80% of the black carbon produced remains on ground after the fire with the rest being emitted with the smoke. This black carbon mainly in the fine residue may significantly contribute to the background concentrations of black carbon in the atmosphere. From the ratios of black carbon in the residue to carbon exposed to the fire, and to carbon emitted as CO2we estimate the global black carbon formation to be 50–270 Tg yr−l(teragram = 1012g). As this carbon is very resistant to microbial decay, it may well represent a sink for biospheric carbon. This sink may be significantly greater, for example, up to a factor of 2 when reducing the pretreatment temperature to 300°C, if some of the less resistant carbon with higher H/C ratios which is removed by our method would also show significant resistance to microbial breakdown. The fire‐induced sequestration of carbon from the short‐term biospheric to the long‐term geological cycle due to the formation of black carbon may represent a significant sink of atmospheric CO2and source of O2and possibly has influenced today's atmospheric oxygen content.