Stable carbon isotopic composition of biomass burning emissions – implications for estimating the contribution of C3 and C4 plants

Biomass burning on the African continent emits large amounts of CO₂, CO, and aerosols. Our aim is to use measurements of the stable carbon isotope ¹³C in organic carbon, CO and CO₂ in biomass burning smoke to estimate the contribution of C3 plants (trees and bushes) and C4 plants (mainly Savannah grass), which have very distinct ¹³C/¹²C ratios. This is possible, if ¹³C/¹²C ratios are not significantly altered by the combustion process. This assumption is investigated in a series of laboratory experiments, where C3 and C4 plants (corn and willow wood), or C3-C4 plant mixtures are burned. The laboratory results are used to interpret the results of pilot studies of smoke sampled in African savannah fires.

 

First results from the laboratory studies indicate that organic carbon (OC) from combustion of willow or corn shows ¹³C/¹²C ratios comparable to the burned plant material. For combustion of willow (C3), the ¹³C/¹²C ratios in OC tend to be slightly higher than in the wood fuel, depending on combustion conditions. For combustion of corn ¹³C/¹²C ratios of OC tend to be slightly lower than in the fuel. For mixtures of willow and corn the relationship between ¹³C/¹²C ratios in the emitted organic carbon and the fuel mixture is slightly non-linear: For a 50-50% oak wood and corn mixture the ¹³C/¹²C ratio in OC is closer to that of corn than that of willow. First results from pilot field studies indicate that a larger fraction of OC comes from trees and bushes, although mainly Savannah grass is burned in the investigated fires.