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

<p>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.</p><p>&#160;</p><p>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.</p>